A strange signal artefact with a 1970 Fender Vibrolux Reverb.

[pdf64]

Yes, I know it sounds bad.
But are you sure it's due to rev. driver grid conduction?

I’m as sure as I can be of something that I last tinkered with a few years ago.
eg I’d bet a few £ on it, though wouldn’t go as far as the title deeds of my house

[Helmholtz]

Ok, rather than totally lifting the cap, try 470ohms in series with it.

A 470R series resistor noticeably lowers the artefact on sine signals but doesn't completely remove it.
I'll keave it in for now.

I found that the artefact mainly depends on the amplitude of the reverb drive level, so the effect of some unbypassed cathode resistor is essentially lower gain/lower drive output.
And to a lesser extend improved linearity by increased local NFB.