[HBR] Output stage is power oscillator

[Ian Wilson]

I have been using a single pentode output stage. No problems, except
that the output is rather low and I think it could benefit from some
negative feedback to reduce distortion.

So I added a triode using the circuit from the HBR-13, although I am
temporarily using a 6AW8A until I can get the right box out of the garage
to locate a 6CX8.

This output stage oscillates very enthusiastically. I thought it was a
problem with input shielding but with all input wiring removed it still
oscillates around 50kHz. This comes down to audio ranges if I add
a 0.01uf capacitor across the power transformer primary.

I have added stopper resistors; no help (thought perhaps it was
oscillating at VHF and squegging at 50kHz).

Suggestions?

73, ian K3IMW