Need help with 66 Pro Reverb

[BigBoy]

I'm hoping someone out there has more troubleshooting experience than me, and can recognize this problem.

A guy brought his 66 Fender Pro Reverb to me for some work. It had/has this mysterious sound in it. When a low note/chord is struck, the attack is fine, but as the note decays there's this sizzly sound that fizzles out. It sounds like electrical sizzling in a bad monster movie. The sound happens regardless of which channel the guitar is plugged into, and thru any/all speakers plugged into the amp.

OK, here's what I know/have proven that it IS NOT:
- not farty bass syndrome
- not a bias issue (he asked me to change the amp from fixed to cathode bias)
- new tubes all around, so it's not a tube problem
- not a speaker/baffle problem
- not an OT problem--OT replaced with new Heyboer OT
- not PI plate resistors -- replace them
- not PI or entrance caps -- replaced them
- Power supply filter caps all changed to new Spragues
- voltages are fine all around

Amp chassis and trannies are extremely rusty, and the PT looks as if someone at some time actually sanded it.

I suspect it's a problem in the PT--does anyone agree?

The problem must lie in components both channels share in common, either in signal or power, right?

Any good help would be appreciated.
Thanks

[spudwrench2]

Do you have access to an oscilloscope and a sine wave generator? Feed a signal in, fiddle with your frequency and level until you get some funky distorition and take it from there. Seems like you could scope stage by stage and look at the power supply rails for noise.
Logically, I think you are on the right track- since it happens in both channels, it must be a shared component- power amp, PI, power supply or a ground. You did mention the chassis is rusty. Double check all grounds! If you are scoping throughout the power amp, you may wish to disable the feedback loop so you don't feed back a distorted signal and confuse your logic. It really could be just about any passive component breaking down under load and arcing, etc. Gotta scope it.
How are the tube sockets? A loose socket contact can create all kinds of strange symptoms. (Also, there could be that corrosion issue again!)

Good luck,

Nathan

[phsyconoodler]

Did you change the 100k plate resistors on ALL the preamp tubes?
Also when the amp makes that noise,whack the top near the preamp side.If it stops you have a bad tube.Just because they are new doesn't mean they're good.If the whack works,take V1 and V2 and swap them with the reverb driver and PI tubes and see what happens.Super Reverbs are famous for this.

I just had the exact problem in a marshall bluesbreaker;I found a coupling cap loose on the board.You need to tug on a few things and poke and prod some more before you condemn a PT.