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Yay 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.

Yay 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 hot or even normal it may not continue to work. The plate voltage is high (520-560V) and the floating paraphase phase inverter gets very asymmetrical the farther into clipping you go, driving the grids of two power tubes with a much larger signal while the signal going to the other half of the push-pull actually gets smaller.

Gotta 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).

Gotta 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 graphic 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. Lots of great info — Atomium Amps V-4 Analysis (post restoration).

Ampeg V4/VT22 schematic. VT22 is the same amp but the combo version.

Ampeg V4/VT22 schematic.
VT22 is the same amp but the combo version.