Ampeg B-200R Dc on output

[goofenstein]

Hello everybody. So I've been working on an Ampeg B-200R that has had issues in the past and has killed a couple speakers before it found its way to me. When i first got it there was around 300mvdc on the output and if the volume controls and tone controls were turned up then the dc would jump up to around 10-15vdc. After reflowing all the solder joints and replacing the mosfets and drivers and biasing it, the dc on the output now sits around -270mv. But i cant get it to go any lower without altering the circuit or by hitting it with an audio signal. With a 120mv 1khz input i can get the -270mv to swing up to about +50mv. The +- rails are unregulated so they aren't perfectly matched and they sit about 10v above what the schematic calls for. When i scope the mosfets i get a pretty large saw wave on the gates and source of the mosfets(.5v p-p opposite slope on either supply). But it doesnt show on the output and doesnt show at all in the bias supplies which are tapped from the +-rails. I would assume that the filter caps are causing the saw wave but scoping the rails with no mosfets showed little ripple. And i didnt have any replacement filter caps on hand so i just switched the two around thinking if one were causing an offset it would swap polarity if i swapped the caps. It didnt. The only thing that i could do to lower the offset was to lower the NFB resistors value. The amp appears to be working fine other than the offset. When scoping the output sine wave looks perfectly fine to me. No big crossover notches or anything. The only other thing i noticed about the amp was the preamp would oscillate if everything was turned up all the way with no signal in. I havent really been looking at the preamp though since it has coupling caps between pre and power stages and im looking an offset. Am i off track? Zobel network tests fine. Replaced all electros except filter caps. Replaced diodes around oc1. Tried it loaded as well no change in the dc. Bias has no effect on offset. Pulling 12Au7 has no effect. Also the positive ballast resistors seem to heat up quicker than the negative ballasts but they are located next to the 330ohm resistors that drop the 16v supply. And Q6 feels hotter than Q7 after it runs for a bit. Not sure why.
Heres voltages
Q11 G:55.6
D:-270mv
S:59.5

Q4 G:55.6
D:-270mv
S:59.5

Q8 G:-55.7
D:-270mv
S:-59.3

Q5 G:-55.7
D:-270mv
S:-59.3

Q7 B:3.72
C:56.4
E:3.19

Q6 B:-3.66
C:-56.3
E:-3.16

Q3 B:-3.05
C:3.72
E:-3.66

Anything seem way off there that im missing? Thanks everybody.

OK Link Fixed!

Ampeg B-200R.pdf

[Jazz P Bass]

That link does not work.
I'll bet it is a copy from this site.
You need to change the file size & then upload it again.
Anyway, I think you need to concentrate on the preamp oscillation.
That cannot be good for the amp or the speakers.
You stated that turning all of the controls up triggers it.
So with all controls up, which control turns the oscillation off. (if any)

[goofenstein]

Thanks for the reply Jazz P Bass! So i checked into the oscillation and i traced it back to the 12au7, put a new one in and now it doesnt oscillate as easily. Still does though. But if i plug headphones in the oscillation stops(everywhere, not just at the output). But i still get the dc on the output regardless. Accidentally toasted the mosfets when trying to read their current draw and now only have one pair in so mismatched mosfets arent the cause. Also dont know if this matters or not but i still get the offset even with a lightbulb limiter in place. Still stumped as to where the offset is coming from. Thanks again! Will try to reupload the schem tonight.

[goofenstein]

Any ideas regarding the offset? Could it just be due to the design and the fact the rails are unregulated and have drifted apart? Also how quickly would 270mv effect a 300 watt 15" speaker?

[mozwell]

Only 270mV DC offset on the output of a pwoer amp, no problem. Ignore it & just concentrate on the oscillation.
Check all the power supply rails, 180VDC, +/-50V, +/-16V, +/-15V, are all these smooth and free of ripple ?
Measure the rails on your multimeter, on DC and AC volts ranges
A bad power supply filter capacitor could be your problem.

If the power supplies are good, using an oscilloscope, trace you way through the circuit, starting at the input jack (no guitar lead plugged in) to see where the oscillation starts. Check the soldering, it coudl be a bad solder joint on a capacitor causing your oscillation. If you find out at which stage in teh amp the oscilaltion starts, we shoudl be able to get rid of it.