So I finally finished building my guitar amp. It sounds great, except for one thing that occurs when gain is too high. What happens is that when I turn one of the preamp volume pots high enough strange ringing occurs. The funny thing is that on the other channel (which is identical, because it is a stereo amp with switchable guitar add-on, so you have two totally independent and identical guitar channels), no such thing happens.
Switching tube doesn't help, so I would rule out bad tube.
Here's the part of the circuit that most likely causes the problem (deduced by checking what pots change this weird effect):
(yes I use power tube in preamp, because I like power tube distortion and I don't like playing very loud for most of the time).
I am pretty sure that oscillation must come from this part, because the 'preamp volume' pot has certain position, beyond which the oscillation occurs regardless of position of other pots (actually even regardless of the volume from previous preamp stage), and increasing master volume doesn't change anything, it just makes the singnal louder.
At first I thought that amp is osciallating in 200khz region or something like that (and I'm unfortunately unable to check for sure because I don't have oscilloscope), so I added capacitor in negative feedback loop to fix phase shift for large frequencies, but nothing changed. And I think if it was RF oscillation, master volume would change somethinhg and it doesn't.
Anyone has any idea what could it be? Perhaps it is a known issue?
Please tell me if you need more information about the amp.