5 watt SE guitar amp head with sidetone buzzing problem when strings are strummed

[rock soderstrom]

I have modded a China 5 watt SE guitar amp head and I have a constant buzzing background noise sound (when I strum the strings) that I can't get under control. It sounds like something is vibrating with the strings, but it's not a mechanical problem with the speaker or tubes. I've swapped them all to isolate the problem, it does seem to be coming from the amp and I'm running out of ideas now.

Does anyone know the problem or have similar experiences. What could be responsible?

The circuit now is a really good sounding hot Mini Z clone (but with Master Pot) but the problem was already there with the donor amp. Tubes are 1x 12AX7 and 1x EL84.

 

[rock soderstrom]

This is the circuit at the moment

 

[CJ]

Sounds like oscillation, try moving wires unless it is on a PC board, your gain went up with the deleted tone circuit, you could try a divider to attended the signal. Maybe a NFB resistor from the xfmr secondary,

 

[rock soderstrom]

Sounds like oscillation, try moving wires unless it is on a PC board,

I think you're right, I've already opened up all the wiring harnesses but I can't influence the behavior. I think the problem originates on the circuit board.

your gain went up with the deleted tone circuit

That's true, but the problem already existed in the original version. By the way, the tone stack had very little effect, sounded terrible and choked the amp. That was a very good decision to take it out. The voltage divider in front of the gain pot had originally also swallowed a lot of gain.

you could try a divider to attended the signal. Maybe a NFB resistor from the xfmr secondary,

I'm afraid it will change the character of the amp too much, I'm very happy how it sounds now, really great, wild and super loud for 5 watts. This is not my first 5 watt amp, but I really like it now - good recipe!

I'm going to try a grid stopper before V1b, see what that does and I'll contact the transformer primary to the 240V winding to get the secondary voltages down, they're all too high!

Maybe it has something to do with the poorly filtered DC heating?