Classic smoke show. Had a blast a few weeks back at Empty Bottle blowing the ass out of my Sunn Concert Lead. Has been cranked mostly full volume for the roughly 12 years I’ve owned it but I guess this time it was too much. Smoked for about 10 minutes and almost made it through the whole set. In the end there was a burnt 22 ohm bias resistor which is pictured, an open output transistor and a shorted output transistor. The whole Empty Bottle smelled like burning 40 year old electronics, which was actually pretty glorious compared to the usual — something fails shorted and the circuit breaker trips within a couple seconds, turning the amp off quickly, without smoke or slow death.
Concert Lead is a bit hard to work on if you need to work on the output section (which is almost always the problem) because the power/output section PCB (right) has a bunch of interconnect cables that need to be removed to get the board out. Most of the interconnects are from the interstage transformer (pictured up top) that couples the driver section of the output section to the output transistors. Since this is a board style that has components on top and solder on the bottom, you almost always have to pull the board to work on it. Since it is time consuming and annoying, you will always see lots of rigged in components on boards like this because people don’t want to take the time to pull the board. I even did this, with the 22 ohm resistor. I don’t care about rigging my own stuff as long as it works good. For a customer I would make it look nice/normal. The interstage transformer that has the mounting brackets broken off and is held in place with a hose clamp was like that before I got it. Chassis is really bent on the bottom so it probably got dropped really, really hard in the past, and I guess it’s possible the transformer broke its mounting brackets and was loose in the chassis. Must have been a good show.
Great amp although it has never been a mainstay of my live rock rig. I’ve gotten good use from it live for some noise/drones and almost always gets busted out at some point for recording though.
Sunn Concert Lead schematic (old version, silverface). This version varies from the red knob Concert series amps in a few ways. One is that the power transformer has 3 taps for seperate voltage windings and 3 rectifiers, where as the red knob series has a PT with a single secondary, and all the lower PS voltages are tapped off from there.