Scratchy, Popping, Distortion in new Pro-Reverb build.

[PfeifferElectronics]

Hi,

I've been working on this Pro Reverb build for a few months and I've got all the bugs worked out except this last one that really has me stumped. I get a "popping fizzy distortion" at high volumes. It almost sounds like the speaker breaking up but too harsh. Sounds to me like something is arcing but everything in this amp is brand new. It only occurs on the Vibrato channel when the intensity knob is NOT clicked off. It seems to be more noticable when I play humbuckers. I have swapped tubes but this doesn't seem to do anything.

I'm thinking that the trem circuit is interfering some how even when the footswitch has it turned off. So to summarize, I get bad distortion when the intensity knob is on, doesn't matter at what level. It only happens when volume is up past 7. Only happens on vibrato channel. The bias voltage coming into the trem circuit is -70V My other amp it is -60V

Thanks for any ideas....

[g1]

Sometimes those kind of symptoms may be caused by parasitic oscillation. It sounds like there is a switch on the intensity pot? Maybe the lead dress of the wires to that switch have some effect on the problem?