I made the Angela Instruments single 6V6 amp project, it worked fine for about a year until I experimented with a different tone stack, I didn't like the variation and now after I've put it back to how I had it originally it oscillates when it makes any sound at all. At the same time I added a switch to add an extra cathode bypass cap to the second 12AX7 stage. That cap is now disconnected from the tube socket.
Symptoms: no sound most of the time, if I tap the circuit board or a tube or anything it'll usually come on with a slow oscillation like a Fender tremolo.
If I turn the volume knob past the halfway point the sound stops.
I can measure the plate voltages on the 12AX7 swinging from around 150 to 250 volts when it's oscillating. It oscillates slowly enough that the Fluke has time to flash the readings, the cycle is maybe twice a second.
Anywhere I poke with a chopstick seems to get a reaction, I tried moving wires as much as I can do see if some adjacent wires are causing some kind of problem and that's having no effect.
The problem stops with the 12AX7 removed from the socket.
I've substituted the 5Y3, 6V6, and the 12AX7 and that hasn't cured it.
Any suggestions for things to check? So far I don't have any better ideas than just replacing stuff till it stops.