Fender Vibro Champ

[The Dude]

......What was really throwing me was the fact that it only oscillated with the volume all the way down or all the way up......

You mean you only heard the oscillation with the volume all the way up or all the way down (or did you look with a scope?). I suspect it was oscillating all the time and the volume changes brought the frequency down into audible range.

[pdf64]

Just to note that another contender for the original symptoms may be a dodgy grid leak path for the 6V6 control grid.
ie if the grid loses its DC reference, the valve can pass lotsa current and redplate etc.

[stokes]

You mean you only heard the oscillation with the volume all the way up or all the way down (or did you look with a scope?). I suspect it was oscillating all the time and the volume changes brought the frequency down into audible range.

Didnt look at it on the scope but I was monitoring the cathode voltage and only saw drastic changes when the control was all the way up and all the way down.