Black Iccy wrote:



I think I looked at the right schematic. I don't do repairs these days
but going back in time, it was not uncommon for spurious oscillations from
the output stages. Not denying that overall looping may be a problem but
*some* strange ultrasonics etc would get tamed by tiny (small value) chokes
in the plate connections and/or adding resistors into the grid line (so-called
grid stoppers). The circuit shows that there are some there. Maybe
replace/upgrade? Have you checked for RF band oscillations (in addition to
your audio group)?



** The OP's Marshall is unstable, oscillating at a few kHz, when gain settings are high. This strongly suggests that positive feedback is the culprit.

It is surprising how little capacitive coupling from the output valve plate wiring to input grid wiring can cause this to happen - with a voltage gain of over 200,000 between the two, even 1/100th of a pF will do the job.

The input sensitivity to full power of Marshall models fitted with a master volume is typically 60 microvolts at 6kHz, with all controls maxed. Just plugging an open circuit jack into the input results in oscillation.


.... Phil