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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 probably mainly because they’re from a different era where tubes were more robust, but also because with the tubes under the chassis they run hot when all the heat just rises up into the chassis. To top it off, the power transformer and power tubes (all the hottest parts of the amp) are all shoved together in the corner of the chassis right next to each other. Really not a good idea. Very surprising really that Ampeg’s thermal design for this amp is so terrible and pretty much all other aspects of the design are totally awesome.  They even floated the chassis to isolate it from cab vibration with rubber grommet mounting brackets. So cool. 
These amps are biased very cold from the factory, in the 45-50% plate dissipation range. If you bias it hot or even “normal” it will probably be too hard on the tubes and it may break. The plate voltage is high (around 530-540V) 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.