Blackface Vibro Champ
Repair/Mod. The repair to this was simple. Anytime I get a customer telling me they are getting intermittent static from an amp that has a plate driven tone stack such as most fenders, I automatically assume its the 250pf cap right into the tone stack. When these start to fail, they leak DC onto the grid of the next stage and the static occurs. In this case........I never got the amp to reproduce the noise. I still recapped the tone stack and haven't heard back from the customer sooooooo I'm assuming that did it?
Mod-ish
I did do a slight mod to this amp. First off, these single ended amps have an overdrive tone that mostly comes from cranking the volume all the way, which means most of the overdrive they are producing come from the power amp rather than the pre.
HOWEVER... these amps have a volume control right before the second (or last) gain stage. With that being said, the second stage (if hit hard enough) can produce a little bit of grit. One simple way to achieve this is to either move the Volume control after that stage or simply add another means of attenuation. In this amp I added another means of attenuation after the second stage (just before the single ended power section) and kept the original Volume control in its original place.
Keep in mind, this mod is very slight. It does allow these amps to have a bit more grit than it would have otherwise, but where this mod really shines are in amps such as the AB763 or similar fender circuits that have a phase inverter installed. In these configurations you are getting even more out of your tone by driving the phase inverter a little harder.