Well, here's my reasoning: the fact that the oscillation happens only when the vol and master are maxed tells me that the feedback is from the output stage to the input stage. The fact that the oscillation frequency is slow means that the path is probably through the PS. The PS caps and dropping resistor are essentially a low pass filter, so that path only supports low frequency oscillations. It's not necessarily the only low frequency path, but other paths are usually higher frequency. That's why I suggested looking at the PS filter.
But, the fact is the entire loop needs to support the oscillation frequency, not just the feedback path but also the usual signal path forward through the amp too. Now, the PS caps and dropping resistor make a low pass filter while the coupling caps and vol controls make a high pass filter. There needs to be some overlap in the filter where oscillations can happen.
Making the PS caps bigger lowers the corner frequency of the low pass filter thus reducing the overlap. Alternatively, making the coupling caps smaller raises the corner frequency of the high pass filters, also reducing the overlap.
Looking at your schematic, you have 0.1uF coupling caps. Those are relatively huge for the application. Those coupling caps with the 1M pots puts the cutoff frequency at 1.6Hz, which is probably lower than the oscillation frequency. Try replacing both caps with something much smaller, like 0.02uF. That might help.
Having said all of that, it still surprises me that 33uF and 22K in the PS dosn't work well enough, even with the big coupling caps. Which makes me wonder if that 33uF cap might be pooty.
Topic: Oscillation Issue Sounds like a tremolo (Read 4768 times)