"Ringing" oscillation in Hot Rod Tweed

[aamatruda]

I built a Hot Rod Tweed (Blue Guitar schematics) which is based on modifications to a Bassman head. I built it from scratch, and although it works fairly well, I get a static like buzz at higher volumes. When I trace a 420 Hz signal after the Master Volume pot, I can see the envelope of non-musical distortion. It seems like it's a resonant frequency issue since it starts at about 25% Master Volume, is worst at ~50% Master Volume, then is eliminated by 90% Master Volume.

I have tried swapping caps, cap values, tubes (even put 12AT7's vs 12AX7's to reduce the gain at the Phase inverter), but I can't eliminate it. The photos show the signal with the volume at ~25% and the Master Volume at various levels.

It seems other folks have run into oscillation problems from wiring location / proximity issues. Any suggestions on what to try are welcome.

[Enzo]

That looks like parasitic oscillations. I would first suspect your lead dress. Move the outpu ttube grid wires awat grom other wires as much as possible.

[bbartilson]

It seems other folks have run into oscillation problems from wiring location / proximity issues. Any suggestions on what to try are welcome.

Use an insulated something to move wires around while the thing is exhibiting the symptom, and see what you can do to make it worse/better.

In particular, I'd like to know where the NFB wire returning from the OT to that 56k resistor is located, and how exactly the master volume is wired... that is, how the connection is made between the wiper of the master, the 100k, and the .02 coupling cap to the grid of V4A.

Best,
Bill