https://irationaudio.com/dean-costello/https://irationaudio.com/wp-content/uploads/2018/02/img_2115.jpgIMG_21152024-06-14T20:41:21+00:00weekly0.6https://irationaudio.com/amp-repair-chicago-il/https://irationaudio.com/wp-content/uploads/2015/03/836-world-headquarters.jpg836 World Headquarters2024-06-14T20:39:48+00:00weekly0.6https://irationaudio.com/2017/08/27/sears-silvertone-1466/https://irationaudio.com/wp-content/uploads/2017/08/silvertone1465chassis185-12010.jpgsilvertone1465Chassis185-12010I was never able to find Silvertone 1466 schematic but I did find this 1465. It is the guitar version and has tremolo and reverb added but I would guess much of the rest of the circuit is the same.https://irationaudio.com/wp-content/uploads/2017/08/silvertone-1466-front.jpgSilvertone 1466 frontSears Silvertone 1466 bass amp. Keeping it very simple with just volume, treble and bass. PCB is visible behind the front panel on the bottom, so I suppose you could actually just reach in and touch it. https://irationaudio.com/wp-content/uploads/2017/08/silvertone-1466-chassis.jpgSilvertone 1466 chassisInside the chassis is a mix of point to point and PCB. Preamp PCB is mounted to front panel and power amp and power supply are mostly hard wired. This just needed a grounded 3-prong and some mods for versatility. The amp comes with a speaker cable out that wires directly to the 6x10 speaker cab. The owner wanted to be able to use this head with different cabs and this cab with other amps, so I added a 1/4" speaker out jack to the head and added a 1/4" input jack to the speaker cable that goes to the cab. There were a few old crusty electrolytics that were replaced also. https://irationaudio.com/wp-content/uploads/2017/08/silvertone-1466-back.jpgSilvertone 1466 backBack panel has cutout for head to fit inside. And little white stoppers that flip down so it doesn't fall out when you move it around.2020-04-04T07:07:16+00:00monthlyhttps://irationaudio.com/2017/08/11/ampeg-gemini-gv15/https://irationaudio.com/wp-content/uploads/2017/08/ampeg-gv15.jpgAmpeg GV15Time to go into the archives. Old Ampeg Gemini GV15 from late 60s or so. Has a single 15" speaker to handle the 50W or so. Nice wheels since it's a backbreaker. Also always makes me cringe when I see people rolling amps down the sidewalk so I'm actually not a fan of them. More a fan of broken back and unjostled amp.https://irationaudio.com/wp-content/uploads/2017/08/ampeg-gv15-chassis.jpgAmpeg GV15 chassisR73 470 ohm 2W power supply dropping resistor was burnt and the amp had a bias problem that was making the power tubes red plate. R73 probably got toasty because of bias problem causing excessive plate and screen current when tubes were redplating, I'd guess. Interesting that R73 should be damaged because that's the same resistor in Ampeg V series that sometimes has trouble. In V4 it's R53 470 ohm 7W and V2 has R55 1K 10W (higher resistance and higher power rating in V2 because since it has 2 power tubes compared to 4 in V4 and same transformers, the power supply voltages are not as loaded down and power tubes are in the 580-590V territory instead of 530-540V for V4). This resistor in all these amps is a power supply dropping resistor that takes place of where a choke would be in amps that have them, between the B+ and screen supply. There is a heavy current that flows in this resistor when the screen current increases, essentially when you're playing loud. Play loud enough and enter into power amp distortion territory and the screen current increases exponentially. You probably won't run into this too much with Gemini series player since maybe they'll be more clean and lighter rock players (2W resistor) but with V series powerful rock amps definitely. So I guess they needed 7 and 10W resistors for V4 and V2. I kind of think 10W is a good minimum for either of these amps. Some Traynor amps had 20W resistors in this position FTW.https://irationaudio.com/wp-content/uploads/2017/08/ampeg-gemini-gv15-schematic.jpgAmpeg Gemini GV15 schematicWell, not the greatest focus but you gotta appreciate the realness of this Ampeg GV15 schematic. Pasted in the chassis -- the usual Ampeg-style nice touch for service people. 2020-04-04T07:03:38+00:00monthlyhttps://irationaudio.com/2017/03/14/sunn-concert-lead/https://irationaudio.com/wp-content/uploads/2017/03/sunn-concert-lead-silver-face.gifSunn Concert Lead Silver facehttps://irationaudio.com/wp-content/uploads/2017/03/sunn-concert-lead-resistor.pngSUnn concert lead resistorhttps://irationaudio.com/wp-content/uploads/2017/03/sunn-concert-lead-front.pngSunn concert lead fronthttps://irationaudio.com/wp-content/uploads/2017/03/sunn-concert-lead-chassis.pngSunn COncert lead chassis2020-04-04T06:57:10+00:00monthlyhttps://irationaudio.com/2016/05/30/fender-pa100/https://irationaudio.com/wp-content/uploads/2016/05/fenderpa100schematic.pngFenderPA100schematicFender PA100 schematic. Cool amp with four identical channels. You can bridge them together for a more aggressive sound if you desire. https://irationaudio.com/wp-content/uploads/2016/05/fender-pa100-head.jpgFender PA100 headPA100 with 4 channels and some lush reverb. Still running on four old RCA 6L6GCs. https://irationaudio.com/wp-content/uploads/2016/05/fender-pa100-chassis.jpgFender PA100 chassisVery deceiving repair because it would put out no signal at all. V3 12AT7 was not working, all the flash material had turned white. Also the 470 ohm cathode resistor was burnt to a crisp. Replacing both and the amp worked again but the reverb was kind of weak. At first I thought it was weak because the amp was sitting on my desk instead of on top of a cab, getting the physical vibration, but actually it was because of the reverb transformer. There was still high voltage on the primary but in some way the secondary wasn't able to drive the reverb tank. It didn't test strange with a meter but it was definitely the problem. Clipping in a new one and the reverb was back to full strength.2020-04-03T09:07:20+00:00monthlyhttps://irationaudio.com/2016/02/19/sundown-a-50/https://irationaudio.com/wp-content/uploads/2016/02/sundown_a50.pngsundown_a50Sundown A-50 schematichttps://irationaudio.com/wp-content/uploads/2016/02/mg_2542.jpg_MG_2542A build which I guess you could call better than average but still janky in some ways. The caps in the power supply are not held down or strapped down in any way. This one had a power supply cap flapping in the breeze which had broken its lead, due to vibration. Heavy components need to be secured in some way. This build has lots of heavy components flying around in the air without support. Will not withstand the test of vibration over time.https://irationaudio.com/wp-content/uploads/2016/02/mg_2550.jpg_MG_2550Sundown A-50 which is a mid 80s amp made by Dennis Kager. Kind of a cool little amp with two channels for clean and gain. Has an RMS knob adjusting output from 50 to 15W and a Governor adjustment which controls a post phase inverter compression circuit.2020-04-03T06:37:33+00:00monthlyhttps://irationaudio.com/2016/01/15/soldano-avenger/https://irationaudio.com/wp-content/uploads/2016/01/soldano-avenger-2-preamp.jpgSoldano Avenger-2 preampSoldano Avenger power supply/output sectionhttps://irationaudio.com/wp-content/uploads/2016/01/soldano-avenger-power-supply-output-section-1.jpgSoldano Avenger power supply output section-1Soldano Avenger preamp/power supplyhttps://irationaudio.com/wp-content/uploads/2016/01/mg_2533.jpg_MG_2533This thing sounds great and also is really good with pedals. Not many amps that can be gainy enough to used outright but also work well with pedals. Sadly the power transformer died for what seemed to be a fluke, some accident, or no reason. There was nothing else wrong with the amp. This thing was doing 98W at clipping with 5881s. Pretty impressive considering most 100W 6L6GC amps usually only do 80-85W with 5881s. What a beast. https://irationaudio.com/wp-content/uploads/2016/01/mg_2537.jpg_MG_25372020-04-03T06:31:56+00:00monthlyhttps://irationaudio.com/2016/01/10/hiwatt-dr103/https://irationaudio.com/wp-content/uploads/2016/01/img_0203.jpgIMG_0203The quintessential Hiwatt DR103 finding its way on to my cluttered workbench. Just having a quick look at this classic piece. Look how the speaker output 1/4″ jacks are isolated…………https://irationaudio.com/wp-content/uploads/2016/01/img_02071.jpgIMG_0207Looks rugged.https://irationaudio.com/wp-content/uploads/2016/01/img_02091.jpgIMG_0209Phenolic coating added to each solder connection so it does not corrode over time. High grade. If you want to learn more about the venerable DR103 and Hiwatts in general vist Mark Huss’s wonderful site — Mark Huss Hiwatt Information Pages. Tons of info and schematics for all the different eras/builds.https://irationaudio.com/wp-content/uploads/2016/01/img_02061.jpgIMG_0206Looks great and very organized just as you would expect. Someone did a great job.2020-04-03T06:10:31+00:00monthlyhttps://irationaudio.com/2015/12/02/traynor-ygl-3-mark-3/https://irationaudio.com/wp-content/uploads/2015/12/traynor-ygl3_3a_mk3.gifTraynor YGL3_3A_Mk3Traynor YGL-3 / 3A Mark-3 schematic. Check it out, power tubes are 6CA7/EL34 and rather than having the suppressor grid tied to ground like is usually the case it is connected to the negative bias voltage at the control grid. https://irationaudio.com/wp-content/uploads/2015/12/mg_2387.jpg_MG_2387Alright cool old Traynor I believe from late 60s. Maybe a Twin Reverb competitor since it has reverb and vibrato on two channels, high and low inputs, treble boost, etc, the whole deal. https://irationaudio.com/wp-content/uploads/2015/12/mg_2380.jpg_MG_2380Quite a long tag strip in this one. The brown boxes front and center are thyrectors, a part I have never encountered before. They are dual directional diodes used in this case for output transformer protection. They bridge across the two plate leads and if you're unfamiliar with them and they short it will make you think the OT has gone. Nice signal right up to the power tube grids but at the plates its weak and terribly distorted. When the amp is pushed the B+ drops dramatically. Barely any sound comes through the speakers. Power tubes are burning hot as if they're putting out 100W. Seems like in other OT protection schemes when the protection fails the amp blows a fuse and shuts down but in this case it does not. I replaced the shorted thyrectors with R3000 diodes from plate lead to ground a la modern Fenders and whatnot. Don't want to use more thyrectors, they are selenium rectifiers. A component of yesteryear --selenium is toxic.2020-04-03T06:06:33+00:00monthlyhttps://irationaudio.com/2015/10/25/mesa-boogie-f-50/https://irationaudio.com/wp-content/uploads/2015/10/mg_2450.jpg_MG_2450Mesa Boogie F-50. Seems like this is a little known amp. Has two 6L6GCs in the output section for 50W. Also comes in a four 6L6GCs F-100 version for 100W and a F-30 with two EL84s doing 30W. I like how it's pretty stripped down and doesn't have a billion knobs and buttons and switches like a lot of other Mesas. Has a clean sound which people seem to claim is one of Mesa's best ever, and a gain channel with boost, so really only two channels. With the boost it gets pretty gainy, but doesn't quite reach into the gain territory of the rectifier series. Had a string attached to the knob for what I would assume was a price tag. I should have removed it before taking the pic oh well.https://irationaudio.com/wp-content/uploads/2015/10/mg_2432.jpg_MG_2432Pretty neat and tidy like they usually are. Always nice when the pots are not PCB mounted. This amp worked great other than an intermittent connection at the AC cord, causing it to cut out or sometimes not even turn on. Check out the full schematic for Mesa Boogie F-50/F-100 available as PDF and get into it.2020-04-03T06:03:46+00:00monthlyhttps://irationaudio.com/2015/10/24/ampeg-v4/https://irationaudio.com/wp-content/uploads/2015/10/vt22-v4-74.gifvt22-v4-74Ampeg V4/VT22 schematic. VT22 is the same amp but the combo version.https://irationaudio.com/wp-content/uploads/2015/10/img_0402.jpgIMG_0402Gotta love how all the old Ampegs came with the schematic pasted to the chassis. This one is in good shape and has most of it's screen printed front panel still intact. Lots of these have worn off over the years. I could write about the V4/VT22 but there is an awesome write up at the Atomium Amps site, might as well just peruse that and get into it -- Atomium Amps V-4 Analysis (post restoration). https://irationaudio.com/wp-content/uploads/2015/10/img_0401.jpgIMG_0401Yay my favorite amp Ampeg V4. These amps are notoriously unreliable but it's not because they're cheap or poorly designed. They're from a different era where tubes were more robust. They run hot and the tubes hang under the chassis, so all the heat just rises up into it. If you bias this at 70% you will probably be in for a headache -- not recommended. 50-55% is much better for reliability. The plate voltage is high (530-560V) and the floating paraphrase phase inverter design gets very asymmetrical the farther into clipping you go. When the amp is driven into distortion it starts running two power tubes really hard while the other two actually draw less current. It's common to see these amps with two power tube logos darkened from the heat while the other two look normal. Also they just usually don't get the love that a lot of other vintage amps get. They fell out of popularity for a while because they have a handful of weird 70s tubes that became hard to find. Now most of them are back in production though, with the exception of the 6K11 -- 7027, 6K11, 6FQ7/6CG7, 12DW7/ECC832. Usually when I get them they've had next to no maintenance.People like to re-tube and re-cap their 70s Marshalls and Fenders but it seems a lot of Ampegs went in the basement for a while. Now they're getting resurrected and prices are going up.2020-04-03T06:00:39+00:00monthlyhttps://irationaudio.com/2015/10/03/marshall-jcm800/https://irationaudio.com/wp-content/uploads/2015/10/mg_24271.jpg_MG_2427Bias kept creeping up and up for about 20 minutes with this amp before finally stabilizing. Gotta let this thing run for a bit (like most amps) before biasing otherwise you will bias it "normal" which will eventually turn into hot/way hot. Marshall JCM800 service manual with schematic and much more available at the amparchives page -- JCM800https://irationaudio.com/wp-content/uploads/2015/10/mg_2422.jpg_MG_2422The quintessential, Marshall JCM800 from 1983. Still has the original power supply caps and they are visibly bulgey. However, AC at the B+ node is only 5V at idle and 19 at clipping -- seems pretty average. Amp still performs normal but the caps are probably getting towards the end of their days. It is year 32 after all. Layout is very similar to Marshall JMP. This thing appears to be untouched other than what look like new 1k/5W screen grid resistors.https://irationaudio.com/wp-content/uploads/2015/10/mg_2416.jpg_MG_2416Broken connection on the ground buss wire for the preamp volume pot.2020-04-03T05:48:15+00:00monthlyhttps://irationaudio.com/2015/09/28/music-city-super-power-amplifier-treble-and-bass/https://irationaudio.com/wp-content/uploads/2015/09/mg_2457.jpg_MG_2457The old Partridge transformers you see a lot from UK amps of the era.https://irationaudio.com/wp-content/uploads/2015/09/mg_2455.jpg_MG_2455Has high and low inputs for Bass and Treble channels as well as an on/off Boost for each. Originally made for KT77s but doing it up with some 6550s just to check it out.https://irationaudio.com/wp-content/uploads/2015/09/mg_2449.jpg_MG_2449Checking out the MATH (FFT) a little bit lately which is kind of fun. The amp is run up to 32.6VAC output in the pic but it really starts to clip at about 31V into 8 ohms for 120W, great.https://irationaudio.com/wp-content/uploads/2015/09/mg_2445.jpg_MG_2445Music City Super Power Amplifier Treble and Bass. Whoo! Bet you will never see one of these again. Made for the Music City Music Store in the UK by Laney some time in the 70s. Supposedly very similar to the Laney Supergroup, but not an exact copy. This thing has been totally recapped, and even most of the coupling caps have been replaced with orange drops. That's too bad, because it probably had the old yellow mustard caps originally. Oh well, sounds great and works nice, who cares.https://irationaudio.com/wp-content/uploads/2015/09/mg_2437.jpg_MG_2437I borrowed this and used it for a recording. It uses KT77s for the power tube, probably because the plate voltage is so high, in the 630-640V range. Afterwards I noticed two power tubes had blackened JJ logos. Flipping it on after the recording the fuse blew in a few seconds, and then continued blowing. One the blackened power tubes would intermittently short. Usually pretty quick, like within 10 seconds. Fortunately it survived recording. https://irationaudio.com/wp-content/uploads/2015/09/img_0254.jpgIMG_0254Fun stuff2020-04-03T05:45:09+00:00monthlyhttps://irationaudio.com/2015/09/24/kustom-250/2020-04-03T05:24:55+00:00monthlyhttps://irationaudio.com/2015/08/08/peavey-classic-vt/https://irationaudio.com/wp-content/uploads/2015/08/img_1993.jpgIMG_1993https://irationaudio.com/wp-content/uploads/2015/08/img_2673.jpgIMG_2673https://irationaudio.com/wp-content/uploads/2015/08/img_2564.jpgIMG_2564This amp reminds me of Musicman somewhat since the preamp is mostly op amp driven and the output section has a transistor phase inverter and then tube output section. The plate voltage is only about 500V, which is still somewhat high, but not as high as the Musicmans which go into the 600 and 700V ranges for that extra bit of output power. I was amazed by the Musicman 2100-RD which did 100W with two 6L6GCs, but that's an aside. The plate voltage was 720V. Peavey Classic VT schematis is available here - https://drive.google.com/file/d/0B01EXvY0__YYWlhsMUdYVnhOTmc/view?usp=sharing Peavey Classic VT user manual here - http://assets.peavey.com/literature/manuals/80343001.pdfhttps://irationaudio.com/wp-content/uploads/2015/08/img_2562.jpgIMG_2562Peavey Classic VT from some time in late 70s or early 80s. Pretty cool little 2x12 combo amp. Does 50W with two 6L6GCs and has reverb and "phase." Phase control is awesome. It basically modulates the signal by introducing a variable frequency notch in the amplifiers passband. "This phasor is capable of producing an extremely wide range of phasing effects, including tremolo/vibrato effects. When used with a touch of reverberation, it can closely approximate other popular sound effects such as those produced by rotating speakers, etc."2020-04-03T05:21:42+00:00monthlyhttps://irationaudio.com/2015/07/26/acoustic-150/https://irationaudio.com/wp-content/uploads/2015/07/img_2599.jpgIMG_2599Getting some Acoustic 150s to work on. They do about 100W and have tremolo and reverb. https://irationaudio.com/wp-content/uploads/2015/07/img_2596.jpgIMG_2596Preamp and reverb and tremolo is on the PCBs on the right and the middle PCB in the foreground is the input and driver stage of the output section. Output transistors are mounted on the chassis for heat sinking. They are 2N3055s in typical fashion. From the driver stage of the output section the signal is transformer coupled to the bases of the output transistors, kind of like Sunn Concert series. Since this is a single supply amp and it runs on a single +75V from the power supply, a big 1000uF/50V cap is used at the output to isolate DC from the output section from the speaker. The Acoustic 134, 134, and 150 schematics and user manuals are all merged into one document, 36 pages, literally telling you everything. What a great manual -- https://drive.google.com/file/d/0B01EXvY0__YYVnU1QVRqbktkUUk/view?usp=sharing2020-04-03T05:18:47+00:00monthlyhttps://irationaudio.com/2015/06/13/ampeg-svt-iii/https://irationaudio.com/wp-content/uploads/2015/06/img_2527.jpgIMG_2527This is similarly rack-able like the SVT-II but with SS output section instead of six 6550s it's way smaller and lighter. It did 358W at clipping into 4 ohms. https://irationaudio.com/wp-content/uploads/2015/06/img_2526.jpgIMG_2526https://irationaudio.com/wp-content/uploads/2015/06/img_2525.jpgIMG_2525Ampeg SVT-III, not the SVT-III Pro. This amp is similar to the SVT-II in preamp but the power amp is solid state. Output transistors are mounted to the heatsink and are in the old school TO-3 package. These ones are MJ15023/MJ15022. Preamp is up front with three 12AX7s and the output section and power supply are towards the rear.https://irationaudio.com/wp-content/uploads/2015/06/img_2509.jpgIMG_2509Main problem was the inductor at the output was physically damaged because of a drop, and then electronic damaged ensued from intermittent connections. The red wire wound inductor is in parallel with the 10 ohm/2W resistor. They are in series with the output. In the image you can see the resistor is blackened -- it is burnt open. The inductor was broken loose from the PCB but still making contact. After a little use it rattled loose making an intermittent connection at the output. When the connection went in and out it fried the 10 ohm resistor and also burnt the PCB traces underneath. It's important to scrape all the charred remains off the PCB because it is now carbonized plastic and therefore can be conductive. Full schematic and parts list and all that here -- https://drive.google.com/folderview?id=0B01EXvY0__YYeTZWZnRBang4T1k&usp=sharing2020-04-03T05:13:36+00:00monthlyhttps://irationaudio.com/2015/06/13/fender-bassman-ab165-2/https://irationaudio.com/wp-content/uploads/2015/06/fender_bassman_ab165.giffender_bassman_ab165Fender Bassman AB165 schematichttps://irationaudio.com/wp-content/uploads/2015/06/img_2512.jpgIMG_2512Fender Bassman in the drip edge fashion, with the metal grill going around the silver grill cloth. From around the 32nd week of 1967.https://irationaudio.com/wp-content/uploads/2015/06/img_2506.jpgIMG_2506This amp didn't work because one of the power tubes had fallen out of the socket and was re-inserted in the wrong orientation (center pin to line up the tube was broken off). I see so many old Fenders with extremely loose power tube sockets. This is because the tubes get jostled around for decades when people shove the AC cord into the head chassis. Most can be re-tensioned, some have to be replaced.2020-04-03T05:10:33+00:00monthlyhttps://irationaudio.com/2015/06/03/red-bear-mk-60/https://irationaudio.com/wp-content/uploads/2015/06/6n2p-to-12ax7-conversion.jpg6N2P to 12AX7 conversionThese originally came with Russian style 6N2P preamp tubes. Similar to 12AX7 but harder to find, so it is common to rewire the tube socket for 12AX7, which is not hard to do. https://irationaudio.com/wp-content/uploads/2015/06/6n2p-to-12ax7-conversion-e1452586747918.gif6N2P to 12AX7 conversionhttps://irationaudio.com/wp-content/uploads/2015/06/img_2317.jpgIMG_2317Amp needed power supply recap and broken jacks replaced. Build quality is similar to Sovtek, a mixture of rugged and ultra flimsy. Sounded sweet at least. Oh well. Schematics are at the triode site . They are totally hard to read unfortunately -- http://www.triodeel.com/schindex.htmhttps://irationaudio.com/wp-content/uploads/2015/06/img_2314.jpgIMG_2314Red Bear MK 60. Cool Russian made amp from the 90s, similar to JCM800 as they say. It is crunchy and crisp and it rocks. I only got the chassis, not the whole enclosure. It was distributed by (Gibson of all people) in the US but only for a few years. 2020-04-03T05:03:57+00:00monthlyhttps://irationaudio.com/2015/03/31/traynor-yba1-1a-mkii/https://irationaudio.com/wp-content/uploads/2015/03/img_1720.jpgIMG_1720Traynor output doing about 70V pk-pk into 8 ohms for 76W at clipping.https://irationaudio.com/wp-content/uploads/2015/03/traynor-yba1-1a-mkii.giftraynor yba1-1a MKIITraynor YBA1-1A MKII schematic Also notice the schematic lists 8 ohms as the OT load. Lots of 'net stuff says you can run these at 4 ohms but in my experience the current draw runs off too much, exceeding 200mA, and it will probably destroy the tubes. I wouldn't continue with that. At 8 ohms no problem. Maybe this impedance confusion is just due to YBA-1 vs YBA-1A mixup. Not sure https://irationaudio.com/wp-content/uploads/2015/03/img_1974.jpgIMG_1974Yay Traynory YBA1-1A Bassmaster Mark II. What a long name. Not that familiar with the Traynor amps but this one seems somewhat special because it is powerful for an amp that has only two power tubes. People on the net say anywhere from 65-100W but I remember this one starting to clip around 76W or so. Lots of speculation on this. Kind of a funny image since there are five different brands of electrolytics in this amp. Lots of people have had their hands in this over the years, Oh well. At least it works good and there are no more caps from the late 60s/early 70s still in use.https://irationaudio.com/wp-content/uploads/2015/03/img_2118.jpgIMG_2118And yay again you can pop off the top for easy servicing. The amp is basically layed out like a Fender with the eyelet board so it is easy to trace the circuit and work on. So yes YBA-1A has more clean headroom and power than the YBA-1, and larger transformers. I think the YBA-1 used 7027s and was rated for 45W and the YBA-1A used 6CA7s and was rated for a probably false 90W rating. I am kind of speculating so please correct me if I am wrong.https://irationaudio.com/wp-content/uploads/2015/03/img_2117.jpgIMG_2117Like a lot of the old amps it has two channels with a high and low input which can be jumpered together, blending the two channels.2020-04-03T04:55:57+00:00monthlyhttps://irationaudio.com/2015/03/31/matamp-gt/https://irationaudio.com/wp-content/uploads/2015/03/img_2132.jpgIMG_2132Matamp GT-something. Model is unknown since this is a custom build. Matamp would not relinquish the schematic but they did try to help by offering insight into my questions about the circuit, so cheers to them somewhat. Left-most knob "Voice" is a 6-way switch allowing different "voices."https://irationaudio.com/wp-content/uploads/2015/03/img_2047.jpgIMG_2047There is no information available This is from the Dave Green era of Matamps which is I believe the 1990s. Mirrored image on the back of the front panel shows four blue power supply caps. They are shielded from the power tubes just like in the old Sound City's. In front of them are the two preamp tubes (right), the choke, and the left most tube is the phase inverter. Power switch on the back is for half power -- it shuts off two of the power tubes. I would usually not go into such detail as per the layout and inner workings of an amp, but since there is no service information or schematic available I have decided to do so in case it might help someone. Plate voltage was 482V with a bias setting of ~34mA. https://irationaudio.com/wp-content/uploads/2015/03/img_2044.jpgIMG_2044Circuit is actually pretty easy to follow, but the amp looks weird on a scope because it doesn't get that much clean volume. I was confused by this at first and wondered if something was wrong in the preamp, but Matamp told me "without the footswitch this amp will behave very strangely," and it helped my understanding very much. It can do like 70W clean, about 115 max, but I guess the reason for this is because without the footswitch you cannot choose between the two channels so they basically are blended together. There is a low and high input, but with the high gain channel always on (Channel 2 I'm guessing), it will not get up to 100W clean, or whatever output you would expect from four EL34s. Each channel has a master volume. The EQ is shared between both channels. There is gain knob and an overdrive knob. I would assume overdrive goes with channel 1 and gain with channel 2. Without the footswitch they all work.https://irationaudio.com/wp-content/uploads/2015/03/img_2043.jpgIMG_2043Looks like a Rolls Royce. Gotta be pretty baller to own one like this. There are a handful of circuit board/turret boards here but only four are really important. And also as an aside the boards are PCB turret boards. I don't think I've seen that before. They are routed like a PCB but components are mounted on turrets in a high grade fashion. Very easy to work on and very rugged. Anyway, board on the way left just has a few transistors and opamps but don't let it bum you out they just control channel switching and the LEDs on the front panel. Next to left board (elevated) has resistors and coupling caps for anodes of the preamp tubes. Tone stack is also on this PCB. Middle board has resistors and caps for cathodes of preamp tubes on the lower half of it. Higher up are some caps which are for the DC heater supply for preamp. Next board to the right has big grey resistors. Bottom two are dropping resistors dropping HT voltage down to preamp levels. Other resistors on the board are control and screen grid resistors for EL34s. Yellow caps look like individual coupling caps from anodes of phase inverter to grids of power tubes. The four diodes up higher on the PCB are the rectifier. Right most board (upright) has heater supply for the power tubes as well as the negative bias supply. Big green resistors are 1 ohm cathode resistors (or was it 10 ohm?) to measure bias, as there are 4 individual bias trim pots, one for each power tube. However, measuring across the 10 (?) ohm cathode resistor is not a good method unfortunately in this application I must say. 4-probe bias probe comes up huge here because as you adjust one tubes current draw it changes the draw of the others. Measuring across the resistor and calculating over and over with your TI-83 would take forever. Bias probe reveals current draw per tube with the flip of a switch and frees up your TI-83 for Bowling. 2020-04-03T02:01:24+00:00monthlyhttps://irationaudio.com/2015/03/30/teletronix-la2a-leveling-amplifier/https://irationaudio.com/wp-content/uploads/2015/03/teletronix-la2a_layout.jpgTeletronix la2a_layoutTeletronix LA2A layout Look at the green wire going from fuse to ground. How is that even possible. Don't do that. Or please enlighten me.https://irationaudio.com/wp-content/uploads/2015/03/teletronix-la2a_1968.jpgTeletronix la2a_1968Teletronix LA2A schematic. The schematic is valuable. There is an error or two on the layout and the color coding of things can throw you off.https://irationaudio.com/wp-content/uploads/2015/03/img_1588.jpgIMG_1588A nice piece any studio or recording person will surely be familiar with. This is now at Bricktop Recording compressing and limiting some shit. https://irationaudio.com/wp-content/uploads/2015/03/img_1587.jpgIMG_1587I don't really know anything about recording gear but this is a Teletronix LA-2A kit I partially built. I say partially because someone else started it, abandoned it and sold it and it eventually made it's way to me. I don't know why they stopped. It was probably half way done and done well and correct. It is a rack mount compressor and limiter.2020-04-03T01:50:14+00:00monthlyhttps://irationaudio.com/2015/03/30/jbl-l65-jubal/https://irationaudio.com/wp-content/uploads/2015/03/jbl_l-65_75w_crossover_sm-pdf_1.pngjbl_l-65_75w_crossover_sm.pdf_1JBL L65 Jubal technical manualhttps://irationaudio.com/wp-content/uploads/2015/03/img_1568.jpgIMG_1568Upside down view of the awesome JBL L-65 home stereo speaker enclosure. I tried rotating it upright but the picture becomes very disorienting so oh well. I got a pair of these old JBLs in for repair and seems like they're kind of coveted -- a pair still sells these days for around $1000-2000. Unfortunately both of the 12" woofers were blown as well as a tweeter. It has a 3-way crossover with low frequency woofer, midrange speaker, and a crystal tweeter for the highs. I listened to a good recording on the cab that still had the working crystal tweeter and it sounded like the snare and the high hat were in the room with me. The two control knobs you see in the bottom right are for the presence (mids) and brilliance (highs) volume controls. The woofer just is. Crossover is on the right.https://irationaudio.com/wp-content/uploads/2015/03/img_1567.jpgIMG_1567The cab with the fried crystal tweeter also actually had it's volume control literally burnt to a crisp (like the Carcass song). I could not take the knob off and put it on the new L-pad I installed because it was melted to the shaft. I wondered why in this application it is called an L-pad attenuator and how this was different from a regular old volume or tone pot in an amp, but by looking it up it makes sense. "In its most basic form, the L-pad Attenuator is nothing more than a very simple voltage divider circuit used in many electrical and electronic circuits to generate a lower voltage while maintaining a constant impedance." "A pot (potentiometer) is a single, variable resistive element. An L-pad is two resistive elements; one in parallel with the load and the other in series with it (forming the shape of an L). Two fixed resistors are often used to "pad down" (attenuate), e.g., the output of a tweeter without altering its frequency response." 2020-04-03T01:46:11+00:00monthlyhttps://irationaudio.com/2015/03/30/fender-bandmaster-ab763/https://irationaudio.com/wp-content/uploads/2015/03/fender-bandmaster-ab763-schem-2.jpgfender-bandmaster-ab763-schem-2Fender Bandmaster AB763 layouthttps://irationaudio.com/wp-content/uploads/2015/03/fender-bandmaster-ab763-schem-1.jpgfender-bandmaster-ab763-schem-1Fender Bandmaster AB763 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1787.jpgIMG_1787More cool stuff always rolling through. Can't give enough thanks to all the people for bringing me their gear to work on. This is a Fender Band-Master I think from the year 1967. It is the AB763 circuit which was introduced in July, 1963 and replaced the original Bandmaster circuit, the 6G7-A (blonde era). One of the coolest features of this amp, which I don't have a picture of, is that on the underside of the chassis there is a little groove routed out right in the middle of the bottom panel. It's subtle to notice because the tolex covers it, and the routed portion is not too deep. I eventually realized it's routed out so if you stack two heads, the top one isn't all wobbly, bobbling around as it straddles the rubber strap handle on the amp below it. Really cool. Gotta acknowledge the designers on that one. Imagine how many more millions of people would have been happy over the years if this had become standard feature.https://irationaudio.com/wp-content/uploads/2015/03/img_1783.jpgIMG_1783Pretty standard chassis shot and this amp actually works great because it has been totally recapped sometime in the recent past with all Sprague electrolytics. All the coupling caps and the vast majority of resistors is original. It just had some nagging extraneous noise issues and a poor bias setting. Some of the jacks were corroded and and making poor connection. I replaced a few of the 100K plate load resistors in V1 and V2 and it took the sizzle out of the noise floor. Input jack #2 on the vibrato channel wasn't working and needed to be retensioned. Various tiny problems. I believe this is considered a 40W amp and per my measurements it was doing 47W at clipping into 4 ohms. https://irationaudio.com/wp-content/uploads/2015/03/img_1715.jpgIMG_1715There is a lot that could be said about this blackface era of Fender amps. It's got to be one of the favorite topics among guitar amp tone hounds worldwide. Luckily I can just refer to their ponderings and save my typing. http://fenderguru.com/amps/bandmaster http://en.wikipedia.org/wiki/Fender_Bandmaster http://acruhl.freeshell.org/mga/main/bf_vs_sf.html http://www.unclespot.com/AA763vsAB763.htm2020-04-03T01:41:58+00:00monthlyhttps://irationaudio.com/2015/03/29/hughes-and-kettner-triamp-mkii/2020-04-03T01:38:02+00:00monthlyhttps://irationaudio.com/2015/03/27/ampeg-b-25/https://irationaudio.com/wp-content/uploads/2015/03/ampeg-b-25-b-8-72-schematic1.jpgKMBT_C284-20131024130532Ampeg B-25 schematic the realhttps://irationaudio.com/wp-content/uploads/2015/03/img_1466.jpgIMG_1466Cool old Ampeg B-25. It is the "blue line" version meaning the screen printed front panel is blue, rather than black. The older ones are blue so I would guess this is a late 60s amp. I could be slightly wrong but I believe the switch to black was in 1971. Anyway, this amp was blasted by a short circuit, heater to screen supply. I'm not sure why this happened. The two traces run right next to each other on the PCB but are certainly isolated.https://irationaudio.com/wp-content/uploads/2015/03/img_1464.jpgIMG_1464Pretty simple amp. Has two channels with volume, treble and bass, and some rocker switches to adjust the frequency response in the typical Ampeg fashion. This amp puts out about 50W from two 7027 output tubes, which in todays era are typically replaced with 6L6s since 7027s are not really in production anymore.2020-04-03T01:35:59+00:00monthlyhttps://irationaudio.com/2015/03/27/mesa-boogie-mark-iii/https://irationaudio.com/wp-content/uploads/2015/03/mesa-boogie-mark-iii-preamp.jpg*GothamMesa Boogie Mark III preamphttps://irationaudio.com/wp-content/uploads/2015/03/mesa-boogie-mark-iii-power-section.jpg*GothamMesa Boogie Mark III power amp and power supplyhttps://irationaudio.com/wp-content/uploads/2015/03/img_1469.jpgIMG_1469Mark III in for repair, basically just needing new power tubes. Also had 2 broken push pull pots and a broken fuse holder. Used by such famous rockers as Today is the Day, Weezer, and NOFX, and more. https://irationaudio.com/wp-content/uploads/2015/03/img_1431.jpgIMG_1431High tech amp. Has a simul-class power section which biases two power tubes in class A and two tubes in class AB through the same output transformer. SO, at idle two tubes bias at about 70mA and drop to 30-40mA when cranked (class A), while the other two idle around 30-40mA and rise to about 120-130mA when cranked (class AB). Also has half power option which, instead of limiting the power in the 4 tube simul-class output section, switches you over to another 2 power tubes. This amp had 6L6s for the simul-class section and EL34s for the half power option, although you could use 6L6s for the half power mode as well, from what I've read. A total of 6 power tubes in this amp2020-04-03T01:34:33+00:00monthlyhttps://irationaudio.com/2015/03/27/ampeg-b-15n/https://irationaudio.com/wp-content/uploads/2015/03/img_1412.jpgIMG_1412Ampeg B-15N from 1966 or 1967. The holy grail for recording studios. This one was in storage for 10+ years. When the owner brought it out to sell it it kept blowing fuses, which happens a lot with amps that sit for years and years without being turned on. Usually it is because of capacitors in the power supply. The caps are filled with a liquid electrolyte; when the amp sits for years without being turned on the electrolyte dries out and the cap short circuits when presented with hundreds of volts again. https://irationaudio.com/wp-content/uploads/2015/03/img_1410.jpgIMG_1410Old Mallory power supply cap reads 235-6641. 235 is the Mallory code, 6641 means it was manufactured during the 41st week of 1966. https://irationaudio.com/wp-content/uploads/2015/03/img_1408-e1427475677741.jpgIMG_1408Seems all the old Ampeg amps come with the schematic. This one reads 10-66 in the bottom right corner. https://irationaudio.com/wp-content/uploads/2015/03/img_1405.jpgIMG_1405Nice, totally symmetrical sine wave. Clipping at 31W into an 8 ohm load, as it should. https://irationaudio.com/wp-content/uploads/2015/03/img_1382.jpgIMG_1382Tubes are old Sylvanias with the Ampeg logo screen printed on the base. This amp has an incredibly low noise floor, even with 50 year old caps. Perhaps because the power and output transformers are shielded, completely enclosed in a metal casing with no gaps. https://irationaudio.com/wp-content/uploads/2015/03/img_1380.jpgIMG_1380As far as I can tell this B-15N has never been worked on. It looks to be all original and even all the solder connections look untouched. Some of the caps in the preamp are made by Ducati??? Is this the same Ducati that makes motorcycles? People always want to sell vintage "all original" amps for big bucks, and that is nice for collectors and gear nerds and such, but when an amp is that old and it's all original it usually means it does not work right, if at all. Amazingly, after some time put in troubleshooting, this amp still works right. It was blowing fuses because of dodgey solder connections in the power supply. I really thought power supply caps or the power tubes would have been causing this. They seem to work fine, albeit the power tubes are kind of poorly matched, but not THAT bad. Re-soldering the connections on the power supply caps and the power tube sockets did the trick.2020-04-03T01:33:27+00:00monthlyhttps://irationaudio.com/2015/03/27/weber-8cm100/https://irationaudio.com/wp-content/uploads/2015/03/img_1359.jpgIMG_1359This amp had a bias problem. It is not uncommon for push-pull amps to run one side of the output section harder than the other when pushed into clipping but it seemed excessive in this case since 2 of 4 red JJ EL34 labels were blackened. The phase inverter plate load resistors were reversed. This normally doesn’t matter but in this case it seemed to. Maybe because the coupling caps feeding the two grids of the phase inverter are different values?? Bias was more even when reversing the PI plate load resistors and matching the schematic. It also oscillated at 20K when the master volume was up past 7, and had no grid resistor for V1 so there was lots of radio being picked up by the amp. Adjusting the lead dress of V1, especially the wires coming from the plate leads, rid the 20K oscillation. A grid resistor at pin 2 of V1 got rid of the radio. In the case of no grid stopper to buffer the signal, the wire from V1 to input jack, your guitar cable, and the guitar itself are acting as an antennae.https://irationaudio.com/wp-content/uploads/2015/03/weber-8cm100_layout.jpg8M800_layoutWeber 8CM100 Layout.a Weber 8CM100 schematic -- http://www.tedweber.com/media/kits/8cm100_schem.jpg https://irationaudio.com/wp-content/uploads/2015/03/img_1363.jpgIMG_1363his is a kit amp made by Weber. It is a copy of a Marshall JCM800. Unfortunately expensive repair because the power transformer was shorted. The amp would blow fuses when turned on since all the wall voltage was being immediately dumped to ground.2020-04-03T00:45:16+00:00monthlyhttps://irationaudio.com/2015/03/27/sunn-concert-bass-2/https://irationaudio.com/wp-content/uploads/2015/03/img_1357.jpgIMG_1357Early version of the Sunn Concert Bass. Seems like these earlier ones with the silver panel were 1972-74 and the red knobs started appearing in 1975. This amp is actually very similar to the red knob version in the preamp and output section but the power supply is totally different. Red knob version has a single secondary winding meaning there is only one voltage out from the power transformer and successive voltages are stepped down with dropping resistors and then tapped off the power supply. The early version actually has three power transformer secondaries each supplying different voltages to different parts of the amp. Each supply has a rectifier so it is kind of a triple rectifier LOL. Anyway in this case the rectifier for the preamp supply shorted and amp would continually trip its circuit breaker. https://irationaudio.com/wp-content/uploads/2015/03/img_1356.jpgIMG_1356Concert Bass internals Full schematic and service info documentation here -- https://drive.google.com/file/d/0B01EXvY0__YYRWVWWC1mMy0tS3c/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/03/img_1336.jpgIMG_1336Guess it was only a matter of time before this thing failed since basically all caps in the amp with the exception of the two biggest, the 3450uf/75V cap and the 1000uF/75V cap, looked like this. Seems like 90% of the time short circuits happen in the power supply or the output section. Since this short circuit happened in the preamp supply there was no other component damage other than the rectifier itself. In most other cases of solid state short circuit you probably will be replacing output transistors and more. 2020-04-03T00:31:14+00:00monthlyhttps://irationaudio.com/2015/03/27/acoustic-470/https://irationaudio.com/wp-content/uploads/2015/03/img_2544.jpgIMG_2544And from the other sidehttps://irationaudio.com/wp-content/uploads/2015/03/img_2542.jpgIMG_2542As an aside there are two different versions of the power amp/output section PCB. I backlit them one day when I had multiple 470s in for repair. https://irationaudio.com/wp-content/uploads/2015/03/img_1191.jpgIMG_1191470 circuitry is braced between metal brackets on front, back and sides, so the whole top and bottom of the PCB panels is accessible. Makes it kind of easy to work on, until you have to access the component side of the preamp boards. Anyway, word of warning to 470 owners -- the huge power supply cap mounted to the back panel chassis is centimeters away from the reverb tank, which is at ground potential. Acoustic placed a piece of foam on the reverb tank here so the cap doesn't short to ground, but that was like 40 years ago and the foam is probably dust, like it was in this 470. If the amp gets really shaken up or the cap becomes loose in the bracket it will eventually make contact and short. Sparks will fly. Bad stuff may happen. Put some electrical tape or something on the reverb tank to isolate it just in case. Also make sure you drain the cap of any remaining voltage before doing this so you don't possibly die. https://irationaudio.com/wp-content/uploads/2015/03/img_1188.jpgIMG_1188Damaged in shipping. The A channel would cut in and out and also the B channel was very noisy. Very hard to find the intermittent, which turned out to be the treble pot on the A channel IIRC. Replacing a few transistors in the B channel preamp seemingly eliminated noise but unfortunately it was not banished completely. Certain very particular control setting combinations would still bring noise on the B channel and the source was never found.https://irationaudio.com/wp-content/uploads/2015/03/img_1187.jpgIMG_1187Amazingly putting out 166W into 4 ohms. This mystified me at first as it was about 10 more watts than my 370 put out at the same impedance. The key is the 370 can go to 2 ohms and yield more power still. The 470 cannot. No more need to wonder ?why?. Complete service manual and owners manual here. It literally tells you everything -- https://drive.google.com/folderview?id=0B01EXvY0__YYVmhxVFF5MV9kOWM&usp=sharing 2020-04-02T07:54:27+00:00monthlyhttps://irationaudio.com/2015/03/26/sound-city-120-mark-iv/https://irationaudio.com/wp-content/uploads/2015/03/img_1173.jpgIMG_1173Sound City Mark IV has an active preamp so if volume is up but all tone controls are turned down the amp puts out no sound. If you are testing this amp for output power and you put all tone controls on 5 like you would with most amps, you will think you have weak output for some reason. This amp will ONLY put out full power if all tone controls are on 10.https://irationaudio.com/wp-content/uploads/2015/03/sound_city_lb_120_mk_iv_11-73_schematic.jpgKMBT_C284-20131029150301Sound City 120 Mark IV alternate schematichttps://irationaudio.com/wp-content/uploads/2015/03/sound-city-120-mark-iv1.jpg*GothamSound City 120 Mark IV schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1502.jpgIMG_1502Here is a picture of another Mark IV Sound City 120 and I'm including it because of its better lighting but also because the preamp tubes go from top to bottom of the image V1, V2, V3, V5, V4. Yes, the last two tubes are out of order. If you are getting strange voltage measurements in the V4 and V5 position this is why. Check to see which tube socket couples to the output section via the .047uF coupling caps. After googling chassis shots of Sound City 120s I did find some more that were wired this way, so I know this is not the only one. Big trap for young players.https://irationaudio.com/wp-content/uploads/2015/03/img_1166.jpgIMG_1166Poor lighting but you get the idea. Turret board is rugged. All 12AX7 tubes are in order from top to bottom, V1 to V5, with V5 as phase inverter.https://irationaudio.com/wp-content/uploads/2015/03/img_1161.jpgIMG_1161Sound City 120 in for repair. Main problem was a poor solder connection somewhere around the first or second preamp tube causing a crackling sound. This was somewhat hard to find because it only comes up when the amp is vibrating on top of a cab. I had to chop stick the hell out of it.2020-04-02T07:38:58+00:00monthlyhttps://irationaudio.com/2015/03/26/sound-city-200/https://irationaudio.com/wp-content/uploads/2015/03/img_1053.jpgIMG_1053Outrageously doing 29.2V into 8 ohms for about 210W at clipping dayumhttps://irationaudio.com/wp-content/uploads/2015/03/sound-city-lb-200-plus.jpg*GothamSound City LB 200 Plus schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_6590.jpegIMG_6590Behemoth Sound City 200+. Huge power section with four KT-88s was doing about 210W at clipping as per my measurement. https://irationaudio.com/wp-content/uploads/2015/03/img_1133.jpegIMG_1133Partridge transformers in this one. you can tell because of the yellowish tint in the metal -- they have been dipped in some type of epoxy. Other Sound City's came with Electro Voice (not the same as the speaker company) transformers. They are not dipped and maintain a silver metallic look. Otherwise they look almost identical to the Partridge brand.https://irationaudio.com/wp-content/uploads/2015/03/img_4919.jpegIMG_4919Some clever person along the way has totally gutted the preamp and built a very simple Fender-style one. It has one. Notice the preamp has four tube sockets and the middle two are empty, except for the heater winding. This amp uses a single 12AX7 preamp tube. The other 9-pin tube is the phase inverter, which feeds the signal the the four KT-88s in the output section.2020-04-02T06:26:08+00:00monthlyhttps://irationaudio.com/2015/03/26/sovtek-mig-100-u/https://irationaudio.com/wp-content/uploads/2015/03/img_0315.jpgIMG_0315https://irationaudio.com/wp-content/uploads/2015/03/img_0314.jpgIMG_0314Sovtek MIG100-U. Sounds very raw and gnarly. I always like amps that have enough balls to potentially start sounding bad. This one has the gain ability to go 'too far.' Amp was blowing fuses because of a shorted power tube.https://irationaudio.com/wp-content/uploads/2015/03/img_0309.jpgIMG_0309Back to the primitive measuring bias via transformer shunt methodhttps://irationaudio.com/wp-content/uploads/2015/03/img_0289.jpgIMG_0289Internals2020-04-02T06:24:42+00:00monthlyhttps://irationaudio.com/tek-knowledgey/2019-07-31T19:12:19+00:00weekly0.6https://irationaudio.com/2019/07/14/fender-reverb-6g15/https://irationaudio.com/wp-content/uploads/2019/07/fender-reverb-6g15.jpgFender Reverb 6G15https://irationaudio.com/wp-content/uploads/2019/07/fender-reverb-back.jpgFender Reverb backhttps://irationaudio.com/wp-content/uploads/2019/07/fender-reverb-chassis.jpgFender Reverb chassishttps://irationaudio.com/wp-content/uploads/2019/07/fender-reverb-6g15-schematic.pngFender Reverb-6G15-Schematic2019-07-14T08:38:57+00:00monthlyhttps://irationaudio.com/2015/03/26/126/https://irationaudio.com/wp-content/uploads/2015/03/img_0800.jpgIMG_0800Three inputs and one volume knob. The volume knob controls mic volume only. Instrument volume must be controlled from the instrument.https://irationaudio.com/wp-content/uploads/2015/03/gibson-eh100-1940-schematic.jpgGibson eh100 (1940) SchematicGibson EH-100 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_08291.jpgIMG_0829https://irationaudio.com/wp-content/uploads/2015/03/img_0794.jpgIMG_0794https://irationaudio.com/wp-content/uploads/2015/03/img_0802.jpgIMG_0802Long speaker cable allows the speaker to be placed up to about 3" away from the amp.https://irationaudio.com/wp-content/uploads/2015/03/img_0801.jpgIMG_0801The Gibson EH-100 model. There were a few different versions of this amp from about 1936-1940 I believe. This one in stained mahogany is a 1940 version. A cool feature of this amp is that the top and bottom section are detachable. The bottom section is the amplifier and top section holds the speaker. The Gibson logo in the corner is screen printed on. The EH-100 was advertised as "the" amp for Hawaiian electric guitar.https://irationaudio.com/wp-content/uploads/2015/03/img_0793.jpgIMG_0793Some modern caps are visible in the pic but other than that this thing was all original. For some reason a previous tech started this repair and then abandoned it. The power supply was recapped but the amp sounded terrible. This amp runs two 6V6GTs in push pull so they should be running at an equal current draw. When I got the amp one tube was at 70mA and the other was 21mA! Coupling caps linking the phase inverter and the power tube grids were replaced and the bias was evened to 41mA per tube. This was still too hot though so the cathode resistor was adjusted to reach a nice bias of 20mA at 363Vp. Drifted resistors and a few cathode bypass caps were replaced and it sounded clean again.https://irationaudio.com/wp-content/uploads/2015/03/img_0829.jpgIMG_08292019-07-12T15:57:37+00:00monthlyhttps://irationaudio.com/2018/01/05/hiwatt-dr405/https://irationaudio.com/wp-content/uploads/2018/01/hiwatt-400.jpgHiwatt 400The white whale as it might be called. 400W Hiwatt bass amp with 6 6550s behind the metal panel that shields the preamp tubes from the power tubes. Really rare amp. Has 4 speaker jacks so you can run a boat load of cabs.https://irationaudio.com/wp-content/uploads/2018/01/hiwatt-400-chassis.jpgHiwatt 400 chassisSuper neat and minimal build as you would expect. It has two bridge rectifiers like the SVT style so it can do a stacked (or whatever you would call it) power supply node that supplies plate voltage and screen voltage from same node. Screen voltage is roughly halved and is at a reasonable 410V and plate voltage is in the 730V range. This one did about 340W at clipping with its intense high voltage. SVT will do 280-300W with a ~660V plate voltage. There is a lot of info and schematics on these amps on Mark Huss's Hiwatt page and I really recommend it if you would like to peruse -- 2018-01-05T17:44:58+00:00monthlyhttps://irationaudio.com/2017/12/21/peavey-ultra-plus/https://irationaudio.com/wp-content/uploads/2017/12/peavey-ultra1.jpgPeavey ultraPeavey Ultra Plus just needed some various re-soldering so it could work without cutting out occasionally. Needed a quick fix so a touring band could continue their saga.https://irationaudio.com/wp-content/uploads/2017/12/peavey-ultra-chassis1.jpgPeavey Ultra chassisCan't think of any other amps that have preamp tubes inside the chassis other than this amp and 5150s. Seems like a strange idea since tubes get hot and you would want to minimize heating inside the chassis because it reduces electrolytic capacitor lifetime. Wonder why they do it. Maybe it makes the build easier in some way. Here is Peavey Ultra Plus schematic and parts list in all 13 pages of it's glory. Not hard to get schematics from PV. 2017-12-22T01:17:35+00:00monthlyhttps://irationaudio.com/2017/12/16/peavey-valve-king-100/https://irationaudio.com/wp-content/uploads/2017/12/pv-vk.jpgPV VKLesser known Peavey rock amp Valve King 100. Everyone always thinks of 5150. This one has done the rounds so many times it eventually needed new power tubes and a few new screen grid resistors. https://irationaudio.com/wp-content/uploads/2017/12/peavey-vk.jpgPeavey VKScreen grid resistors are located under power amp PCB so you have to remove the board and some of the back panel stuff to get it out. VK100 has a strange value of screen grid resistor 330 ohm. Most screen grid resistors for power tubes tend to be in the 470 ohm to 1.5K range. Anyway one screen grid resistor was burnt out and 330 is a strange value. Not wanting to mix and match and not having any 330 ohm in stock I decided to replace all four with 470 ohms which is the most standard value for 6L6GC power tube. It's a pretty negligible difference anyway. To my surprise the customer said it sounded way different! Not in a good way, more "bad" and "thin" with "weak palm mutes," so typically not what you're looking for. I ordered up some 330 ohm / 5W and he was happy as can be. 2017-12-16T20:34:55+00:00monthlyhttps://irationaudio.com/2016/06/16/hiwatt-sa112/https://irationaudio.com/wp-content/uploads/2016/06/hiwatt-sa112-power-supply.jpghiwatt-sa112-power-supply.Hiwatt SA112 power supplyhttps://irationaudio.com/wp-content/uploads/2016/06/hiwatt-sa112-power.pngHiwatt SA112 preampHiwatt SA112 preamphttps://irationaudio.com/wp-content/uploads/2016/06/hiwatt-sa112-2.jpgHiwatt SA112-250 Watt Hiwatt from I believe early 80s. Getting into that PCB Hiwatt era but still a neat build. Looks like along the way someone put in some orange drops for the phase inverter coupling caps. Screen grid resistors for power tubes were what looked like 100 ohms 1W (2W?) strangely. I replaced with 1K / 3W. Anyway this was brought in because the power tube melted the speaker wire. It is a 1x12 combo but I forgot to get a pic of the whole amp. Good to use one of those nylon retaining brackets so the speaker cable doesn't get too close to the power tubes and melt in these scenarios, and it's good to go. Here is Hiwatt SA112 preamp schematic and Hiwatt SA112 power supply schematic.2017-12-12T08:03:31+00:00monthlyhttps://irationaudio.com/2015/02/02/re-source/https://irationaudio.com/wp-content/uploads/2015/02/columquartz_5193.jpgcolumquartz_5193https://irationaudio.com/wp-content/uploads/2015/02/cdac54c6-bbea-462b-84bb-e710d446ac0a.jpgcdac54c6-bbea-462b-84bb-e710d446ac0ahttps://irationaudio.com/wp-content/uploads/2015/02/48news_azozeolemurians.jpg48News_AzozeoLemurians2017-11-30T23:07:57+00:00monthlyhttps://irationaudio.com/2017/11/30/gomez-amplification-the-tahitian-reverb-amp/https://irationaudio.com/wp-content/uploads/2017/11/gomez-the-tahitian.jpgGomez the tahitianGomez Amplification The Tahitian Reverb-Amp. Basically works but had some damaged power tubes in shipping. I think they were just left in the tube sockets when shipped and some got rattled loose and were rolling around inside the chassis when it arrived. Two had broken guide pins and one did not work at all, but still had the guide pin. New power tubes and it's ready to go. Essentially some sort of Fender derivative but they decided to go a little untraditional with a sky blue tolex, cool.https://irationaudio.com/wp-content/uploads/2017/11/gomez-the-tahitian-chassis.jpgGomez The Tahitian chassisInside the chassis is what you'd expect based on the external look, eyelet board and lots of braided wire in the Fender style of build. Grounds are haphazardly mounted anywhere on the chassis that's convenient which is not a good build technique, but I guess it's worked good enough for many old amps over the years, and even this one. Noise floor is not offensive and the amp seems to work fine/normal in that regard. The tremolo circuit is the bias modulating style, so when it's in use the bias voltage for the power tubes varies up and down for an amplitude modulation in the sound. Since the bias is switching from normal to very cold to make the signal vary volume, there is a bit of distortion you can hear in this type of tremolo circuit because it's biasing the power tubes momentarily cold into crossover distortion territory. It's pretty easy to view on an analog oscilloscope. Anyway, a slight drawback of that type of tremolo circuit but I guess it has a unique sound that people appreciate in other ways. Here's a little article I came across that talks about it -- Bias Modulating "Tremolo" Circuits2017-11-30T22:47:05+00:00monthlyhttps://irationaudio.com/2017/08/28/roland-chorus-echo-sre-555/https://irationaudio.com/wp-content/uploads/2017/08/roland-sre555.jpgRoland SRE555Rackmount Roland SRE-555 which is essentially tape echo, reverb, and chorus in one. I never really get too much studio gear to work on and I'm not that familiar with it but this just needed the bearings lubed because the tape would not turn, or would vary speed.https://irationaudio.com/wp-content/uploads/2017/08/roland-sre555-front.jpgRoland SRE555 frontNot the best image but you get the idea. I didn't have to get much into it but Roland provides the owners manual on their site to try to at least get you going in the right direction -- Roland SRE-555 Chorus Echo owners manual2017-08-29T05:24:30+00:00monthlyhttps://irationaudio.com/2016/01/14/silvertone-1483/https://irationaudio.com/wp-content/uploads/2016/01/silvertone_1483.pngsilvertone_1483Silvertone 1483 schematic. Look how it lists the resistance of the transformer windings. I would give this amp an A+ at being cheap, simple and good. https://irationaudio.com/wp-content/uploads/2016/01/mg_2568.jpg_MG_2568Always cool, old stuff rolling through. Silvertone 1483 from I believe 1966. It is a 23W bass amp and the two channels are identical. This one does 17W with the original tubes which are RCAs labeled "Silvertone" so not too bad. https://irationaudio.com/wp-content/uploads/2016/01/mg_2567.jpg_MG_2567I think this one had been sitting for a while. V1 was totally crackley but there weren't any cracked solder connections like I expected (it was pretty bad), the tube socket was just all old and crusty and corroded. Worked nice after a good cleaning of the pots and jacks and tube sockets. Kinda noisey with the old caps and possibly also because of the old carbon comp 100K plate resistors in the preamp but it will stay that way and stay original because it works pretty good. I would give this amp an A+ at being cheap, simple and good. 2017-08-27T18:06:09+00:00monthlyhttps://irationaudio.com/2015/03/26/sears-40xl-silvertone-1422/https://irationaudio.com/wp-content/uploads/2015/03/img_0550.jpgIMG_0550https://irationaudio.com/wp-content/uploads/2015/03/img_1028.jpgIMG_1028Here is another one of those old department store amps that was almost a toy for it's time, but is now coveted in this era of cheap, mass produced transistor amps. I believe it's fron the late 60s. Has tremolo and reverb, although the reverb tank is actually a toy I think. Tremolo sounds tite though. This one has been through Vietnam it seems. Sears 40XL is the same as the Silvertone 1422 amp. This one was converted into a head but they were actually 1x12 combos. Much better pics and info here -- http://www.tonegems.com/amps/gallery-69-silvertone-1422-sears-40xl-tube-amp/https://irationaudio.com/wp-content/uploads/2015/03/img_1010.jpgIMG_1010All you need is a few tubes, some coupling caps, and a few electrolytics for the power supply and you're basically good. Original caps that were removed are sitting on the desk in front of the amp. On the casing the manufacturer printed that these caps were GUARANTEED to work for 1 year. I don't know how they ever sold any in the first place with that kind of guarantee but regardless they lasted 45. They were actually fine, and were only replaced for reliability because they were 45 years old and probably should have failed long ago. I never found the schematic for this one. 2017-08-27T17:58:33+00:00monthlyhttps://irationaudio.com/2017/08/21/traynor-bass-master-yba-1a-mkii/https://irationaudio.com/wp-content/uploads/2017/08/traynor-yba1-1a-mkii.pngtraynor yba1-1a MKIITraynor Bass Master YBA-1A MKII schematic. So as you can see, this YBA-1A MKII has 537V B+ for the power tubes. With this B+ it will do about 65W at clipping. Original Traynor YBA-1 from the previous post only has a 430V supply and will do about 40W at clipping. Check out the flyback diodes they have going from plate leads of the power tubes to ground. They are listed as what seems to be 6RS20SP20820 and 6RS20SP787 and are actually called thyrectors (6RS20SP series). They are selenium and should be probably just be removed or replaced. They will eventually fail because selenium rectifiers increase in resistance over time. So as the decades go on they run slightly warmer and warmer, until they usually short. Selenium is toxic and burnt/eploded/releasing the smoke selenium rectifiers should be avoided. You can just replace them with those R3000 diodes or three 1N4007 in series.https://irationaudio.com/wp-content/uploads/2017/08/traynor-yba1-1a-mkii.jpgTraynor YBA1-1A Mark II70s-style Traynor Bass Master YBA-1A Mark II. The B+ on this amp is slightly higher than the original since it has a replacement power transformer, Hammond 274BX.Look at how small this Hammond 274BX is compared to the footprint of the original Hammond PT that used to be there. This seems normal for Hammond Transformers of today to have same or better specs but be much smaller than the old 60s 70s transformers. Original power tubes in this amp are 7027 but the owner likes to use KT88 and use it for bass. 7027 has 0.9A heater current draw per tube, but when you switch to KT88 the heater current draw is 1.5A per tube. So adding in the preamp tubes at 0.3A each current draw per tube this amp is has a 3.9A heater current draw. With original 7027s it would have been only 2.7A total. This is the main thing to look out for when trying to use different power tubes. If the power transformer can't handle the heater current it will get really hot and eventually fail. You could see how this would be a bigger issue with 4 power tube amps because switching from 6L6 to 6550 for instance would increase the heater current draw in the output section alone from 3.6A to 6A, which is a big jump! Anyway, 274BX is a fine choice since it is rated for 6A heater current. I guess the other thing that is important when switching to big tubes is physical space. The power tube sockets on this amp are too close together so the KT88s are probably heating each other a bit, but I guess it gets away with it. Ideally there would be more room between the two power tubes so there is better airflow and heat dissipation. This amp was designed to have 7027 power tubes which are not as wide so it would have not been as much of an issue with those tubeshttps://irationaudio.com/wp-content/uploads/2017/08/traynor-yba1-1a-mkii-chassis.jpgTraynor YBA1-1A MKII chassisInside the chassis someone has installed a bias trimpot and also recapped the amp by drilling the chassis and installing clamps to hold the capacitors. Drilling a steel chassis is not that fun and some people would get made if you did it, but it's probably the best, neatest way to do it. If you're lucky like me you have access to an air compressor to try to blow all the metal debris out of the chassis after drilling. Don't be a bum and leave it in there. Even if you have to hold the chassis up and blow in it. Close your eyes before doing it in case of flying metal debris.https://irationaudio.com/wp-content/uploads/2017/08/traynor-yba1-1a-mkii.giftraynor yba1-1a MKIITraynor Bass Master YBA-1A MKII schematic. So as you can see, this Traynor YBA-1A MKII has 537V B+ for the power tubes. With this B+ it will do about 65W at clipping. Original Traynor YBA-1 from the previous post only has a 430V supply and will do about 40W at clipping. Check out the flyback diodes they have going from plate leads of the power tubes to ground. They are listed as what seems to be 6RS20SP20820 and 6RSP202P787. They are selenium and should be probably just be removed or replaced. They will eventually fail because selenium rectifiers increase in resistance over time. So as they decades go on they run slightly warmer and warmer, until they usually short. Selenium is toxic and burnt/eploded/releasing the smoke selenium rectifiers should be avoided. You can just replace them with those R3000 diodes or 3 1N4007 in series.2017-08-22T14:43:51+00:00monthlyhttps://irationaudio.com/2017/08/17/traynor-bass-master-yba-1/https://irationaudio.com/wp-content/uploads/2017/08/traynor_bassmaster_yba1.jpgtraynor_bassmaster_yba1Traynor Bass-Master YBA-1 schematichttps://irationaudio.com/wp-content/uploads/2017/08/traynor-yba-1.jpgTraynor YBA-1Traynor YBA-1 from and quite an old school one. From some time in the late 60s I think. Has holes routed in the front for ventilation which I'd think is helpful especially with power tubes hanging upside down underneath the chassis. It's a little 40W amp. The B+ is only about 430V.https://irationaudio.com/wp-content/uploads/2017/08/traynor-yba-1-chassis.jpgTraynor YBA-1 chassisPretty much laid out like a Fender. Tag strip holds everything and that old cloth pushback wire is routed all over pretty haphazardly. Amp had been already ben partially recapped with a few Sprague caps and I finished it off adding a few F&Ts. Not the most sightly recap but it is solid. Always nice to work on simple old amps like this that can be repaired and revamped and made to work good for decades longer.2017-08-17T20:38:14+00:00monthlyhttps://irationaudio.com/2016/10/09/tektronx-2215/2016-10-09T20:21:57+00:00monthlyhttps://irationaudio.com/2015/03/27/bk-precision-1477-dual-trace-oscilloscope/https://irationaudio.com/wp-content/uploads/2015/03/img_1285.jpgIMG_1285The screen on this is really nice for what seems like kind of a cheap scope. You can find these on ebay for under $100. It's not as bright as my Tektronix CRT scope but the traces are very clear. They don't move around or cut in and out anymore so the clean up really did it. Digital scopes are very accurate and self calibrating and offer a lot of numerical data on the screen which is valuable, but there is something kind of magical about that old analog trace on the display. Back to the primitive I suppose. BK Precision 1477 Dual-Trace Oscilloscope Service Manual -- https://drive.google.com/file/d/0B01EXvY0__YYbVN4Mlh2YllTbUU/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/03/img_1283.jpgIMG_1283 saw this scope on craigslist for $30 and it was not far away so I went to go grab it. It turned on but actually it did not work, so the guy just gave it to me. The trace would cut in and out on certain settings and also float up and down on the screen. Most of the time it was very hard to even get anything to show on the display. It is a BK Precision 1477 which is a 15Mhz dual-trace scope. It is an old analog scope probably from the mid 80s and has a CRT screen which is actually just one big tube. CRT means cathode ray tube. https://irationaudio.com/wp-content/uploads/2015/03/img_1280.jpgIMG_1280With the scope standing upright and the screen facing down you can see big CRT probably takes up half the space in the scope chassis itself. From this side view you can see the power supply on top, the vertical deflection board on the left behind the CRT and the horizontal deflection board on the right. These PCBs connect to the knobs and jacks and switches on the front panel that control your vertical and horizontal scales of the V/t graph. I cleaned all the pots and cleaned and retensioned the BNC input jacks on the front panel. Also re-soldered a bunch of connections where a knob or switch connects to the PCB. These solder connections are always problems in guitar amps because of all the jostling. So mostly just cleaning and touch up stuff for this BK. No parts were replaced.https://irationaudio.com/wp-content/uploads/2015/03/img_1261.jpgIMG_1261Also I noticed a cap in the power supply (?) was disconnected on one lead.2016-10-09T17:03:24+00:00monthlyhttps://irationaudio.com/2016/10/08/hammond-preamp/https://irationaudio.com/wp-content/uploads/2016/10/hammond-preamp.pnghammond-preampA little preamp I could only assume was salvaged from a Hammond organ because the rectifier tube has Hammond silk screened on it. It has a 6X4 rectifier and one 12AX7 per channel for stereo. Interestingly the Mallory can capacitor has a low voltage cap in it along with three high voltage caps. The low voltage cap is for the cathode bypass for the 12AX7s.https://irationaudio.com/wp-content/uploads/2016/10/hammond-preamp-scope.pnghammond-preamp-scopeWorked nice once the broken connection was fixed and the preamp tubes were replaced. One was really weak compared to the other one so I put in a new 12AX7 which then made a much bigger signal compared to the remaining old RCA 12AX7. So I guess one was weak and one was really weak. With two new 12AX7s the signals are equal. https://irationaudio.com/wp-content/uploads/2016/10/hammond-preamp-chassis.pnghammond-preamp-chassisPreamp did not work because of a broken connection on the cathode resistor pin 3. Both 12AX7s have shared cathodes so both channels didn't work since each channel had one gain stage that would not pass signal.2016-10-08T18:57:00+00:00monthlyhttps://irationaudio.com/2016/10/07/engl-tube-preamp-530/https://irationaudio.com/wp-content/uploads/2016/10/engl-530.pngengl-530Engl tube preamp in a single rack mount space. Has 4 channels kind of but really channel 1 switches between a clean and crunch setting and channel 2 switches between gain and more gain. Really has a large range of rock tones.https://irationaudio.com/wp-content/uploads/2016/10/engl-530-chassis.pngengl-530-chassisSince it is 1 rack space and tube it has the tubes lay horizontal in the chassis. They have a foam pad underneath them to pad them from vibration and also to isolate them from the PCB. The foamy pad makes the 12AX7 squeeze into the tube socket at an angle and it wore out the tube socket pins, making them become loose and untensioned. Because of this the signal would cut out occasionally and if you tapped on the chassis you could usually get it to cut back in. Re-tensioning the tube socket had good results but I wonder if one day it will have to be replaced.2016-10-07T13:18:22+00:00monthlyhttps://irationaudio.com/2016/10/04/epiphone-ensign-ea-14-rvt/https://irationaudio.com/wp-content/uploads/2016/10/gibson-ga45rvt-schematic.jpggibson-ga45rvt-schematicClosest schematic is I believe Gibson GA-45 RVThttps://irationaudio.com/wp-content/uploads/2016/10/epiphone-ensign.jpgepiphone-ensignHas two channels with one being essentially plain and the other having tremolo and reverb. Does 50W with two 6L6GCs in the output. The preamp uses 6EU7s so sometimes you will see the tube sockets converted to 12AX7s. Has that old school bright switch sound that is too bright to deal with for most people of this era. Here it is called presence.https://irationaudio.com/wp-content/uploads/2016/10/epiphone-ensign-chassis.jpgepiphone-ensign-chassisEpiphone Ensign EA-14 RVT from mid 60s-ish. Funny rats nest but hey. Had been partially recapped in the past but now needed the job completed so I threw in some F&Ts. There was a problem with the tremolo too but unfortunately I don't remember what I did to fix it.2016-10-05T14:15:30+00:00monthlyhttps://irationaudio.com/2015/10/04/vht-deliverance-60/https://irationaudio.com/wp-content/uploads/2015/10/img_2238.jpgIMG_2238VHT Deliverance 60 and in the usual fashion VHT really likes to tell you a lot and even tell you off on the PCB. Has bias settings printed and various remarks on build quality/preference and other various things relating to tone mysticism. Pretty cool little amp. Has three preamp tubes with the third controlled in our out of the circuit by the More/Less switch. Does 60W with two 6550s/KT88s as the name implies. Looks neat and tidy.https://irationaudio.com/wp-content/uploads/2015/10/img_2239.jpgIMG_2239This thing had no real problems other than some noisy preamp tubes. I was just giving it a once over since it is a rental unit for Big Iron. I was able to find a page with some pics and hand drawn shematics. I cannot verify their 100% accuracy because I didn't go too far into this amp but I'm sure they are useful -- VHT Deliverance 60 schematics2016-10-05T01:31:24+00:00monthlyhttps://irationaudio.com/2016/10/02/kalamazoo-model-two/https://irationaudio.com/wp-content/uploads/2016/10/kalamazoo-model-2-schematic1.jpgkalamazoo-model-2-schematicKalamazoo Model Two schematic thanks to Miles O'Neal. Has one 12AX7 for input stage and to drive the tone stack (V1B), while V1B side drive the power tube. It is not a phase inverter since it is a single ended amp, just has a 6BQ5/EL84 for output. V4 12AX7 is the tremolo oscillator. 6X4 is the rectifier. Check out the Kalamazoo Model Two amp field guide if you want to get into it a little more.https://irationaudio.com/wp-content/uploads/2016/10/kalamazoo-model-2.jpgkalamazoo-model-2Kalamazoo Model Two which has been through the shit. Just a little 1x10 combo with tremolo. Seems to have even been underwater at some point. Oh well, it lives on. Very similar to Gibson GA-5T Skylark amp but with a few slight component value differences. Kalamazoo was a Gibson budget brand, more geared towards smaller practice amps.https://irationaudio.com/wp-content/uploads/2016/10/kalamazoo-model-two-chassis.jpgkalamazoo-model-two-chassisSomeone has kept it going with a new Jensen C10 speaker (underwater incident) and some NTE caps to keep the power supply going. It would not turn on because of a broken connection at the on/off switch, needed a 3-prong, and V4 cathode bypass cap needed to be replaced to make the tremolo work good. 2016-10-05T01:20:40+00:00monthlyhttps://irationaudio.com/2016/09/25/magnatone-480/https://irationaudio.com/wp-content/uploads/2016/09/magnatone-480.jpgmagnatone-480All point to point with everything mounted to pots and jacks, tag strips, tube sockets, etc. Has those old varistors that give it that unique Magnatone vibrato. There are some awesome articles on it if you want to read up by RG Keen and Tim Robbins.https://irationaudio.com/wp-content/uploads/2016/09/magnatone-480-schematic.jpgmagnatone-480-schematicThe real Magnatone 480 schematic. This amp has a transistor. It is in the reverb recovery and actually needed to be replaced !! Was missing the two phase inverter 12AU7s and needed some new power tubes, a good, good cleaning of all pots and jacks and it worked pretty good. Someone wanted their dad's old amp fixed up, cool. https://irationaudio.com/wp-content/uploads/2016/09/magnatone-480-amp.jpgmagnatone-480-ampThe flagship of the Magnatone line, Magnatone 480. Has two inputs for channels 1 and 2 and can be used in stereo or mono. Has two separate output sections with 12AU7 phase inverters, two 6973 power tubes per side and two output transformers. 2016-10-01T23:44:46+00:00monthlyhttps://irationaudio.com/2016/09/19/fender-hot-rod-deluxe-limited-edition/https://irationaudio.com/wp-content/uploads/2016/09/fender-hot-rod-deluxe.jpgfender-hot-rod-deluxehttps://irationaudio.com/wp-content/uploads/2016/09/fender-hrdeluxe-le-board.jpgfender-hrdeluxe-le-boardHas an unsoldered connection. Looked like a factory error at first but actually I doubt it since you can tell that someone has replaced the relay next to it. They must have unsoldered that resistor for some reason and forgot to re-solder it. It is pressure fit into the circuit, just a human error.https://irationaudio.com/wp-content/uploads/2016/09/fender-hrd-cracked-solder.jpgfender-hrd-cracked-solderCracked solder connections are the name of the game a lot of times for these amps. You can see the two most bottom and most right most connections on the input jack are cracked. Also the top two solder connections are damaged and barely hanging on.https://irationaudio.com/wp-content/uploads/2016/09/fender-hot-rod-deluxe-le.jpgfender-hot-rod-deluxe-leHot Rod Deluxe with the cool sunburst paintjob. Has a very thick layer of clearcoat on it. As far as I know the amp is not any different from a regular tolexed HRD other than snazzy exterior.2016-09-20T16:56:42+00:00monthlyhttps://irationaudio.com/2016/09/18/acoustic-230/https://irationaudio.com/wp-content/uploads/2016/09/acoustic-230.jpgacoustic-230Challenging Acoustic 230 repair that just kept going. Amp was bought DOA for super cheap. Was received not able to be turned on. Had some rigged on/off switch that didn't work but I actually had a identical or near identical switch as the original. Then didn't put out full power because of a broken connection on an interconnect wires from preamp to EQ board, then reverb didn't work because of broken connection in reverb tank, then I realized 70 and 125Hz sliders in EQ weren't working and L4 and L5 inductors needed to be replaced, then it worked great for a while but eventually would start to drift weaker and weaker in volume over time as the amp warmed up because of a thermal drift issue with Q305 transistor in the output section, wow.https://irationaudio.com/wp-content/uploads/2016/09/acoustic-230-chassis.jpgacoustic-230-chassisKind of a hard amp to photograph but preamp board is held in place by mounting to the volume and tone controls on the front panel of the amp. EQ board is up top, output section is mounted to a large heat sink that is mounted to the bottom of the chassis. Output transistors are on the fins on the left and right of the output PCB, just like Acoustic 370. If you want to see a cool trick I will pass it along. I was told what to do by some old gray beards when I was stumped so it's not like I thought of this myself, by any means. Anyway, thermal drift problem had been narrowed down to output section somewhere but unsure where or what. Transistors I guess would be first culprit, then caps and resistors, I would guess. Anyway, you can get a q-tip, get the end wet, sandwich it between some ice packs for a few dozen seconds, it gets really cold and you can use it as a cold probe. Without shocking yourself or shorting out other things in the circuit with this wet, conductive probe, touch the transistor bodies to cool them down and see if the signal recovers. You will have to let the amp run for a while, like 20 minutes in this case, so it can get warm and the signal is collapsed.2016-09-19T17:04:15+00:00monthlyhttps://irationaudio.com/2016/08/28/mesa-boogie-studio-22/https://irationaudio.com/wp-content/uploads/2016/08/mesastudio-22chassis.jpgMesaStudio.22+chassisNot too much to see since all the components are on the other side of the board. This would not be a fun one to troubleshoot with component side inaccessible. The PCB is held in place by plastic standoffs that mount to the chassis. If they are anything like the plastic standoffs in the Sunn Concert Series amplifiers (they seem to be exactly the same) they will most likely dry out and become brittle over the decades and when you need to unclip them to work on the circuit board they will probably snap off without too much trouble. Maybe I'm wrong hopefully. One power tube had a thermal runaway condition so it would eventually red plate if you left the amp on too long. https://irationaudio.com/wp-content/uploads/2016/08/mesaboogiestudio-22.jpgMesaBoogieStudio.22+Old Mesa Studio .22+, not that old though because the older ones had metal switches. It's a 1x12 combo that runs two EL84s in the output section for around 20W of power.2016-08-29T02:54:24+00:00monthlyhttps://irationaudio.com/2015/03/26/acoustic_370/https://irationaudio.com/wp-content/uploads/2015/03/acoustic-370-schematic.jpgAcoustic 370-SchematicAcoustic 370 schematic Schematic and parts list -- https://drive.google.com/folderview?id=0B01EXvY0__YYSXplOHIzZ0Z1SlE&usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/03/img_0713.jpgIMG_0713After image. Symmetrical clipping trimpot is adjusted and the 370 is doing 24.8V into 4 ohms for 153.8W.https://irationaudio.com/wp-content/uploads/2015/03/img_0698.jpgIMG_0698Huge power transformer down below, preamp board on the top left, power amp board in the rear (bottom) of the chassis and connected to output transistors and large gold heat sink, and three massive can-style electrolytic capacitors in the middle. Biggest cap (silver) powers the power amp, top left can cap (smallest one) goes to the preamp, and the one on the right couples the power amp to the speaker, blocking the DC power amp voltage and passing only the AC signal voltage to the speaker jack. If you look close you can see the purple circular trimpot on the power amp board towards the lower half of the PCB. It is slotted so you can turn it with a screwdriver. This is the symmetrical clipping offset adjustment (R304 on the schematic). It's used to adjust the DC voltage on the capacitor connected to the speaker jack to HALF the power amp supply voltage. In this case the power supply voltage 86.6V. Even when cranking the trimpot all the way the lowest voltage within reach was 51.2V. A check of the resistors in the circuit revealed some drifted ones that needed to be replaced. For instance R303, a 180K resistor connected directly to R304 trimpot measured 223K. The previous picture shows the amp after repair powered on at idle with multimeter displaying voltage at the cap coupling power amp to speaker jack. Not exactly half but i got the most symmetrical clipping IMO with 43.8VDC.https://irationaudio.com/wp-content/uploads/2015/03/img_0708.jpgIMG_0708Heavy 370 on the bench getting a lot of little 1uF caps replaced. Also it is a single supply amp meaning the output sits at half the supply voltage the signal is fed to the speaker through a large cap that blocks this DC. This voltage must be half the supply voltage for symmetrical clipping.https://irationaudio.com/wp-content/uploads/2015/03/img_0705.jpgIMG_0705Before image of 370 output at the onset of clipping. The top of the waveform is fattening and flattening and the bottom is still nicely rounded. Doing 21.1V into 4 ohm load for 111W.https://irationaudio.com/wp-content/uploads/2015/03/img_0704.jpgIMG_0704And if you really want to let it rip into heavy distortion. 32.4V for 262W into 4 ohms. Don't forget, this amp can go down to 2 ohms and yield more power still.https://irationaudio.com/wp-content/uploads/2015/03/img_0469.jpgIMG_0469Chassis image before re-conversion to 6L6 mode.https://irationaudio.com/wp-content/uploads/2015/03/img_0468.jpgIMG_04686L6 typically has 470 ohm screen grid resistor and EL34 uses 1.5K. 470 ohm screen grid resistors as well as a power supply dropping resistor are toasty. Also a 6L6 has no connection on pin 1. The suppressor grid is internally tied to ground via pin 8, the cathode. On an EL34 the suppressor grid is tied to pin 1 and since the suppressor grid must be grounded it must be physically wired to ground on the tube socket by tying pin 1 to pin 8.https://irationaudio.com/wp-content/uploads/2015/03/img_0462.jpgIMG_0462Mig 50 that had an EL34 conversion that didn't work out. The screen grid resistors were not changed when it was switched from 6L6 to EL34. 2016-08-28T23:52:13+00:00monthlyhttps://irationaudio.com/2015/03/26/gibson-br-9/https://irationaudio.com/wp-content/uploads/2015/03/gibson-ga9-br9.jpg*GibsonThere are many versions of this amp and this one resembles most closely the Gibson GA-9 circuit. The earlier version of this amp has a 6SN7 preamp tube that is transformer coupled to the output section. This one had a 6SJ7 like the GA-9 which is coupled to the output section via C3 and R7. R7 is the volume knob. The cathode bypass caps C1 and C4 were replaced, as well as the power supply filter caps C5, C6 and C7. https://irationaudio.com/wp-content/uploads/2015/03/img_0509.jpgIMG_0509ou can pull the chassis from the amp but it can only be removed so much because the output transformer windings mount directly to the speaker. Wish I took a proper image of the back of the amp assembled. The control panel has just 2 input jacks, a volume knob, and a fuse. Guess tone was not invented yet back then…...https://irationaudio.com/wp-content/uploads/2015/03/img_0498.jpgIMG_0498https://irationaudio.com/wp-content/uploads/2015/03/img_0496.jpgIMG_0496Image before recap.https://irationaudio.com/wp-content/uploads/2015/03/img_0494.jpgIMG_0494Nice relic from late 40s/early 50s. Vintage Gibson BR-9 lap steel amp in for a recap.2016-08-28T23:36:38+00:00monthlyhttps://irationaudio.com/2015/03/26/acoustic-g100t/https://irationaudio.com/wp-content/uploads/2015/03/acoustic-g100t-g100t-112-service-schematics-6.jpgAcoustic G100TAcoustic G100T parts listhttps://irationaudio.com/wp-content/uploads/2015/03/acoustic-g100t-g100t-112-service-schematics-5.jpgAcoustic G100TAcoustic G100T parts listhttps://irationaudio.com/wp-content/uploads/2015/03/acoustic-g100t-g100t-112-service-schematics-4.jpgAcoustic G100TAcoustic G100T schematichttps://irationaudio.com/wp-content/uploads/2015/03/acoustic-g100t-g100t-112-service-schematics-3.jpgAcoustic G100TAcoustic G100T schematichttps://irationaudio.com/wp-content/uploads/2015/03/acoustic-g100t-g100t-112-service-schematics-2.jpgAcoustic G100TAcoustic G100T schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_0771.jpgIMG_0771https://irationaudio.com/wp-content/uploads/2015/03/img_0770.jpgIMG_0770There was an unfortunate squeal in the gain channel. Power supply caps were not decoupling.https://irationaudio.com/wp-content/uploads/2015/03/img_0773.jpgIMG_0773100 watt Acoustic G100T TUBE amp. One of the last amps Acoustic made in the early 80s before going out of business. I know they also made the G60T which is a 60 watt version of this amp as a combo. there is more info at the link below but notice it is a different version of the G100T than the one pictured here. It has one input jack instead of 2, 4 switches on the front, switchable FET and tube distortion, etc. http://acoustic.homeunix.net/twiki/bin/view/Acoustic/GuitarTubeHead1602016-08-28T23:36:06+00:00monthlyhttps://irationaudio.com/2015/03/26/193/https://irationaudio.com/wp-content/uploads/2015/03/img_1047.jpgIMG_1047Doing 22.3V into 8 ohms for 62W at clippinghttps://irationaudio.com/wp-content/uploads/2015/03/kustom-replacement-parts-list.jpgVintageKustom.comChicago Bill throwing down for the greater goodhttps://irationaudio.com/wp-content/uploads/2015/03/full-k100-1-schematic.jpgFull K100-1 SchematicKustom K100-1 Schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1039.jpgIMG_1039Chassis is kind of funny because it's almost empty. This amp is very light. The only thing that weighs anything is the power transformer. Two big caps for the power supply, and a single PCB mounted on the front panel which holds all the components for the preamps for both channels. Output transistors are mounted on the heatsink bolted to the bottom of the chassis -- as with most 60s and 70s solid state amps it seems they are 2N3055's.https://irationaudio.com/wp-content/uploads/2015/03/img_1044.jpgIMG_1044Kustom K100-1. As far as I know Kustom amps typically had a power output of 1/2 the model number of the amp. In this case a Kustom K100 has an output of about 50W. This one scoped out to 57W before clipping. Naugahyde vinyl covering is still in nice shape. This one had a broken input jack, dirty pots, and a bad SE4002 transistor in channel 2. This is the first transistor after the input jack so the channel basically didn't work at all. SE4002 is long out of production and was replaced with transistor equivalent NTE123. A handful of drifted resistors were replaced as well. 2016-08-28T19:29:01+00:00monthlyhttps://irationaudio.com/2015/03/26/fender-bassman-ab165/https://irationaudio.com/wp-content/uploads/2015/03/fender_bassman_ab165.giffender_bassman_ab165Fender Bassman AB165 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_0943.jpgIMG_0943Nice F&T replacements for strength and stability in this modern era.https://irationaudio.com/wp-content/uploads/2015/03/img_09021.jpgIMG_0902https://irationaudio.com/wp-content/uploads/2015/03/img_0918.jpgIMG_0918Looking down the chassis. Various cathode bypass capacitors were replaced as well as the 470 ohm screen grid resistors. One measured 390 ohms and the other was 712 ohms. There is a bias trimpot attached to the hum balance pot installed by some previous tech visible just to the left of the upside down "70" written on the chassis.https://irationaudio.com/wp-content/uploads/2015/03/img_0849.jpgIMG_0849Old bulgy, leakey Mallory caps in the power supply needed to go. They are original and are still kinda working after about 45 years! The 4th from the left is visibly oozing it's electrolyte (an ionic conducting liquid)! Looking from left to right you can see the 4th and 5th cap have 235-7018A and 235-7015A (it looks like) printed on the paper covering, indicating they were manufactured during the 15th and 18th weeks of 1970.https://irationaudio.com/wp-content/uploads/2015/03/img_0845.jpgIMG_0845Old silverface Bassman from 1970. One power tube was totally burnt out. The amp was nearly all original other than a few modern tubes. It was in need of a recap and some various other maintenance.2016-08-28T19:27:42+00:00monthlyhttps://irationaudio.com/2015/03/26/sunn-model-t-fender-version/https://irationaudio.com/wp-content/uploads/2015/03/sunn-modelt-reissue-p2.jpgsunn-modelT-reissue-p2Sunn Model T schematic page 2https://irationaudio.com/wp-content/uploads/2015/03/sunn-modelt-reissue-p1.jpgsunn-modelT-reissue-p1Sunn Model T schematic page 1https://irationaudio.com/wp-content/uploads/2015/03/img_0403.jpgIMG_0403https://irationaudio.com/wp-content/uploads/2015/03/img_0401.jpgIMG_0401The problem was one of intermittent volume drop out. If you bang on the amp with your fist it would come back to full volume for a bit but definitely not reliable. Power supply dropping resistor has burnt PCB underneath it.https://irationaudio.com/wp-content/uploads/2015/03/img_0399.jpgIMG_0399The problem was one of intermittent volume drop out. If you bang on the amp with your fist it would come back to full volume for a bit but definitely not reliable. Power supply dropping resistor has burnt PCB underneath it.https://irationaudio.com/wp-content/uploads/2015/03/img_0398.jpgIMG_0398One huge single PCB what a hasslehttps://irationaudio.com/wp-content/uploads/2015/03/img_0396.jpgIMG_0396Viewing the same area as the previous pic but from the bottom side of the board you can see the trace that connects to the power supply dropping resistor has broken off the PCB. Between the two traces you can see the charring of the board on the other side. In cases like this where the PCB has been damaged the component must be hardwired.https://irationaudio.com/wp-content/uploads/2015/03/img_0391.jpgIMG_0391Unfortunately if you actually have to replace anything you have to pull out at least a dozen push-on connectors coming from the power transformer and more. Very time consuming and still nearly impossible to get the board out.https://irationaudio.com/wp-content/uploads/2015/03/img_0404.jpgIMG_0404https://irationaudio.com/wp-content/uploads/2015/03/img_0298.jpgIMG_0298Sunn Model T blasphemously called a reissue even though it is just a beefed up Fender Evil Twin. Obviously it has nothing in common with the original Sunn Model T -- it is just a modern 2-channel amp with clean and high gain channel.2016-08-28T19:24:46+00:00monthlyhttps://irationaudio.com/2016/07/23/acoustic-model-165/https://irationaudio.com/wp-content/uploads/2016/07/acoustic-165.jpgAcoustic Model 165Acoustic 165 from back in the day, probably early 80s. Some of the last Acoustics in production where they actually made tube amps, wow. This one is 100W with four 6L6GC. Someone put the front baffle on crooked and I didn't straighten it, because a lot of the hardware for the amp was lost (not by me) and any old screw that was around was used to keep it holdin together, and now I kind of regret it. Has two channels with master volumes for each, tone stack is shared between both channels, reverb, 50W / 100W mode, graphic EQ, etc.https://irationaudio.com/wp-content/uploads/2016/07/acoustic-165-chassis.jpgAcoustic 165 chassisHas lots of different brands of electrolytics -- Nichicon, Ilinois Capacitor, Mallory, NTE, a Cornell Dubilier (CDE) that someone has put in along the way. Other than the CDE, wonder if this has to do with being made towards the end of Acoustic. Just speculation, don't really know. The PCB was missing a few screws to hold it in place and when the amp would vibrate it would stress wires that go from the PCB to the power amp in 1/4" jack. When the connection would break, the amp would cut out occasionally since the signal could not get to the output section. 2016-08-27T14:36:59+00:00monthlyhttps://irationaudio.com/2016/07/27/earth-stagemaster-pa-2000/https://irationaudio.com/wp-content/uploads/2016/07/earth_pa2000_stagemaster1.pngearth_pa2000_stagemasterEarth Stagemaster PA2000 schematichttps://irationaudio.com/wp-content/uploads/2016/07/earth-pa-back-panel.jpgEarth PA back panelHas thirteen tubes to in all. Need lots of preamp tubes for all those tone stacks. Can push up to four cabs but really it is just a non-selectable 4 ohm minimum output no matter how many speaker jacks you use. Has reverb but you can hook up an external unit and select between internal and external reverb on each channel.https://irationaudio.com/wp-content/uploads/2016/07/earth-pa-head.jpgEarth PA headAnother amp I bought from Enzo as part of his shop liquidation. He is a legendary tech, if you didn't know. Guess someone never followed through with getting this thing going and it was abandoned piece that hung around the shop for many years.https://irationaudio.com/wp-content/uploads/2016/07/earth-pa-chassis.jpgEarth PA chassisBottom pops off for servicing and it's all point to point with components mounted to tag strips, pots, tube sockets, etc. Old Sprague caps are from 1973 and still going. Had a bit of an uneven output because the 82K plate load resistor for the one side of the phase inverter was about 50K. 100K was ok. Mostly just needed some cleaning and a bias.https://irationaudio.com/wp-content/uploads/2016/07/earth-pa-chassis-1.jpgEarth PA chassis 12016-07-28T16:29:32+00:00monthlyhttps://irationaudio.com/2015/03/25/fender-blues-deville/https://irationaudio.com/wp-content/uploads/2015/04/fender-blues_deville.gifFender blues_devilleFender Blues Deville schematichttps://irationaudio.com/wp-content/uploads/2015/04/img_0353.jpgIMG_0353Once you've seen one of these amps you've pretty much seen them all, Blues Deville, Hot Rod Deville, Blues Deluxe, Blues Jr. etc. The input jacks are PCB mount and typically break their solder connection from jostling. Same sometimes happens to the tone and volume knobs which are PCB mount also. These amps are also known to have broken or cracked solder connections on the tube sockets, especially power tubes. Re-flow them for stability, in these times.https://irationaudio.com/wp-content/uploads/2015/04/img_0350-e1428206779489.jpgIMG_0350Fender Blues Deville 2x12. The volume knob is linear taper so it seems like this amp is really loud because the whole range of volume resides in about the 1-4 setting. You can change this to audio taper to give it more use-able range.2016-07-28T01:31:02+00:00monthlyhttps://irationaudio.com/2016/07/14/ampeg-svt/https://irationaudio.com/wp-content/uploads/2016/07/ampeg_svt_preamp.gifampeg_svt_preampAmpeg SVT preamp schematichttps://irationaudio.com/wp-content/uploads/2016/07/ampeg_svt_power.gifampeg_svt_powerAmpeg SVT power amp/power supply schematichttps://irationaudio.com/wp-content/uploads/2016/07/ampeg-svt-12bh7.jpgAmpeg SVT 12BH7Old, old, old 12BH7 tubes that are the drivers for the 6550s in the output section were very weak and had exhausted most of their silver flash material. Also had a few old 12AX7s in tube sockets that were supposed to have 12DW7s. https://irationaudio.com/wp-content/uploads/2016/07/ampeg-svt.jpegAmpeg SVTAmpeg SVT from back in the day, probably late 60s. One of the big dropping resistors in the power supply broke its solder connection after about 50 years of rattling. The intermittent connection burnt the circuit board a little but it was not a big deal to patch back up. https://irationaudio.com/wp-content/uploads/2016/07/ampeg-svt-chassis.jpgAmpeg SVT chassisYou can pull the chassis and get the preamp and power amp/power supply out to work on. 90% of the time your problems will be in the output section or power supply. Can be hard to bias these amps because monitoring current draw for 6 tubes isn't easy. There is a 1 ohm cathode resistor that allows you to bias the amp to .072V (~24ma current draw per power tube with ohm's law conversion) but it really only gives you an averaged current draw of three tubes on either of the push/pull side. Most of the time this is ok, but it is possible some tubes are weak or strong, so the average is pulled up or down to what it needs to be but some tubes are working harder than others. This is where the bias probe comes in handy. 2016-07-18T05:26:38+00:00monthlyhttps://irationaudio.com/2016/06/28/marshall-jtm30/https://irationaudio.com/wp-content/uploads/2016/06/marshall-jtm-30-power-output.jpgMarshall JTM 30 power.outputMarshall JTM30 schematic. Kind of an interesting thing is the bias supply for the 5881s is set by ZD101, ZD102, ZD103, a string of voltage regulating zener diodes in series setting it at -39V. If you want to adjust the bias you need to get some zener diodes out of different values. https://irationaudio.com/wp-content/uploads/2016/06/marshall-jtm30-preamp.jpgMarshall jtm30 preampMarshall JTM30 schematichttps://irationaudio.com/wp-content/uploads/2016/06/marshall-jtm30.jpgMarshall JTM30Marshall from the 90s. Has clean and gain channel with shared tone stack and reverb and maser volume.https://irationaudio.com/wp-content/uploads/2016/06/marshall-jtm30-chassis.jpgMarshall JTM30 chassisKind of crammed together with preamp board on the left and power amp/power supply on the left. Someone has recapped the power supply with axial capacitors where it used to have radial, I could only assume. 2016-06-29T03:52:06+00:00monthlyhttps://irationaudio.com/2015/03/26/sunn-concert-bass/https://irationaudio.com/wp-content/uploads/2015/03/img_1249.jpgIMG_1249Sunn Concert Basshttps://irationaudio.com/wp-content/uploads/2015/03/img_1242.jpgIMG_1242Sunn Concert Bass output after repair. Large sine wave is at the speaker jack. Small sine wave is at the end of the preamp before the output section. Really using that dual trace potential. Putting out 24V into a 4 ohm load -- doing 144W at clipping. I received this amp in pieces. A previous shop declined the repair and returned the unit to the customer disassembled.https://irationaudio.com/wp-content/uploads/2015/03/img_1238.jpgIMG_12384 transistors in the output section were shorted and many parts on the output section PCB were burnt. This was a labor intensive repair so I could see why the other shop turned it down. The output section uses an interstage transformer to couple the signal from the preamp to the bases of the output transistors. You have to disconnect many wires from the PCB to be able to get it out and access the solder side, which is a hasslehttps://irationaudio.com/wp-content/uploads/2015/03/img_1236.jpgIMG_1236Output section after repair. Lots of shorted emitter resistors and 2N3055s. Some of the 22 ohm bias resistors were fried too HUGE lot of service info here, basically the mother load if you ever need to service one of these amps. It has schematics from all eras, parts list, and mods -- https://drive.google.com/file/d/0B01EXvY0__YYRWVWWC1mMy0tS3c/view?usp=sharing2016-06-23T14:39:02+00:00monthlyhttps://irationaudio.com/2015/03/30/yamaha-g100-2x12/https://irationaudio.com/wp-content/uploads/2015/03/img_1702.jpgIMG_1702An old little Yamaha combo amp. I don't really know anything about this amp. I believe it is from the 80s. I think there are various versions of the Yamaha G100. This is a 1x12 combo but I have also seen heads that I believe are the same amp. Not too sure. Had a few issues of poor connections at a ribbon cable interconnect and at the input jack causing it to cut in and out, but mainly just needed a new speaker.https://irationaudio.com/wp-content/uploads/2015/03/img_1711.jpgIMG_1711Super neat sine wave and the amp sounded very clean indeed. Wish the intensity of the display wasn't up so high so we could see it even neater. Yamaha G100 schematic -- https://drive.google.com/file/d/0B01EXvY0__YYekhiM3QtWmZxODA/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/03/img_1708.jpgIMG_1708G100 puts out about 100W - that single 12" speaker needs to be able to handle it. Beware, the master volume knob only works for one channel and not the other. If you switch channels with channel volumes up and master volume down thinking you are in control, you may be in for a rude surprise.2016-06-23T14:38:12+00:00monthlyhttps://irationaudio.com/2015/09/30/sunn-concert-controller/https://irationaudio.com/wp-content/uploads/2015/09/img_0266.jpgIMG_0266This is the reason all the knobs aren't broken off. Thank you Sunn.https://irationaudio.com/wp-content/uploads/2015/09/img_0189.jpgIMG_0189Once you take this out of the chassis and remove screws holding the two chassis panels together, it folds open kind of like a clam. Preamp is on the left with three identical PCBs with volume and tone circuitry for two channels each. PCB at top left is for reverb and tone controls. In the back is the EQ and the ins and outs for each channel. Right side is the power supply and output section. This was worked on in my DIY novice days but even back then it wasn't hard to find a broken solder connection inside the reverb tank and an intermittent jumper wire going from one of the preamp PCBs to the power amp section, making the two channels cut in and out. https://irationaudio.com/wp-content/uploads/2015/09/img_0183.jpgIMG_0183Preamp shot. Compared to Sunn Concert Lead this amp is much more clean. Makes sense since it is a PA. It's actually really awesome In that way though because if you put an overdrive pedal in front of it it maintains a very clear, full sound. Concert Lead tends to get a little shrill when cranked.https://irationaudio.com/wp-content/uploads/2015/09/img_0177.jpgIMG_0177Power amp is identical to Sunn Concert Lead and Concert Bass amps. Reference their schematicshttps://irationaudio.com/wp-content/uploads/2015/09/img_0264.jpgIMG_0264Sunn Concert Controller from back in the day. Bought this for $75 about six years ago. The reverb didn't work and two of the six channels didn't work. An awesome feature of this amp is that is has a tone knob for the reverb -- you can control if it's shimmery and bright or underwater, submersive sounding. 2016-06-23T14:37:15+00:00monthlyhttps://irationaudio.com/2016/05/25/qsc-4-2-power-amp/https://irationaudio.com/wp-content/uploads/2016/03/qsc-frong-panel.jpgQSC frong panelCool font and design. All you need is volume and power and you're all good. https://irationaudio.com/wp-content/uploads/2016/03/qsc-chassis.jpgQSC chassisAh yes, QSC 4.2 power amp. Old school 70s-style with the Motorola MJ15015 power transistor in the output section. Paired with the MJ2955, equivalent to the 2N3055 you see in all the old 70s solid state amps. Does 50W per side with 2 channels. This old school power amp still works good other than cracked solder connections on the input jack/speaker jack PCB. Backlit to show the component side of the PCB but also the traces on the back side. So easy to service since the top and bottom panel come off, making component and solder side so easy to access. 2016-06-23T14:36:40+00:00monthlyhttps://irationaudio.com/2016/06/23/sunn-beta-bass/https://irationaudio.com/wp-content/uploads/2016/06/sunnbeta.jpgSunnBetaSunn Beta Bass with the blendable channels and gain and level on each channel, AND a master volumehttps://irationaudio.com/wp-content/uploads/2016/06/sunn-beta-lead.jpgSunn Beta leadOutput transistors are attached to a big heat sink which also holds the board for the output section. Pretty neat amp. I didn't get much into it, mostly just cleaning pots and jacks and fixing the blend input jack. The Sunn Beta Series Service Manual will tell you everything you need to know. Read the circuit descriptions if you want to learn a lot. Great stuff.2016-06-23T14:33:52+00:00monthlyhttps://irationaudio.com/2016/06/18/musicman-2100-b/https://irationaudio.com/wp-content/uploads/2016/06/musicman-2100-b-2.jpgMicrosoft Word - covertoc.docMusicman 2100-B schematic. Doing the Musicman thing with a tube power section in the grounded grid arrangement and and a really high 700V plate voltage to get lots of power. 100W out of two 6L6GCs. Power tubes are driven at the cathode by a pair of JE1692 transistors. Since the cathode is elevated and the grid is grounded the tube still has a negative bias voltage. Everything from the phase inverter back is all op-amps.https://irationaudio.com/wp-content/uploads/2016/06/musicman100b-chassis.jpgMusicman100B chassisNeat build that is all op-amp driven all the way up the phase inverter. https://irationaudio.com/wp-content/uploads/2016/06/musicman-100b.jpgMusicman 100BLook at this EQ. It has knobs for treble and bass and then the mid is frequency selectable with a level control. Also has a graphic EQ for after the fact. Awesome tone controls. After that all you need is volume.2016-06-18T18:51:27+00:00monthlyhttps://irationaudio.com/2016/05/27/sunn-sentura-ii/https://irationaudio.com/wp-content/uploads/2016/05/sunn_sentura2.pngsunn_sentura2Sunn Sentura II schematichttps://irationaudio.com/wp-content/uploads/2016/05/sunn-sentura-ii.jpgSunn Sentura IISunn Sentura II bought from the venerable ENZO. I bought a few amps from Enzo when he closed down his shop and this is one of them. He had a handful of awesome things that were abandoned by customers. Either they turned down the estimate for repair or they never picked it up, or something. Not exactly sure but this one was a good one to get, only needing minor work.https://irationaudio.com/wp-content/uploads/2016/05/sunn-sentura-ii-top-chassis.jpgSunn Sentura II top chassisCan't see the whole thing in the pic but this one is labeled as being from 10/17/67. Has Hammond replacement PT. Does about 50W with two 12AX7s in the preamp, 6AN8 phase inverter and two KT88s in the power section. Reverb sounds nice. https://irationaudio.com/wp-content/uploads/2016/05/sunn-sentura-ii-chassis.jpgSunn Sentura II chassisA lot of these old Sunns are basically the same, either adding a feature or two such as tremolo, reverb, Hi and Lo boosts, etc. This one has tremolo and reverb however the tremolo does not work because the little tremolo photocell unit has been robbed of it's lamp. Sunn Sonaro in the background is basically the same amp but with no tremolo and reverb, and it has a solid state rectifier rather than tube rectifier like the Sentura II has. 2016-06-16T05:54:44+00:00monthlyhttps://irationaudio.com/2016/02/03/sunn-200s/https://irationaudio.com/wp-content/uploads/2016/02/sunn_200s.pngSunn_200sSunn 200S schematichttps://irationaudio.com/wp-content/uploads/2016/02/img_0616.jpgIMG_0616You can usually tell an early Sunn because the logo has no circle R (trademark logo) after the word after the name. I could stand to be corrected but I believe the trademark logo started getting added at some point in 1969. https://irationaudio.com/wp-content/uploads/2016/02/img_0614.jpgIMG_0614Almost the exact same amp as the Sunn Sonaro in the previous post but from 1967 instead of 1971. Same tone stack with Hi and Lo boosts, a 12AX7 for the preamp and a 6AN8 phase inverter, and two 6550/KT88s in ultralinear in the output. https://irationaudio.com/wp-content/uploads/2016/02/img_0608.jpgIMG_0608Layout is nearly identical to Sonaro but instead of a 1.5K cathode resistor for each side of V1 the 200S has a 1.5K cathode resistor shared for both sides of V1 and it is bypassed with a 250uF cap. It also has a GZ34 tube rectifier instead of solid state rectifier and the B+ is was only 458V compared to 585V for the Sonaro. So, yeah this thing will have more gain, less volume, and less clean output. It only did 40W at clipping.2016-06-16T05:53:41+00:00monthlyhttps://irationaudio.com/2016/06/16/orange-rockerverb-50/https://irationaudio.com/wp-content/uploads/2016/06/orange-rockerverb.jpgOrange RockerverbJust when you think it has that whole one-big-PCB phenomenon going on it's really just two but you can't see the break because of the rainbow interconnect cable. Everything is PCB mounted. Kind of a drag but it does seem at least pretty ruggedly built. PCB isn't flimsy. Here is the Orange Rockerverb 50 schematic if you want to look at it.https://irationaudio.com/wp-content/uploads/2016/06/orange-rockerverb-50.jpgOrange rockerverb 50Rockerverb 50 doing about 50W with four 6V6s in the power section. Just getting some new power tubes since it was bought used and the power tubes seemed really old. Has clean and gain channel and reverb. 2016-06-16T05:17:57+00:00monthlyhttps://irationaudio.com/2015/03/26/gibson-ga-19-rvt-maestro-m-216/https://irationaudio.com/wp-content/uploads/2015/03/gibson-falcon-ga-19-rvt-chassis.jpgGibson Falcon GA 19 RVT chassisAs per usual the Gibson schematic does not match the amp so you have to consult the Maestro M-216 RVT schematic for further knowledge. There are a few different versions of this amp. this one has a 6C4 reverb tube and 6EU7 preamp tubes. I forgot what pre tubes the other version has but I know it has a 7199 for reverb. In this case V3 plate voltage was 0 rather than the needed ~220V. The reverb tube was lit up because it was getting heater voltage but it was getting no high voltage at the anode to draw electronics with.https://irationaudio.com/wp-content/uploads/2015/03/img_0589.jpgIMG_0589https://irationaudio.com/wp-content/uploads/2015/03/img_0585.jpgIMG_0585Has lots of tubes that are no longer in production today so you gotta hope they still work. They do -- the problem was no reverb, which was traced to an open reverb transformer.https://irationaudio.com/wp-content/uploads/2015/03/img_0587.jpgIMG_0587Gibson Falcon GA 19 RVThttps://irationaudio.com/wp-content/uploads/2015/03/gibsonga19maestrom216.jpgGibsonGA19MaestroM216Gibson GA 19 RVT / Maestro M-216 schematic 2016-05-31T17:30:10+00:00monthlyhttps://irationaudio.com/2016/05/29/univox-u45b/https://irationaudio.com/wp-content/uploads/2016/05/univox_u45bschematic.jpgunivox_u45bschematicUnivox U-45B schematichttps://irationaudio.com/wp-content/uploads/2016/05/univox-u45b-full.jpgUnivox U45B fullCool, old Univox. Keeping it simple with minimal everything. Volume, tone and tremolo are the knobs and that's it. Input jacks are guitar, accordian, and aux. Old school chrome dust cap on the 12" speaker. Let's the high frequencies roll.https://irationaudio.com/wp-content/uploads/2016/05/univox-u45b-chassis.jpgUnivox U45B chassisSuch a surprise to see but someone has totally rebuild this amp and has done a totally good job. They even put the isolating washers on the 1/4" input and speaker jacks. The nut on the tremolo knob had become loose so the knob/pot would just spin around forever, eventually ripping the leads from the pot to the PCB out. A quick resolder after finding the schematic and this old dog was ready to rip again, with tremolo.2016-05-29T22:33:20+00:00monthlyhttps://irationaudio.com/2016/05/28/sovtek-mig100b-bassov-blues-boy/https://irationaudio.com/wp-content/uploads/2016/05/sovtek-mig100b.jpgSovtek MIG100B2016-05-28T23:45:48+00:00monthlyhttps://irationaudio.com/2016/05/27/aims-eclipsor/https://irationaudio.com/wp-content/uploads/2016/05/aims-eclipsor.jpgAIMS EclipsorWhoops these pictures are pretty bad. Grainy old iphone pics that are horribly lit sorry. AIMS Eclipsor is a rare beast. There is really no info that I know of on these amps. Some people say they are made by ex-Fender engineers who started their own company in Phoenix. Kind of in a Twin Reverb style with reverb and tremolo and some boost switches. https://irationaudio.com/wp-content/uploads/2016/05/aims-eclipsor-chassis.jpgAIMS eclipsor chassisHas those big bottle 6550/KT88s in the output section rather than 6L6s like the Fenders have for more clean headroom. Needed a recap because it was blowing fuses. Let the good times roll. 2016-05-28T22:22:11+00:00monthlyhttps://irationaudio.com/2016/05/26/mojotone-janice-dual-showman/2016-05-27T05:14:16+00:00monthlyhttps://irationaudio.com/2016/05/25/hartke-lh500/https://irationaudio.com/wp-content/uploads/2016/05/hartke-lh500-full.jpgHartke LH500 fullHartke LH500 bass amp. One of those hybrid bass amps you always see that has a preamp tube or two and then a big, powerful solid state output section. Big heat sink on the right with fan to keep those output transistors cool when trying to push 500W. Not an easy task. Airflow and fan placement are very important in amps like this if they are to be reliable.https://irationaudio.com/wp-content/uploads/2016/05/hartke-lh500-damage.jpgHartke LH500 damageThis board got all burnt up when this thing failed. The amp did not fail, but the fan, and output transistor, and a 10 ohm resistor did. Not sure if the fan burn out the transistor or if it was the other way around, but this thing needed a little hardwiring because the board is so burnt I didn't know if it would be reliable otherwise. 2SC3117 transistor seemed impossible to find but as a last resort I called Hartke and they supplied it without a problem. Awesome support! Full schematic is available here for Hartke LH500.2016-05-26T01:40:58+00:00monthlyhttps://irationaudio.com/2016/05/25/gibson-super-goldtone-30rv/https://irationaudio.com/wp-content/uploads/2016/05/gibson-super-gold-tone-full.jpgGibson Super Gold Tone fullCranking out about 30W with 4xEL84 output section. Ventilation on top which is a good idea I'd say because EL84s get really hot and they are buried deep in that chassis. Here is the full service manual for Gibson Super Goldtone. https://irationaudio.com/wp-content/uploads/2016/05/gibson-super-gold-tone-chassis.jpgGibson Super Gold Tone chassisGibson Super Gold Tone with the Les Paul style knobs. Didn't know Gibson still made amps but this one is from 2002. It has one 10" and one 12" speaker. Had this one in for a retube. Kind of a classy amp with all its gold trim.2016-05-25T21:25:32+00:00monthlyhttps://irationaudio.com/2016/05/25/hiwatt-l100r/https://irationaudio.com/wp-content/uploads/2016/05/hiwatt-l50r-preamp-circuit-diagram.jpghiwatt-l50r-preamp-circuit-diagramHiwatt L100R and L50R schematic. Not the full schematic but the best I could ever find. Parts values are not filled in and there is no output section or power supply :(https://irationaudio.com/wp-content/uploads/2016/05/hiwatt-l100r-frong.jpgHiwatt L100R frongHiwatt L100R from the mid 80s or so. Not a typical Hiwatt like you would expect because it is a channel switching amp -- it has a clean channel and distortion channel, and reverb. Transformers are absolutely massive. Clean sounds like a Hiwatt but the gain channel is pretty whatever. Seems like they tried to get down with the 80s metal trend but it didn't work good. Oh well, still has that Hiwatt clean sound that people love.https://irationaudio.com/wp-content/uploads/2016/05/hiwatt-l100r-chassis.jpgHiwatt L100R chassisKind of a weird PCB era of Hiwatt. The build is kind of janky but I don't think know if it came from the factory that way or if someone did a sloppy recap on it. Also look how the plate leads for the output transformer are clipped short and then there is another wire attached to make it reach to the tube sockets. Same thing around the AC input. Also this amp has a bunch of PCB interconnects but it looks like Hiwatt ran out of them and they are hardwired instead? Not what you would typically expect from Hiwatt but this one is from a dark era I guess. It works and sounds pretty good, but not a high quality job like the 70s ones are known for. 2016-05-25T20:40:29+00:00monthlyhttps://irationaudio.com/2016/04/02/orange-or120/https://irationaudio.com/wp-content/uploads/2016/04/orange-or120-graphic-mkii-schematic.gifOrange OR120 graphic MKII schematicOrange OR120 Graphic MKII schematichttps://irationaudio.com/wp-content/uploads/2016/04/or120-scope.jpgOR120 scopeOR120 on the bench getting a once over before recording. It was cutting out a lot after being banged around in the car on the way to recording. Oh well, all worked out.https://irationaudio.com/wp-content/uploads/2016/04/or120-close-up.jpgOR120 close upClose up of screen grid resistor solder connection on pin 4 of power tube. https://irationaudio.com/wp-content/uploads/2016/04/or120-chassis.jpgOR120 chassisYay Orange OR120 from back in the day. Caps say 1975. Has trimpots for bias and for hum balance. Just one preamp tube and a cathodyne phase inverter and then 4xEL34 in the output. Hard to see in the picture but the bias was set so hot (and these amps run so hot in general, not great ventilation in the chassis) that the solder on pin 4 of the two right output tubes is kind of vaporized. The screen grid resistor looks like it's just sitting in the socket, barely connected. 2016-04-14T18:12:07+00:00monthlyhttps://irationaudio.com/2016/02/21/george-dennis-the-blue/https://irationaudio.com/wp-content/uploads/2016/02/george-dennis-the-blue-100-schematic.jpgGeorge Dennis The Blue 100 schematicGeorge Dennis The Blue schematichttps://irationaudio.com/wp-content/uploads/2016/02/george-dennis1.jpgGeorge DennisDon't really know too much what to say about this one. I have never seen or heard of this amp before and I didn't get too far into it because it just had a reverb problem. It would squeal, which I eventually realized was caused the the RCA jack for the reverb pan being somewhat smashed and not making good connection. This is one of those do everything kind of amps I guess since it has four channels, reverb, various switched cuts, boosts, sweeps, contours, mid shifts, etc. I only played it for a minute or two but it seemed like you might be able to get a good Rust in Peace tone out of this thing, among other things.https://irationaudio.com/wp-content/uploads/2016/02/george-dennis-the-blue1.jpgGeorge Dennis The BlueKind of a nice build. I like how the wires coming from the tone and volume pots are looped through the PCB for extra support. I don't even want to think about wiring all those pots though. No schematic that I know of for this one. Sort it out.2016-03-05T02:03:30+00:00monthlyhttps://irationaudio.com/2016/02/27/ampeg-v4b/https://irationaudio.com/wp-content/uploads/2016/02/ampeg-v4b-chassis.jpgAmpeg V4B chassisAmpeg V4B. Nearly identical to V4 but has an Ultra Lo switch for extra bass and no reverb. Recapped by someone else and pretty nicely maintained. Clean too. There is a big good write up on V4/V4B on the Electronic States site that says more than I would ever write, check it.https://irationaudio.com/wp-content/uploads/2016/02/ampeg-v4b-schematic.jpgAmpeg V4B schematicAmpeg V4B schematichttps://irationaudio.com/wp-content/uploads/2016/02/ampeg-v4b.jpgAmpeg v4bMaybe the tone secret to the V4 is that almost all of them have the silkscreen wore off the front panel which makes the musician actually have to dial in a good sound by ear.2016-02-27T17:38:38+00:00monthlyhttps://irationaudio.com/2016/02/21/knight-93-320/https://irationaudio.com/wp-content/uploads/2016/02/knight_93_320_769052.jpgknight_93_320_769052Stock imagehttps://irationaudio.com/wp-content/uploads/2016/02/knight-93-320-schematic.jpgKnight 93-320 schematicKnight 93-320 schematichttps://irationaudio.com/wp-content/uploads/2016/02/knight_93_320_818070.pngknight_93_320_818070https://irationaudio.com/wp-content/uploads/2016/02/knight-93-320.jpgKnight 93-320Knight radio made by Allied Radio Corp. in Chicago. If you peruse the radiomuseum.org listing of this amp it says it's from "1948?" This one was bought at an estate sale with no power tubes and no rectifier tube. It uses a 5U4GB and two 6L6s in push-pull for the output. Who knows when is the last time it was turned on so I brought it up on the variac and the bulb limiter to limit possible smoke and burning. It actually worked. Pretty noisey but had some volume. This would be a great fixer-upper, however I didn't do this because the owner just wanted some tubes and confirmation it worked so he could sell it as a fixer-upper. I adjusted the bias for the power tubes but that's really it, other than cleaning all pots, jacks, and tube sockets. https://irationaudio.com/wp-content/uploads/2016/02/knight-93320.jpgKnight 93320Cool image if you want to enjoy the rats nest. Awesome ground bus wire and old wax caps, and old domino caps! Wow, this thing is old. I wish I remembered to read the caps to see when it's from, but I would guess it's from sometime in the 1950s.2016-02-24T22:56:32+00:00monthlyhttps://irationaudio.com/2016/02/20/marshall-jcm900-2100/https://irationaudio.com/wp-content/uploads/2016/02/marshall-jcm900-2100-preamp1.jpgMarshall JCM900 2100 preampMarshall JCM900 2100 preamphttps://irationaudio.com/wp-content/uploads/2016/02/jcm900-11.jpgJCM900-1JCM900 on the bench after it has been somewhat abandoned. I think it was shelved over 5 years ago for sounding too trebley and blowing a fuse. When I got it it worked actually but had weak output. Power tubes were weak and one was not working at all because the 2.2K screen grid resistor was burned open. A lot of opamps in this amp. They are used for the input and first gain stage, reverb drive and reverb recovery, and for the boost circuit. V1 is a 12AX7 to drive the tone stack, it's a cathode follower in the typical fashion. V2 is a gain stage and V3 is phase inverter.https://irationaudio.com/wp-content/uploads/2016/02/jcm9001.jpgJCM900Has two channels with reverb on both. Since this amp was too trebley I did a mod that affected the negative feedback, to try to give the amp more bass response. Putting a .001uF capacitor in series with paralleled C6 and R17 in the negative feedback circuit creates a shelving filter that does not feed back lower frequencies. So when lower frequencies are not fed back, they are not phase canceled in the phase inverter, and the amp has more bass response.https://irationaudio.com/wp-content/uploads/2016/02/jcm900-4100-2100-power-supply.jpgJCM900 4100, 2100 power supplyMarshall JCM900 2100, 2101, 4100, 4101 and 4102 power supply schematichttps://irationaudio.com/wp-content/uploads/2016/02/marshall-jcm900-4100-preamp.jpgImage1.pspMarshall JCM900 4100 preamp schematichttps://irationaudio.com/wp-content/uploads/2016/02/jcm900-1.jpgJCM900-1https://irationaudio.com/wp-content/uploads/2016/02/jcm900.jpgJCM9002016-02-20T20:18:13+00:00monthlyhttps://irationaudio.com/2016/02/20/marshall-jcm900-4100/https://irationaudio.com/wp-content/uploads/2016/02/marshall-jcm900-2100-preamp.jpgMarshall JCM900 2100 preampMarshall JCM900 2100 preamp schematichttps://irationaudio.com/wp-content/uploads/2016/02/jcm9004100.jpgJCM9004100A lot of opamps in this amp. They are used for the input and first gain stage, reverb drive and recovery, and for the boost circuit. It essentially has an overdrive pedal inserted into the front of the amp before the V1 12AX7 drives the tone stack. It is a cathode follower in the typical fashion. V2 is a gain stage and V3 is phase inverter.https://irationaudio.com/wp-content/uploads/2016/02/mg_2592.jpg_MG_2592Somehow I lost my chassis image of this amp and all I have is a pic of the crappy re-soldering job someone did in the past. This thing had intermittent crackle and it would cut out occasionally. After re-flowing many solder connections like these ones on the volume and tone pots it worked reliably. It was also extremely dirty. I'm sure cleaning the pots and tube sockets helped too.2016-02-20T20:17:30+00:00monthlyhttps://irationaudio.com/2016/02/03/sunn-sonaro/https://irationaudio.com/wp-content/uploads/2016/02/sunn-200s_i40_sorado_sonic-i40.jpgThis is not the right schematic since it has a tube rectifier and lower power supply voltages. I believe the preamp, phase inverter, and output section are accurate though. 2016-02-04T02:54:32+00:00monthlyhttps://irationaudio.com/2016/01/10/mojotone-fender-princeton/https://irationaudio.com/wp-content/uploads/2016/01/mg_2563.jpg_MG_2563Mojotone kit amp I built that is the circuit of a tweed Fender Princeton. I photographed it next to this big saw because I almost wanted to cut it in half when I was done. The layout had the output transformer winding color code wrong and it would squeal badly, because the output transformer primary was wired backwards. Oh well, every kit has various errors in the build instructions it seems. https://irationaudio.com/wp-content/uploads/2016/01/mg_2561.jpg_MG_2561Fender Prinecton, all 25 parts. When I got the amp it was started but never completed. The components were installed in the tag board but none of the transformers, input jacks, tone pots, etc., were wired up. Actually sounded pretty good when it was done. Let the good times roll.2016-01-23T09:37:37+00:00monthlyhttps://irationaudio.com/2016/01/18/1545/https://irationaudio.com/wp-content/uploads/2016/01/peavey_mx_vtx-1-schematic.jpgMX.pdfPeavey MX VTX schematic2016-01-18T11:02:29+00:00monthlyhttps://irationaudio.com/2016/01/11/knight-mono-amplifier/https://irationaudio.com/wp-content/uploads/2016/01/mg_2601.jpg_MG_2601Really a nice looking piece. Gotta love those vintage tube radios from back in the day.https://irationaudio.com/wp-content/uploads/2016/01/mg_2600.jpg_MG_2600Knight mono amplifier, model UNKNOWN. Made by Allied Radio Corp. in Chicago in the 60s. Has two 6L6GCs in push-pull in the output section. This one did 27W at clipping, pretty damn loud. Nas sounded awesome through it into a 2x12, blasting. On/off switch is incorporated into the treble knob.https://irationaudio.com/wp-content/uploads/2016/01/mg_2599.jpg_MG_2599Transformers are huge. Makes it easy to just flip it over and work on without any fear of crushing tubes. Amazingly this thing still works other than just needing a good cleaning of the pots and jacks, and tube sockets. Bias was set at about 90% with the stock 200R cathode resistor so I increased it to 250R which brought the bias down into a reasonable level. If this were mine I would recap and let it run for another 50 years, but this one will remain mostly untouched, as the owner just wants a confirmation that it works good so it can be sold. 2016-01-16T06:09:37+00:00monthlyhttps://irationaudio.com/2015/06/04/vht-sig-x/https://irationaudio.com/wp-content/uploads/2015/06/img_2459.jpgIMG_2459A doube fuck you. The FU gesture + that solder connection next to it is just hanging by a thread, how rude.https://irationaudio.com/wp-content/uploads/2015/06/img_24621.jpgIMG_2462No worries, whack in a 100 ohm resistor and she'll be right.https://irationaudio.com/wp-content/uploads/2015/06/img_2458.jpgIMG_2458No schematic that I know of for this modelhttps://irationaudio.com/wp-content/uploads/2015/06/img_2463.jpgIMG_2463VHT Sig: X with all the bells and whistles. It has 3 channels which are individually switchable between 40 and 100W. The power tubes were replaced and with its two new KT88s it did 90W at clipping in 100W mode. It can do 100W with 42W plate dissipation power tubes but the new ones were only 35W. This is all fine though no worries. This amp performed nice.https://irationaudio.com/wp-content/uploads/2015/06/img_2462.jpgIMG_2462https://irationaudio.com/wp-content/uploads/2015/06/img_2461.jpgIMG_2461Lots of little bodges on this one and I don't know if this amp went through many revisions over the years or if they just printed the PCB totally errorful and had to make corrections. All 9 pin tube sockets were wire wronged and were factory corrected.https://irationaudio.com/wp-content/uploads/2015/06/img_2460.jpgIMG_24602016-01-14T09:23:33+00:00monthlyhttps://irationaudio.com/2016/01/10/supro-corsica-s6622/https://irationaudio.com/wp-content/uploads/2016/01/supro_corsica_s6622.jpgsupro_Corsica_s6622Supro Corsica S6622 schematichttps://irationaudio.com/wp-content/uploads/2016/01/mg_2495.jpg_MG_2495Old Supro Corsica made in Chicago back in the day by Valco. This is the opposite of the Hiwatt DR103 in the last post in regards to layout and wiring -- a total rat's nest! Layed out in true point to point with all components mounted to pots, tube sockets and tag strips, but what a mess. Oh well it works good and sounds good, and it's just a cheap amp anyway. This thing needed a new power tube and reverb tank so it could rock like it did in its old glory days. Mallory caps are from the year 1966.2016-01-14T07:44:51+00:00monthlyhttps://irationaudio.com/2015/10/31/magnatone-m8/https://irationaudio.com/wp-content/uploads/2015/10/img_0047.jpgIMG_0047Magnatone M8 from back in the day. These were made in 1963 and 1964. Has two channels with reverb and vibrato. Vibrato is special because it uses a type of varistor that is no longer in production. You can mimic this type of vibrato design but it cannot truly be copied without the varistor of the era. It has a very unique sound. There is an excellent article on this by Tim Robbins that describes this vibrato circuit and why it is unique -- Magnatone Vibrato Designhttps://irationaudio.com/wp-content/uploads/2015/10/img_1995.jpgIMG_1995Flash ruined my pic but maybe I can't even say that because no flash was even worse. Oh well. The top pops off and all the electronics are on one side of the chassis while all the tubes are on the other. All components are mounted to tag strips or to the tube sockets or pots. Power supply is propped up on the left of the chassis but it actually sits at the bottom of the enclosure screwed down into the cabinet. Amp had a torn speaker, AC cord inlet was broken, and had a wrong tube in the 12DW7 position. It's an untouched old relic but it still works. https://irationaudio.com/wp-content/uploads/2015/10/img_2673.jpgIMG_2673Magnatone M8 schematic. Like a lot of old gear the schematic is pasted inside the chassis so a service person can reference it one day. There is a site with a wealth of knowledge committed to Magnatone amps. More info and better schematic for the Magnatone M8 here.2015-10-31T19:47:12+00:00monthlyhttps://irationaudio.com/2015/10/17/fender-princeton-reverb-aa764/https://irationaudio.com/wp-content/uploads/2015/10/fenderprincetonreverb_aa764.gifFenderPrincetonReverb_AA764Fender Princeton Reverb AA764 schematichttps://irationaudio.com/wp-content/uploads/2015/10/mg_2371.jpg_MG_2371Silverface Princeton Reverb with the drip-edge style grill. This thing is all original except the speaker which has been replaced with a Kendrick 10" speaker. https://irationaudio.com/wp-content/uploads/2015/10/mg_2367.jpg_MG_2367Had some issues with the noise floor and also had farty, rattley bass response. Wondered if the speaker was the cause but it was not. Decreasing the V1 cathode bypass cap to 4.7uF and biasing a little cooler helped with that bass distortion. However, once I had it back in the enclosure and played it through its own speaker rather than my listening speaker it still sounded bad. The speaker baffle was rattling. Took care of that, then there was still a strange ticking noise that could be heard, which was coming from one of the power tubes. I didn't hear it at first because there was a fan on close to my bench masking it. This was one of those scenarios where you fix something, realize there's still more problems, find something else, still sounds weird for some reason, dig through it again, eliminate more extraneous noise, etc. Eventually vanquished all the demons and in the end it sounded nice, phew!2015-10-25T20:38:13+00:00monthlyhttps://irationaudio.com/2015/10/03/blue-matamp/https://irationaudio.com/wp-content/uploads/2015/10/img_00571.jpgIMG_0057BLUE Matamp made by Radiocraft for GTV (GCV?). Gotta love these custom Matamps. No schematic available so you just have to sort it out yourself.https://irationaudio.com/wp-content/uploads/2015/10/img_0013.jpgIMG_0013BLUE Matamp on the bench made in UK by Radio Craft. These amps are pretty nice builds with their PCB turret board construction. Seems rugged, and you can tell there is time taken into making something special. Could easily just dump it on a huge PCB and eliminate much of the hand wiring and soldering but they chose not to do this, great. This layout is very similar to the Blue Matamp I had in a previous post, if you would like to peruse for more in depth info.https://irationaudio.com/wp-content/uploads/2015/10/img_0056.jpgIMG_0056Original voltage divider ratio for line out was 6.8K:470K. Adjusting it I found 510R:10K to be much better. Much better reproduction of the output signal and when the amp is cranked doing 30.5V into 8 ohms (116W output) the line out is now at 1.28VRMShttps://irationaudio.com/wp-content/uploads/2015/10/img_0052.jpgIMG_0052I was wrangling the line out which had been somehow manipulated (added?) at some point in the past. When you view it on a scope it looks pretty bad. The output of the amp is in yellow and the line out is in blue. As you can see it is very distorted from the original signal, and only about 200mV RMS.2015-10-20T06:32:43+00:00monthlyhttps://irationaudio.com/2015/10/17/fender-super-bassman/https://irationaudio.com/wp-content/uploads/2015/10/whos-the-winner.jpgWhos the winnerFender Super Bassman in the lab. Kind of an interesting Bassman because the B+ is only about 400V. Typical Bassmans are around 450-480V. Because the B+ is so low it only does about 70W at clipping. Can be kind of misleading at first since most Bassmans with four power tubes do around 100W.https://irationaudio.com/wp-content/uploads/2015/10/fender_bassman-super_cfa7002_sch-2.jpgfender_bassman-super_cfa7002_sch-2Fender Super Bassman layouthttps://irationaudio.com/wp-content/uploads/2015/10/fender_bassman-super_cfa7002_sch-1.jpgfender_bassman-super_cfa7002_sch-1Fender Super Bassman schematichttps://irationaudio.com/wp-content/uploads/2015/10/img_2295.jpgIMG_2295Big chassis with not much in it. Measuring bias with the transformer shunt method. Doing it up with a recap so it will be reliable for an active musician. Power tubes are 70s RCA 6L6GC and they perform just as well as new power tubes.https://irationaudio.com/wp-content/uploads/2015/10/mg_2362.jpg_MG_2362Huge head enclosure that is almost empty. Wonder why this was the trend of the time??2015-10-18T18:16:15+00:00monthlyhttps://irationaudio.com/2015/09/24/fender-bassman-ten/2015-10-12T16:12:39+00:00monthlyhttps://irationaudio.com/2015/07/26/fender-hot-rod-deville/https://irationaudio.com/wp-content/uploads/2015/07/fender-hot-rod-deville-2layout.jpgMicrosoft Word - HRDeville.docFender Hot Rod Deville layouthttps://irationaudio.com/wp-content/uploads/2015/07/fender-hot-rod-deville-schematic-11.jpgMicrosoft Word - HRDeville.dochttps://irationaudio.com/wp-content/uploads/2015/07/fender-hot-rod-deville-schematic-1.jpgMicrosoft Word - HRDeville.docFender Hot Rod Deville schematichttps://irationaudio.com/wp-content/uploads/2015/07/img_2168.jpgIMG_2168Fender Hot Rod Deville. This one had been through the shit. Power tubes were still functioning but were totally rattley. No doubt elements in the tube rattled loose from the vibration of the speakers. Lots of screws in the enclosure and screws holding the speakers were also loose, adding to the terrible rattley, distorted sound. Reverb tank didn't work and needed to to be replaced. https://irationaudio.com/wp-content/uploads/2015/07/mg_2166.jpg_MG_2166Whoops out of focus shot. New camera guess I didn't use it well in the beginning. If you don't have to pull the board you are lucky because it is time consuming. All knobs and nuts that hold the pots in place must be removed to get the board out. Always good to search for poor solder connections in these. You may find some. 2015-10-11T07:31:38+00:00monthlyhttps://irationaudio.com/2015/09/06/sunn-model-t-2/https://irationaudio.com/wp-content/uploads/2015/09/img_0347.jpgIMG_0347Super clean Sunn Model T on the bench and although I've posted the classic Model T before I had to post this one because it' so damn clean and "originial," a word I usually dislike. Seems like a lot of times the metal plate for the front panel gets bent from the amp being taken in and out of the chassis for servicing. This one is so perfect I wonder if it has ever even been wrangled from the chassis. https://irationaudio.com/wp-content/uploads/2015/09/img_0341.jpgIMG_0341Little work bench shot. The other Model T to the left looks like a junkyard dog compared to this one. https://irationaudio.com/wp-content/uploads/2015/09/img_0340.jpgIMG_0340Kind of a museum piece. Old caps and transformers date it to 1973. Sadly the streak of original is over though. A power tube died and took out the plate and screen grid resistors with it. You can see they are replaced with the heat sink style 10W resistors where my multimeter lead is clipped on the tube socket. Other than that this thing is totally original. it's not even dusty inside, because the owner has the Sunn slip cover that came with the amp.2015-10-11T07:27:51+00:00monthlyhttps://irationaudio.com/2015/09/25/peavey-butcher/https://irationaudio.com/wp-content/uploads/2015/09/peavey-butcher-schematic-1.jpgBUTCHER.pdfPeavey Butcher schematichttps://irationaudio.com/wp-content/uploads/2015/09/peavey-butcher-layout.jpgBUTCHER.pdfPeavey Butcher layouthttps://irationaudio.com/wp-content/uploads/2015/09/mg_2272.jpg_MG_2272It's rocking timehttps://irationaudio.com/wp-content/uploads/2015/09/img_2266.jpgIMG_2266Really modular like a lot of Peavey designs seem to be. Main board in the middle is the preamp and phase inverter. Board behind it in the back is for the output section and the PCB on the left has power supply and bias supply. There is no trimpot for bias supply so you either need to install one or adjust the R52, R53 voltage divider ratio.https://irationaudio.com/wp-content/uploads/2015/09/img_2263.jpgIMG_2263Peavey Butcher, the lower gain predecessor of the VTM 60/120. The Butcher logo is extremely metal, but it does more rock-style tones. It's all good it's from the mid 80s or so.2015-10-11T07:23:20+00:00monthlyhttps://irationaudio.com/2015/03/26/fender-vibro-champ/https://irationaudio.com/wp-content/uploads/2015/03/fender-vibro-champ-page-2.jpgFender Vibro Champ Page 2Fender Vibro Champ Amp AA764 layouthttps://irationaudio.com/wp-content/uploads/2015/03/fender-vibro-champ-page-1.jpgFender Vibro Champ Page 1Fender Vibro Champ Amp AA764 schematic https://irationaudio.com/wp-content/uploads/2015/03/img_0122.jpgIMG_0122Fender Vibro Champ Amp from 1969. I’m sure true aficianados can chime in but I believe this style/era with the aluminum trim that goes around the grill cloth started in 1967 and ended sometime in 1969.https://irationaudio.com/wp-content/uploads/2015/03/img_0090.jpgIMG_0090https://irationaudio.com/wp-content/uploads/2015/03/img_0083.jpgIMG_0083This is the cathode bypass cap and cathode resistor for the 6V6. 470 ohm resistor looks a little darkened from heat stress and measures 517 ohms. The serial number on the cap date it to 1969. 235-6907KB. 235 (Mallory manufacturing number) 6907 (07th week of 1969)https://irationaudio.com/wp-content/uploads/2015/03/img_0081.jpgIMG_0081This amp was biased so hot in the stock setting that it unfortunately had a lot of distortion and didn't really have much clean volume. After adjusting the cathode resistor it sounded much better.https://irationaudio.com/wp-content/uploads/2015/03/img_0079.jpgIMG_00792015-10-03T16:01:35+00:00monthlyhttps://irationaudio.com/2015/03/27/airline-62-9025a-gretsch-6152-supro-6422/https://irationaudio.com/wp-content/uploads/2015/03/gretsch-6152-catalog-page1.jpgGretsch 6152 catalog page1Gretsch 6152 catalog pagehttps://irationaudio.com/wp-content/uploads/2015/03/gretsch6152-airline-62-9025a.pnggretsch6152 airline 62 9025ASchematic for Airline 62-9025A / Gretsch 6152 / Supro 6422https://irationaudio.com/wp-content/uploads/2015/03/img_0930.jpgIMG_0930This Airline 62-9025A was manufactured by Valco in Chicago in the 60s. From my understanding it is the same circuit build as the Gretsch 6152 and the Supro 6422 amps which are single ended class A amps with tremolo and reverb. It has been living ruff n rugged.https://irationaudio.com/wp-content/uploads/2015/03/img_0861.jpgIMG_0861More old Mallory caps for easy approximate date code referencing 235-6605 indicating capacitor manufacture date during the 5th week of 1966.https://irationaudio.com/wp-content/uploads/2015/03/img_0860.jpgIMG_0860A capacitor in the power supply was disconnected, so the circuit was open, so the amp would not turn on. Aged electrolytics were replaced, old weak or noisy tubes were replaced and after biasing it sounded nice. Tremolo was great, reverb did not work and unfortunately was left unrepaired. The cardboard reverb tank is glued to the chassis and it was crumbling when I tried to remove it. No circuit board in this beast. All components are mounted to terminal strips, tube sockets and potentiometers.2015-10-01T17:25:04+00:00monthlyhttps://irationaudio.com/2015/03/27/fender-quad-reverb/https://irationaudio.com/wp-content/uploads/2015/03/fender-twin_reverb_sf_100_schem.jpgFender twin_reverb_sf_100_schemFender Twin Reverb, Quad Reverb, Super Six schematichttps://irationaudio.com/wp-content/uploads/2015/03/fender-quad-reverb-ad.jpgFender Quad Reverb adFender Quad Reverb. Info straight from the horse's mouth. Heavy 4x12 combo. Not as heavy as the Super Six though, which was the 6x12 Twin Reverb combo. I believe this amp was available from 72-79. Early versions were 100W and later versions had an ultra linear output transformer capable of 135W. Not sure what year this changeover took place, however.https://irationaudio.com/wp-content/uploads/2015/03/img_1299.jpgIMG_1299The realhttps://irationaudio.com/wp-content/uploads/2015/03/img_1296.jpgIMG_1296Well, kinda poor lighting but I guess if you've seen one you've pretty much seen them all. Amp had cracked solder connections making one channel quieter than the other. Also the screen grid resistors were 100,000 ohms instead of.............470. Output power in general was weakhttps://irationaudio.com/wp-content/uploads/2015/03/img_1292.jpgIMG_1292After repair doing 113W at clipping into 8 ohms. Pretty beastly. Not what I usually think of when I think "Fender."2015-10-01T17:20:14+00:00monthlyhttps://irationaudio.com/2015/03/27/sovtek-mig-60/https://irationaudio.com/wp-content/uploads/2015/03/sovtek-mig-60-schematic.jpg*GothamSovtek MIG 60 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1254.jpgIMG_1254Cool Sovtek. Sounds gnarly. Wish I got to play it cranked.https://irationaudio.com/wp-content/uploads/2015/03/img_1252.jpgIMG_1252Transformers are shielded to limit hum, but also there are gaps in the shielding so I don't know how well this will work. If you look at the Ampeg B-15 the shields around the transformers are one solid piece with no gaps. I believe this is the ideal https://irationaudio.com/wp-content/uploads/2015/03/img_1227.jpgIMG_1227Nothing special about this repair. Just needed new power tubes. Mostly just posting to propagate schematics.2015-10-01T17:16:51+00:00monthlyhttps://irationaudio.com/2015/03/30/sunn-concert-slave/https://irationaudio.com/wp-content/uploads/2015/03/img_1365.jpgIMG_1365Sunn Concert Slave from early 70s. Same output section as the Concert Lead and Concert Bass. Not sure if it was the same as the Concert Controller, which was the PA version. https://irationaudio.com/wp-content/uploads/2015/03/img_1353.jpgIMG_1353Not too much going on here, just needed a few new caps and a broken AC power cord replaced. Never found the schematic, but you could just consult the output section of one of the other Concert series amps of this era.2015-10-01T00:19:58+00:00monthlyhttps://irationaudio.com/2015/03/30/sovtek-mig-50h/https://irationaudio.com/wp-content/uploads/2015/03/sovtek-mig-50h-schematic.gifSovtek mig-50h SchematicSovtek MIG 50H schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1732.jpgIMG_1732Actually a nicer build in this MIG-50H compared to the MIG-50 I'd say because it has actual traditional style 1/4" solder lug pots for the volume and tone controls that are attached to the front panel and are much sturdier than the crappy plastic PCB mount 1/8" pots in the MIG-50 that always break and then you need to buy new knobs too. Wonder if that's original or if a previous tech installed those. Definitely a step up. This MIG-50H has metal on/off and standby switches too, nice. Too bad the input jacks and speakers jacks are still the cheap plastic cliff-hanger type though.https://irationaudio.com/wp-content/uploads/2015/03/img_1730.jpgIMG_1730Cool, tiny little amp, Sovtek MIG-50H. The high gain version of the MIG-50. As if the MIG-50 was not already a very small amp this is squashed together and is even smaller. Looks like the knobs have been replaced with Marshall-style ones on this piece.2015-09-30T23:19:36+00:00monthlyhttps://irationaudio.com/2015/03/31/fender-bassman-50/https://irationaudio.com/wp-content/uploads/2015/03/fender_bassman50.jpgfender_bassman50Fender Bassman 50 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1973.jpgIMG_1973Chassis image and amp is all original and still works, just needs new power tubes. Recap could be nice for possible reliability but customer decided to let the good times roll. The trimpot more easily visible is a balance adjust to balance current draw for the two power tubes. A bias trimpot was installed right next to it to dial in the new tubes. The 470 ohm screen grid resistors measured about 510 ohms and 890 ohms and were replaced. No longer original oh well. At least it works better now. https://irationaudio.com/wp-content/uploads/2015/03/img_1962.jpgIMG_1962Silverface Bassman 50 from 1974 I believe. Same era amp as the Twin Reverb in the previous post. This amp has a slightly different phase inverter design compared to the earlier era Bassmans. There are a few other variations I'm sure but I'm not all that familiar with them.2015-09-30T23:11:36+00:00monthlyhttps://irationaudio.com/2015/04/17/menatone-king-of-the-britains/https://irationaudio.com/wp-content/uploads/2015/04/img_2332.jpgIMG_2332Ok time to look for things that got too much voltage. Everything looks normal. Ok, maybe plug it in again and probe around. No voltages on any of the IC leads. Ok that won't work. It is a TC1044SCPA. Datasheet says it is a charge pump DC to DC voltage converter, basically just inverting the 9VDC from adapter to -9V for some section of the circuit. It operates with an input voltage anywhere between 1.5-12VDC and it's maximum input voltage rating is 13VDC! Ok so this is probably what burned out after plugging it in for a few seconds. All the caps were fortunately 25V rating so they survived the 18V mistake. I broke a 1N127 glass diode in my fumbling and had to replace that as well. I struggled trying to find one for a little and eventually found that a 1N3070TR is an equivalent. Nothing else was damaged so it is all back in action. Fortunately pretty painless troubleshoot. Look how they got those old carbon comp resistors i in there. Wonder why? http://www.geofex.com/article_folders/carbon_comp/carboncomp.htmhttps://irationaudio.com/wp-content/uploads/2015/04/img_2334.jpgIMG_2334Yay I broke something too. I never document pedal repairs for some reason so I guess now's the time. I played this pedal a few years ago and thought it ruled. I never got one however because it's totally expensive. I saw one used for cheap and impulsively bought it, and broke it in the first 10 seconds. I used a DC adapter that plugs into the wall but it was a variable supply one. You can choose between 3, 4.5, 6, 7.5, 9 and 12 volts. I had it on 12V by accident. Doesn't seem too bad. Unfortunately, by reading the voltages with a meter the actual outputs are 5, 6.75, 8.75, 10.85, 12.9, and 18V. I fried my pedal by giving it double voltage with this cheap adapter damn.2015-09-30T22:58:21+00:00monthlyhttps://irationaudio.com/2015/05/03/matamp-200/https://irationaudio.com/wp-content/uploads/2015/05/img_2161.jpgIMG_2161Hard to know much about these Matamps since there's no schematics available and there's also so much variation between builds. From what I could gather and test this amp is capable of about 120W max with four EL34s. It starts to break up around 75W. The bass and mid controls have a minimal effect. It was biased extremely cold and sounded really grainy and weak, but when put back in a normal region it could get really fuzzy and blown out sounding.https://irationaudio.com/wp-content/uploads/2015/05/img_2157.jpgIMG_2157Stacked PCBs mean you gotta dive in if there's a problem. https://irationaudio.com/wp-content/uploads/2015/05/img_2154.jpgIMG_2154Only two12AX7 tubes in this amp. It has one preamp tube and phase a inverter, that's it before the output section. There is more info here on this amp. This one seems to be the same or very similar -- http://www.planetoftheamps.com/matamp-bass120.html2015-09-30T22:55:44+00:00monthlyhttps://irationaudio.com/2015/06/13/ampeg-svt-3-pro/https://irationaudio.com/wp-content/uploads/2015/06/img_2299.jpgIMG_2299Compared to Ampeg SVT III the SVT 3 Pro has switched to a MOSFET output section with an IRFP9240/IRFP240 combo. Same as Ampeg SVT 4, but smaller since this amp does about 380W at clipping and the SVT 4 is more poweful. Amp B2R also has a very similar output section. A lot of these 90s Ampeg SS output sections are similar, just bigger or smaller depending on the power handing of the amp. Schematic for Ampeg SVT 3 Pro -- https://drive.google.com/folderview?id=0B01EXvY0__YYU0kySjZwOERxeDA&usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/06/img_2214.jpgIMG_2214Ampeg SVT 3 Pro2015-09-30T22:44:54+00:00monthlyhttps://irationaudio.com/2015/07/26/fender-blues-deluxe-reissue/https://irationaudio.com/wp-content/uploads/2015/07/fender-bluesdeluxe_reissue_2004_schematic_rev-a-1.jpgMicrosoft Word - Blues Deluxe Reisu_Svc Man.docFender Blues Deluxe reissue schematic. The Blues Deluxe and Blues Devile reissue schematics are merged. Here is the full schematic https://drive.google.com/file/d/0B01EXvY0__YYM3FqeWtnWGxpRWM/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/07/img_2741.jpgIMG_2741Fender Blues Deluxe reissue from mid 90s. My chassis shot was all blurry but it's ok these amps are all the same inside, Blues Deville, Blues Deluxe, Hot Rod Deville, etc. The 9-pin Fender style input jacks are expensive, so hopefully they are not broken. Touch up the solder on the tube sockets for reliability. This one does not have a bias pot for some reason. A lot of the other similar models do. You either have to use tubes that bias nice by their current draw, or adjust the resistor in the bias supply or install a little trimpot.2015-09-30T22:40:50+00:00monthlyhttps://irationaudio.com/2015/09/26/peavey-vtm-120/https://irationaudio.com/wp-content/uploads/2015/09/peavy_vtm_120-2.jpgpeavy_vtm_120-2Peavey VTM 120 schematichttps://irationaudio.com/wp-content/uploads/2015/09/peavy_vtm_120-1.jpgpeavy_vtm_120-1Peavey VTM 120 layouthttps://irationaudio.com/wp-content/uploads/2015/09/img_0080.jpgIMG_0080Well, very similar to the Butcher but one more 12AX7 in the preamp for an extra gain stage to meet the rock demands of the time. So modular. Pretty easy to work on. No problems here other than needing new power tubes.https://irationaudio.com/wp-content/uploads/2015/09/img_0081.jpgIMG_0081Peavey VTM120 in an Emperor custom head case. Looks pretty awesome. Has a handful of dip switches on the front panel to control how much gain and high and low boosts.2015-09-30T22:23:43+00:00monthlyhttps://irationaudio.com/2015/07/26/mesa-boogie-bass-400/https://irationaudio.com/wp-content/uploads/2015/07/img_0049.jpgIMG_0049Mesa Boogie Bass 400+. Pretty beastly amp with 12 6L6GCs in the output section, doing about 270W. There is another version of this amp with a 6.8K EQ slider, this one doesn't have it. Old school.https://irationaudio.com/wp-content/uploads/2015/07/img_2552.jpgIMG_2552Basically a huge power supply and output section. 6 6L6GCs per side in parallel in push/pull. Buss wire across the whole side with screen grid and control grid resistors from buss wire to tube socket. This one had two shorted diodes in the rectifier so it would blow fuses. The fuse holder was replaced at some point and had excess wire hanging off right next to the chassis. Not sure if this had something to do with it also. Never found a schematic for this. If you have one please share.2015-09-26T02:29:37+00:00monthlyhttps://irationaudio.com/2015/07/07/avo-model-8-mk2/https://irationaudio.com/wp-content/uploads/2015/07/avomk2schematic.jpgAVOmk2schematicAVO 8 MK2 schematic AVO MK1 and MK2 userhttps://irationaudio.com/wp-content/uploads/2015/07/img_0038.jpgIMG_0038No way I could photograph this thing and relay it in all its glory. Anyway you get the idea. It goes without saying, but "they don't make 'em like that anymore." Look at the bottom board, wrapped in wire. I believe those are wire wound "precision" resistors, the precision of the resistor based on how long the length of wire is.https://irationaudio.com/wp-content/uploads/2015/07/img_0034.jpgIMG_0034This thing is ACCURATE even though it's about 60 years old or older. The robustness cannot be overstated. This thing is a brick, and weights probably almost as much as one. Visit the fanboy site here and become one -- http://www.richardsradios.co.uk/AVOmultimeters.htmlhttps://irationaudio.com/wp-content/uploads/2015/07/img_0032.jpgIMG_0032Yay another incredible relic from the past, made in UK sometime between 1956 and 1964 -- AVO Model 8 MK2. AVO stands for Ammeter-Voltmeter-Ohmeter. AC and DC ranges operate on opposite knobs. When a knob is set for AC or DC the meter is connected and working. If neither knobs are set to either AC or DC the meter is disconnected, protecting you from damaging the meter or getting inaccurate measurements by having it on the wrong setting. Has individual knobs to zero out the meter on the resistance setting. I have owned Simpson 260s and Triplett 630s, usually called "the best" analog VOMs ever made, and I gotta say I think the AVO is actually built better and is cooler. 2015-09-07T06:43:32+00:00monthlyhttps://irationaudio.com/2015/03/30/sunn-model-t/https://irationaudio.com/wp-content/uploads/2015/03/sunn_model_t.jpgsunn_model_tSunn Model T schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1801.jpgIMG_1801Back panel shot and although you can't see it in the pic, the impedance selector switch for this amp has selectable 2, 4, 8 AND 16 ohm settings. Can really do it all and I can't recall another amp that has all 4 of those ranges selectable.https://irationaudio.com/wp-content/uploads/2015/03/img_1800.jpgIMG_1800Re-housed 1st generation Sunn Model T from 1973. This is one of the loudest guitar amps ever. It uses an ultralinear output transformer because the output section is modeled after a Dynaco Mark III, meaning that the screen grid supply is fed through the output transformer as well as the B+ voltage -- the output transformer is a dual primary, one for the B+ and the other for the screen grid. Because the screen grid supply and the B+ come from the same power supply node their voltage fluctuates up and down under load together. This connection provides some feedback between the anode and screen grid and has been found to to act in a way that is between pure pentode mode and pure triode mode, with lower distortion than either.https://irationaudio.com/wp-content/uploads/2015/03/img_1739.jpgIMG_1739Chassis shot complete with Stave TV autograph from when they presumably serviced this amp in 1981 (or is it 1987?). The transformers towards the bottom are two paralleled chokes used in the power supply for decoupling and as you can see this amp has been recapped sometime in the recent past with F&T caps. Also has dual bias pots for each side of the push/pull section so you can bias as well as balance each set of 2 power tubes.2015-09-06T16:44:15+00:00monthlyhttps://irationaudio.com/2015/03/30/simpson-260-vom/https://irationaudio.com/wp-content/uploads/2015/03/img_1805.jpgIMG_1805Can't get enough vintage test equipment. I have been on a buying spree of old, old meters that may or may not work. I've been buying the cheapest ones on eBay. The one on the right is from eBay but actually the one on the left was given to me by me uncle and it works very nice. Both are Simpson 260 series 7 analog VOMs. The one of the left is the 7P version. The main difference I can see is that it has a push button reset switch in case of overload rather than a fuse. The meter on the right is a series 7M which has fused overload protection and the nice feature of a mirrored panel display to eliminate parallax error. Allows you to view the needle and interpret the reading on the display accurately from an angle. I am making a rough estimate but I think the Simpson 260 Series 7 was made between about 1975 and 1985 in Elgin, Illinois.https://irationaudio.com/wp-content/uploads/2015/03/img_1797.jpgIMG_17977P on the left and 7M on the right and as you might expect they are not much different. The meter on the left I received working and basically accurate, but the meter on the right needed a little work. The battery terminals were corroded. These need to make good contact with the batteries or the resistance readings will be off. You have to either clean them or hardwire the batteries. It was also out of cal. Not much of a hassle though since these meters have trimpots to adjust the calibration.https://irationaudio.com/wp-content/uploads/2015/03/img_1796.jpgIMG_1796All you need is a voltage and current source and you can dial it in compared to a known calibrated meter. Ohms range uses batteries to put a voltage through the resistance under test to judge its value. A 1.5V battery for the lower resistance settings and a 9V for R x 10,000 scale. As long as the resistors in the ohms range circuit are in cal and the batteries are good, the meter will be in cal. In the image the 260 is reading 433VDC spot on nice. Much better pics and info for all the Simpson 260 meters and more here -- http://simpson260.com/ Simpson 260 calibration procedure -- https://drive.google.com/file/d/0B01EXvY0__YYcVdlanBqR1E5ZlU/view?usp=sharing Simpson 260 3 to 6 repair instructions -- https://drive.google.com/file/d/0B01EXvY0__YYclQ1aTByR3ZFVHM/view?usp=sharing2015-08-10T06:33:50+00:00monthlyhttps://irationaudio.com/2015/03/31/peavey-firebass-700-2/2015-07-27T00:58:46+00:00monthlyhttps://irationaudio.com/2015/03/26/sovtek-mig-50/https://irationaudio.com/wp-content/uploads/2015/03/sovtek-mig-50.jpgSovtek Mig 50Sovtek MIG 50 schematic Schematic posted is master volume version however the amp pictured does not have the master volume option2015-07-27T00:43:07+00:00monthlyhttps://irationaudio.com/2015/03/31/23/https://irationaudio.com/wp-content/uploads/2015/03/img_2013.jpgIMG_2013This amp had a broken footswitch but the main problem was it would blow fuses. That's because of the .001/2000V from EL34 anode to ground was shorted. Not sure exactly why this happens, I guess voltage spikes from the output transformer, but I have seen this cap short before. Seems somewhat common. I believe my anecdote has some relevance because I've asked around.https://irationaudio.com/wp-content/uploads/2015/03/musicman-hd130-w-12ax7-phase-inverter.gifMusicman HD130 (w: 12ax7 phase inverter)Musicaman HD130 schematic (tube phase inverter version)https://irationaudio.com/wp-content/uploads/2015/03/img_2023.jpgIMG_2023solid state preamp drives the 12AX7 phase inverter. Not sure what year it switched over but I know the earlier versions had the 12AX7 phase inverter and the later ones had a solid state one. Bias trimpot is visible right up front between the two big 620 ohm/2W resistors https://irationaudio.com/wp-content/uploads/2015/03/img_2021.jpgIMG_2021https://irationaudio.com/wp-content/uploads/2015/03/img_1979.jpgIMG_1979Got to be one of the smallest, loudest amps available. Actually it's not available because the reissues coming out now do not reproduce the 2x10 version. It does just barely under 120W at clipping. Tremolo sounds amazing, especially when you go past 5 in speed. Wonder if the new reissues HD130s are the same amp. The plate voltage in this is among the highest I've ever encountered, about 720V or so, so I kind of doubt they would reissue that kind of high voltage. But who knows.2015-07-27T00:16:40+00:00monthlyhttps://irationaudio.com/2015/07/26/fender-super-six/https://irationaudio.com/wp-content/uploads/2015/07/img_1721.jpgIMG_17216X10 combo, gotta be one of the biggest combos ever. Had 2 shorted power tubes and needed a recap. Sang nicely after that. You can drown in reverb and it's still bassy.https://irationaudio.com/wp-content/uploads/2015/07/img_1718.jpgIMG_1718Yay, Fender Super Six. This thing sounds awesome and even though the back is open it has such good bass response. 2015-07-26T19:01:01+00:00monthlyhttps://irationaudio.com/2015/03/25/gibson-g10/https://irationaudio.com/wp-content/uploads/2015/04/gibson-g10g20g30.jpgGibson G10,G20,G30Gibson G10, G20, G30 schematichttps://irationaudio.com/wp-content/uploads/2015/04/gibson-g10-parts-list.jpgGibson G10 Parts ListGibson G10 parts listhttps://irationaudio.com/wp-content/uploads/2015/04/img_1032.jpgIMG_1032GIbson G10. This was a failed repair sadly. This amp had poor sound and a buzz. I went on an insane quest to find a MPF161 transistor and eventually found a replacement and the amp sounded very clean with extremely low noise floor! It lasted only 2 weeks and buzz came back, and its origin was never found unfortunately.https://irationaudio.com/wp-content/uploads/2015/04/img_1011.jpgIMG_1011It is a solid state amp from the early 70s or so. Like most of these old SS amps of this era the output transistor is a 2N3055 -- the standard, seemingly.2015-07-26T18:00:55+00:00monthlyhttps://irationaudio.com/2015/06/04/acoustic-370/https://irationaudio.com/wp-content/uploads/2015/06/img_2420.jpgIMG_2420Rogue 370https://irationaudio.com/wp-content/uploads/2015/06/img_2404.jpgIMG_2404Yay https://irationaudio.com/wp-content/uploads/2015/06/img_2401.jpgIMG_2401If you want to trace the circuit and find where things go wrong it's kind of hard because the solder traces are on one side of the PCB and the components are on the other. Very hard to see both at once and you can't flip the board over and over with the amp on while probing the circuit!!! You can prop up the PCB and shine a light behind it to make both visible, then it gets easier. Make sure your prop is good and sturdy so nothing shorts to the chassis! I was led to various cracked solder connections and a nicer sine was obtainedhttps://irationaudio.com/wp-content/uploads/2015/06/img_2399.jpgIMG_2399It would not turn on. There was a break in the AC cord so that was solved but the output still looked nasty.2015-06-18T20:42:21+00:00monthlyhttps://irationaudio.com/2015/03/31/marshall-jmp-1959-2203/https://irationaudio.com/wp-content/uploads/2015/03/marshall-jmp-2203-1959.gifMarshall JMP 2203 1959Marshall JMP 1959 2203 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_2108.jpgIMG_2108Marshall JMP. 100W version with 6550s and master volume. I think this is the most preferred rock amp that preceded the higher gain JCM800. It is really fun to play lead guitar on. This one is from 1979.https://irationaudio.com/wp-content/uploads/2015/03/img_2102.jpgIMG_2102Seems like these amps are known to squeal when the volume and tone knobs are cranked really high. In my limited experience a 1000pF/.001uF cap across V2A's plate resistor will rid that. V1A's 68K control grid resistor is mounted on the PCB with a shielded cable going to the grid. I could still hear some radio coming through with volume cranked so I tried moving the grid resistor to the tube socket with good results.https://irationaudio.com/wp-content/uploads/2015/03/img_2100.jpgIMG_2100100W Master Volumehttps://irationaudio.com/wp-content/uploads/2015/03/img_2098.jpgIMG_2098JJ 50uF/50uF/500V caps fit perfect for recap. Don't even need new brackets2015-06-16T08:00:37+00:00monthlyhttps://irationaudio.com/2015/03/31/fender-twin-reverb/https://irationaudio.com/wp-content/uploads/2015/03/img_1789.jpgIMG_1789Terrible picture sorry for that. This amp has been totally recapped by someone else which is awesome. Works very nice and just needed a new speaker to replace its blown one.https://irationaudio.com/wp-content/uploads/2015/03/img_1840.jpgIMG_1840Nice silver face twin reverb that has the master volume. It is the 100W version not the one with the ultralinear output transformer that is 135W. The amp has master volume so the idea is you can get distortion out of it. The master volume works for both channels but the 2nd channel has more gain so you can try to get a more aggressive tone.2015-06-16T08:00:11+00:00monthlyhttps://irationaudio.com/2015/03/31/mesa-boogie-dual-rectifier-solo-head/https://irationaudio.com/wp-content/uploads/2015/03/mesa-boogie-dual-rectifier-solo-head-amplifier-schematic-3.jpgmesa-boogie-dual-rectifier-solo-head-amplifier-schematic-3Mesa Boogie Dual Rectifier Solo Head power amphttps://irationaudio.com/wp-content/uploads/2015/03/mesa-boogie-dual-rectifier-solo-head-amplifier-schematic-1.jpgmesa-boogie-dual-rectifier-solo-head-amplifier-schematic-1Mesa Boogie Dual Rectifier Solo Head preamphttps://irationaudio.com/wp-content/uploads/2015/03/img_2180.jpgIMG_2180https://irationaudio.com/wp-content/uploads/2015/03/img_2177.jpgIMG_2177I like how the control and screen grid resistors for the power tubes are wired in right between the tube socket and a terminal strip, not PCB mounted.https://irationaudio.com/wp-content/uploads/2015/03/img_2174.jpgIMG_2174This is the old two channel Dual Rectifier. It sounds really awesome, and pretty different from the three channel ones. I have heard people mention this before, so it was cool to experience.2015-06-16T08:00:04+00:00monthlyhttps://irationaudio.com/2015/03/31/kenwood-kr-5400/https://irationaudio.com/wp-content/uploads/2015/04/img_2041.jpgIMG_2041Kenwood KR-5400https://irationaudio.com/wp-content/uploads/2015/04/img_2033.jpgIMG_2033Big 10,000uF power supply caps watch out! Not exactly sure how old this stereo is but it was still working pristinely other than a few AUX and PHONO ins which were cutting in and out. And with service info available should problems arise and you have to dive in further, Kenwood wished this unit to be resurrected. Kenwood KR-5400, KR-4400, KA-4006 Service Manual Supplement -- https://drive.google.com/file/d/0B01EXvY0__YYY1VfTzRqRXhmVWM/view?usp=sharing2015-06-16T07:59:24+00:00monthlyhttps://irationaudio.com/2015/03/30/orange-dual-terror/https://irationaudio.com/wp-content/uploads/2015/03/img_1738.jpgIMG_1738Re-assembled. Could never find the schematic on this one.https://irationaudio.com/wp-content/uploads/2015/03/img_1736.jpgIMG_1736Doing a very symetrical ~44V pk-pk into 8 ohms for 30W power.https://irationaudio.com/wp-content/uploads/2015/03/img_1734.jpgIMG_1734Orange Dual Terror just had a broken input jack. This amp is switch-able for 7, 15, or 30W and it sounds pretty nice. As far as I know, it is basically just the big brother of the Tiny Terror, having two channels rather than one and having four EL84s in the output section for 30W instead of 15W for the Tiny Terror, which only has two EL84shttps://irationaudio.com/wp-content/uploads/2015/03/img_1713.jpgIMG_1713Unfortunately you have to disassemble the amp completely including removing the tubes to work on it since most of the solder connections are on the underside of the board. There is one single PCB that has the whole amp on it including the tube sockets. The on/off switch and the speaker jacks need to be removed to make enough room to lift the PCB out.2015-06-16T07:58:21+00:00monthlyhttps://irationaudio.com/2015/03/27/heathkit-ig-72-audio-generator/https://irationaudio.com/wp-content/uploads/2015/03/heathkit-ig_72.gifHeathkit IG_72Heathkit IG-72 schematic Heathkit IG 72 manualhttps://irationaudio.com/wp-content/uploads/2015/03/img_1332.jpgIMG_1332This is a Heathkit IG-72 tube audio generator. It can do sine waves only. I also only has a handful of parts so it's easy enough to service. The radio museum site has it listed here, in production from 1962 up until about 1977 -- http://www.radiomuseum.org/r/heath_audio_generator_ig_72_ig7.html https://irationaudio.com/wp-content/uploads/2015/03/img_1331.jpgIMG_1331After replacing the pre-tube and calibrating things got symmetricalhttps://irationaudio.com/wp-content/uploads/2015/03/img_1329.jpgIMG_1329From left to right it has a 6CL6 power tube and a 6AU6 for the oscillator. The tube on the right is the rectifier, which is a 6X4. https://irationaudio.com/wp-content/uploads/2015/03/img_1328.jpgIMG_1328I expected to replace the old caps but really it was tubes and calibration that was causing the poor output.https://irationaudio.com/wp-content/uploads/2015/03/img_1326.jpgIMG_1326Unfortunately it could not make a good sine wave2015-06-16T07:57:40+00:00monthlyhttps://irationaudio.com/2015/03/27/ampeg-vt-22/https://irationaudio.com/wp-content/uploads/2015/03/vt22-v4-74.gifvt22-v4-74Ampeg VT-22 schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_15021.jpgIMG_1502https://irationaudio.com/wp-content/uploads/2015/03/img_1211.jpgIMG_1211Ampeg VT-22, combo version of the V4. As if that was not heavy enough. I've never worked on a master volume version so it was cool to see it in person. These are from 76-79 but are not really any different circuit-wise from the non-master volume version that came before it.https://irationaudio.com/wp-content/uploads/2015/03/img_1207.jpgIMG_1207Rugged VT-22 survived the crash with only 1 broken preamp tube and one missing. Worst problem the amp had was that EL34s were installed but it wasn't done right. Original V4/VT-22 power tubes are 7027s but the socket is really wired for a 6L6. 6550s and KT-88s can work too but it can be tricky. V4/VT-22 wiring leaves pin 1 on the tube socket open because there is no connection on pin 1 for 6L6/6550/KT-88 (7027 has pin 1 tied to pin 4, screen grid), but an EL34 has a suppressor grid on pin 1 which must be tied to ground. An EL34, like the 6L6/6550/etc, is a pentode and with the suppressor grid left floating it is only operating in tetrode mode. The amp had toasted screen grid resistors and would only put out 75W at clipping. Too much current flowing in the screen. 1K screen grid resistors were replaced with 1.5K/5W and pin 1 was tied to pin 8 (ground) and the amp put out 105W as it should. Great job and thank you, suppressor grids, for re-routing that secondary emission away from the screen and back towards the anode.https://irationaudio.com/wp-content/uploads/2015/03/img_1205.jpgIMG_1205Different color PCB (early version was green on top and bottom) and maybe a few component values are different but that's about it. This amp needed a once-over after being in a bad car crash. 2015-06-16T07:55:31+00:00monthlyhttps://irationaudio.com/2015/03/27/wurlitzer-4100a/https://irationaudio.com/wp-content/uploads/2015/03/wurlitzer-schematic.jpgwurlitzer schematicWurlitzer 4100A schematichttps://irationaudio.com/wp-content/uploads/2015/03/img_1090.jpgIMG_10904100A. It is a lot louder now and has a fuller sound with more bass. So if you leave your beer on top you can now rattle it off the side onto the floor, spilling it everywhere.https://irationaudio.com/wp-content/uploads/2015/03/img_1084.jpgIMG_1084Close up of output section and power supply. Massive power transformer supplies current for almost 40 tubes.https://irationaudio.com/wp-content/uploads/2015/03/img_1086.jpgIMG_1086With the back panel in the normal upright position. It has 28 12FQ8s in the tone generator circuit. Above are dozens of brown .22uF/4000V coupling capacitors for all the tubes. According to Bogus Magnus "this is Wurlitzer's first electronic spinet organ using vacuum tubes in oscillator circuits for tone generation and also divider circuits." Very nice. To the right of the tone generator is the power supply and output section with their respective tubes facing upright. On the left a 5U4GB rectifier tube and to the right two 6L6s in push-pull for the output section. Beneath those are two 12AX7s. I would guess they are a driver and phase inverter for the 6L6s.https://irationaudio.com/wp-content/uploads/2015/03/img_1078.jpgIMG_1078Tone generator maze of even more coupling caps and an old Utah "special design for Wurlitzer" speakerhttps://irationaudio.com/wp-content/uploads/2015/03/img_1074.jpgIMG_1074Old Wurlitzer organ a friend gave me some years back. Worked for a little bit, then needed a few caps in the power supply replaced to take away the distortion. As cool as distorted organ could sound, it just kept getting quieter and quieter the longer it was on until eventually it cold barely even play for more than a minute or two. Recently I decided to give it another look and I ended up finishing up the recap and any other original capacitors in the power supply and output section were replaced. The 4100A was made from 1959-1963 so they are very old indeed. The whole back panel folds down so it is very easily service-able.2015-06-16T07:55:26+00:00monthlyhttps://irationaudio.com/2015/03/26/ampeg-gemini-g-20/https://irationaudio.com/wp-content/uploads/2015/03/img_0569-e1427661993871.jpgIMG_0569Ampeg Gemini G-20 schematic The real dealhttps://irationaudio.com/wp-content/uploads/2015/03/img_0572.jpgIMG_0572Old, old Linden, New Jersey pre-magnavox corporate buy outhttps://irationaudio.com/wp-content/uploads/2015/03/img_0564.jpgIMG_0564Chassis shot before recaphttps://irationaudio.com/wp-content/uploads/2015/03/img_0563.jpgIMG_0563Evidence of a previous heater to anode short circuit. Probably the most common short circuit.https://irationaudio.com/wp-content/uploads/2015/03/img_0561.jpgIMG_0561https://irationaudio.com/wp-content/uploads/2015/03/img_0558.jpgIMG_0558Tite times with the rarest Gemini, the G-20. You know it's old when it's a BLUE LINE version, rather than the 70's Ampegs which have the black screen printed front panel. Caps and transformers date this old Ampeg to 1968. It was running on GE power tubes and some Amperex 12AX7s. Not sure if these tubes are original but they are from the 70's at least! Some 12AX7s were noisy and were replaced. The power tubes were still running but very unevenly matched, so they were replaced as well. This amp has probably the best tremolo I've ever heard. Pair it with the reverb (which sounds nicer than the reverb on my V4) and it's a killer combo. Unlike the G-12 and G-15 which have one 12" speaker and one 15" speaker respectively, the G-20 has two 10" speakers. If you mention this amp in conversation almost no one will know what you're talking about.2015-06-16T07:55:13+00:00monthlyhttps://irationaudio.com/2015/03/26/marshall-master-lead-combo-2199/https://irationaudio.com/wp-content/uploads/2015/03/marshall-master-lead-combo-2199.gifMarshall Master Lead Combo 2199Marshall Master Lead Combo 2199 schematichttps://irationaudio.com/wp-content/uploads/2015/03/marshall-master-lead-tag.jpgMarshall master lead taghttps://irationaudio.com/wp-content/uploads/2015/03/marshall-master-lead-combo.jpgMarshall Master Lead ComboMarshall Master Lead Combo 2x12 from 1978. Solid state combo. This is a stock image I found on the net I forgot to take one for myself.https://irationaudio.com/wp-content/uploads/2015/03/img_0650.jpgIMG_0650https://irationaudio.com/wp-content/uploads/2015/03/img_0645.jpgIMG_0645A trace which has never been soldered2015-06-16T07:54:50+00:00monthlyhttps://irationaudio.com/2015/03/26/ampeg-svt-ii/https://irationaudio.com/wp-content/uploads/2015/03/img_0902.jpgIMG_0902This is a late 80's Ampeg SVT-II. I found that of the 6 power tubes only 1 was working at full strength. :( With new 6550s installed at idle it seemed fine but when driven some tubes would exceed 200mA while others were only drawing around 80mA. This amp had a bias problem and was destroying power tubes. Coupling caps in the phase inverter were responsible for the uneven drive. SVT at idle is displayed on the meters -- 670Va at 23.6mA per tube.https://irationaudio.com/wp-content/uploads/2015/03/1888560_591533780941430_1844927161_n.jpg1888560_591533780941430_1844927161_nAmpeg SVT II block diagramhttps://irationaudio.com/wp-content/uploads/2015/03/1800265_591533757608099_456385755_n.jpg1800265_591533757608099_456385755_nAmpeg SVT II preamphttps://irationaudio.com/wp-content/uploads/2015/03/1796654_591533804274761_865033612_n.jpg1796654_591533804274761_865033612_nAmpeg SVT II graphic EQhttps://irationaudio.com/wp-content/uploads/2015/03/1653290_591533767608098_1218789037_n.jpg1653290_591533767608098_1218789037_nAmpeg SVT II power amp and power supplyhttps://irationaudio.com/wp-content/uploads/2015/03/img_0912.jpgIMG_0912https://irationaudio.com/wp-content/uploads/2015/03/img_0910.jpgIMG_0910Massive SVT has a preamp board on the right and power amp and power supply on the left PCB. This amp has been worked on a few times before as you can see the red wire which is hardwiring a burnt out trace.https://irationaudio.com/wp-content/uploads/2015/03/img_0906.jpgIMG_0906This is a late 80's Ampeg SVT-II. I found that of the 6 power tubes only 1 was working at full strength. :( Even with new 6550s installed idle seemed fine but when driven some tubes would exceed 200mA while others were only drawing around 80mA. This amp had a bias problem and was destroying power tubes. Coupling caps in the phase inverter were responsible for the uneven drive. SVT at idle is displayed on the meters -- 670Va at 23.6mA per tube.https://irationaudio.com/wp-content/uploads/2015/03/img_0899.jpgIMG_0899After repair doing 33.7V into 4 ohms for 284W at the onset of clipping.2015-06-16T07:51:05+00:00monthlyhttps://irationaudio.com/2015/03/24/marshall-jcm-2000-dsl100/https://irationaudio.com/wp-content/uploads/2015/04/img_0249.jpgIMG_0249View of the same section of the board but from the other side. The total damage of this was the power tubes were destroyed, one tube socket was destroyed and the output transformer was shorted. Expensive repair unfortunately. Here is all the service info -- https://drive.google.com/folderview?id=0B01EXvY0__YYfkc2aEdUMUtXbjlZLVRzYkIwR2l5cGhrTUhvVktCZGJiRWJ1aUZ1Z3l2UjA&usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/04/img_0232.jpgIMG_0232This one suffered a massive blowout due to being turned on with no speaker cab attached. After a minute or so the output transformer destroys itself and more. This tube socket is done. All burnt up.https://irationaudio.com/wp-content/uploads/2015/04/img_0225.jpgIMG_0225And a shot showing the four PCB mount power tube sockets. The two pin 3's that connect directly to the output transformer are burnt off the board and the solder that as once there is now just a vaporized remnant.https://irationaudio.com/wp-content/uploads/2015/04/img_0211.jpgIMG_0211Marshall JCM 2000 chassis2015-06-16T07:50:16+00:00monthlyhttps://irationaudio.com/2015/03/25/laney-vh-100r/https://irationaudio.com/wp-content/uploads/2015/04/img_0737.jpgIMG_0737Guts shot and schematic package for Laney VH-100R --https://drive.google.com/file/d/0B01EXvY0__YYQzhwUWNqNTFlSGs/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/04/img_0726.jpgIMG_0726New tubes and biasing up for Laney VH-100R in the old lab.2015-06-16T07:50:10+00:00monthlyhttps://irationaudio.com/2015/03/31/triplett-630-a-vom/https://irationaudio.com/wp-content/uploads/2015/03/img_1872.jpgIMG_1872Side view. Triplett 630-A manual -- https://drive.google.com/file/d/0B01EXvY0__YYQ3QzN3duamVGaDQ/view?usp=sharinghttps://irationaudio.com/wp-content/uploads/2015/03/img_1871.jpgIMG_1871I bought this on eBay $20 untested and it's actually really accurate to its testament but a few resistors were drifted and were replaced. It takes two batteries for the ohms ranges, a 1.5V for the X1 and X10 range and a 30V for the X1,000 and X100,000. 30V batteries are not available anymore so you have to hack one together with three 9V and two 1.5V in series for 30VDC, and the wonderful old VOM is ready for use again. Resistance measurements on this meter are impressively accurate compared to my Fluke DMM.https://irationaudio.com/wp-content/uploads/2015/03/img_1870.jpgIMG_1870What a great old analog meter, Triplett 630. This is the older version and is from some time in the 50s I believe. The case is bakelite, or polyoxybenzylmethylenglycolanhydride, which is an early plastic material made of phenol and formaldehyde. It is called a VOM because it measures volts, ohms, and milliamps. Well, regular amps also.2015-06-16T07:47:20+00:00monthlyhttps://irationaudio.com/2015/06/12/fender-champ-amp/https://irationaudio.com/wp-content/uploads/2015/06/img_24881.jpgIMG_2488Just a handful of parts and that' really all you need. Amp was blowing the mains fuse because of an intermittently shorting power tube. Look, there is a little mis-step. A 3-prong cord has been added and the green wire is grounded to the chassis, which is good. With 3 prong installed the capacitor that goes from fuse to ground should be removed. This one has been clipped on the wrong lead. It has been lifted from ground but it is still attached on the fuse side. The little length of wire going from the fuse to the capacitor has 120V AC on it. If it makes contact with the chassis it will blow the fuse. They should have clipped that side and left the grounded side connected to the chassis. Schema and other info are available here -- http://vintagefenderamprepair.com/schematics/blackface-schematics/https://irationaudio.com/wp-content/uploads/2015/06/img_2488.jpgIMG_2488Just a handful of parts and that's all you need. Amp was blowing fuses because of an intermittent short in the power tube.https://irationaudio.com/wp-content/uploads/2015/06/img_2507.jpgIMG_2507Old blackface Fender Champ. This amp is basically all original and still works good. A 3-prong was added somewhere along the way and the "death cap" has been removed from the circuit in the proper fashion. Caps date this amp to 1964. 2015-06-12T21:25:48+00:00monthlyhttps://irationaudio.com/2015/06/12/verellen-meatsmoke-preamp/https://irationaudio.com/wp-content/uploads/2015/06/img_2478.jpgIMG_2478Verellan Meatsmoke preamp. From what I gathered this was some sort of kickstarter prototype and only 5 were made. This is #2. There was an error in either the build, or the printing of the PCB, so a 100K resistor needed to be removed, and another one added between two caps in the preamp.https://irationaudio.com/wp-content/uploads/2015/06/img_2473.jpgIMG_2473Basically just a little tube preamp with two 12AX7s. You can see the little power supply for them in the top left corner. I believe these never went into full production because they made it into a pedal instead. Sorry but no schematic for this one. If you have one please share. 2015-06-12T21:21:46+00:00monthlyhttps://irationaudio.com/2015/06/08/trace-elliot-velocette/2015-06-08T22:12:50+00:00monthlyhttps://irationaudio.com/2015/05/03/musicman-2100-rd/https://irationaudio.com/wp-content/uploads/2015/05/img_2310.jpgIMG_2310Musicman 2100-RD schematic -- https://drive.google.com/file/d/0B01EXvY0__YYenBDdkJtXzFDakU/view?usp=sharing https://irationaudio.com/wp-content/uploads/2015/05/img_2307.jpgIMG_2307Musicman 2100RD (100RD) doing 20.2VAC at speaker jack for 100W into 4 ohms. Amazing because it has only two 6L6GCs for it's output section. The output stage is unique it is a grounded grid class B design. The signal is applied at the cathode which sits about 60-70V above ground. The control grid voltage was around +22V and seems strange but it's STILL negative because the cathode is +70V or so. Screen grid is grounded. Plate voltage is extremely high, about 720V to get a huge voltage swing and lots of power. The power tubes are biased nearly at cutoff so almost no current flows at idle. This is a nice article -- http://www.linuxhorizon.ro/test/div/prj/gu50/config_GG/The%20Grounded-grid%20amplifier%20-%20Arrl.QST.-.1961-08-AUGUST.pdf https://irationaudio.com/wp-content/uploads/2015/05/img_2298.jpgIMG_2298I had free reign to do what I wanted on this one and some of the power supply caps were bulgey so I just went ahead and recapped it throughout.2015-06-08T21:20:17+00:00monthlyhttps://irationaudio.comdaily1.02024-06-14T20:41:21+00:00