Ampeg SVT-iii non-pro - how to set the bias

[gui_tarzan]

My search mojo isn't working tonight. I could have sworn there was a thread here that addressed setting the bias on the transistor output section but I can't find it. Does someone happen to have it lying around? I've never checked it on mine and it's around 30 years old. I'm getting a little bit of a weird sound lately that's hard to describe, sort of like it's not tight and crisp.

[nevetslab]

SVT-iii Not the SVT350, I'm guessing. Is it power tube output, or MosFET's. I don't recall an SVT-iii non-pro....didn't find anything in my Ampeg files. I did look thru all the early SVT files I had, which covered most of what they had built from the AC12, B12 and forward thru the early 70's. No SVT-iii. If it has MosFET's.....then typically 25mV across the Source Resistor on both sides of the output stage is typical. With that much age, and if they are MosFETs....probably will have a wide range of voltages across those source resistors (0.47 ohm). When I've had that issue, I try and keep less than 35mV on the one that's sitting way higher than the others.

[gui_tarzan]

It's an actual SVT-iii non-pro, kind of a rare bird, and it's in a rack mount chassis. I've had it since 2004 and have used it extensively in different bands. I have the owner's manual, schematics and a troubleshooting guide but not what I'm looking for. It is MosFET on the power amp side and I've seen where people talk about setting the power transistor bias (which I didn't know existed back then) a couple of times but do you think I can find it now? No!!!

[The Dude]

Was it repaired? If not, I wouldn't see any reason to mess with bias.

[gui_tarzan]

No, no repairs. I just thought it was part of the eventual maintenance needed.

[g1]

The pro is the Mosfet version and the non-Pro is supposed to be bi-polar. Non-pro schematic attached. There is no bias info shown, I would guess you just look with a scope and warm up bias til you just get rid of crossover notch with a low volume signal at output.
As there is no wear or drift with solid-state output devices, you should not have to adjust bias unless power amp repairs are done. That being said, it is possible a bias trimmer could suffer from oxidization, same as any other pot.

Symptom of too cold bias would likely be a fizzy kind of distortion most noticeable on the decay of quiet notes. Too hot bias would lead to overheating.
Loss of 'tightness' like you describe could mean it's time for new power supply caps.

[gui_tarzan]

Ok, I wasn't aware of what the actual difference was between the two, I never looked. So the bias information I remember was probably for the pro version. Good to know.

It's funny, a cap is a cap is a cap but I never think about them in my solid state amps for some reason. There's no explanation for that thinking, it just is.

[Pony]

I love my svt-iii non pro, but yeah, dealing with ground loop issues on mine. I thought the schematics had pot values in it?

[Pony]

Also I just remembered there was a video on YouTube that I came across of someone explaining the biasing on a non-pro svt-iii.

[Enzo]

I see values by each pot in the preamp that I look at