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I've got a Drip Panel Super Reverb (Blackface AB763 circuit) on the bench,that goes into ultrasonic oscillation with signal applied, whenever you crank the treble or turn on the Bright switch. The amp is virtually stock except for the electrolytic caps. Messing with lead dress does not change anything. What else could be causing this?
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Dodgy grounds can do it: are the input & output jacks firmly tightened? Wire jumpers from board to brass ground bus bar behind the pots all soldered solid at both ends? Power supply grounds & PT center taps all solidly wired & bolted down?Originally posted by rf7 View Post
Another possible cause: in many 60's-70's Fenders, there are some preamp cathodes that share resistor/cap combos. If the cap's going bad, signals travel where they shouldn't sometimes resulting in squirrely behavior. So, if they're old replace the preamp cathode C's. And maybe separate R/C for those the factory combined.
How old are those replaced hi V filter caps. If they're getting past 20-25 years maybe it's time again.
There's the time honored trick of putting up to 470 pF cap between the plates of the output drive tube to quell ultrasonic parasites. Let's not do that until you have all the other possibilities covered, as they're seeing to the basic health of the amp. Also it seems your oscillation problem is at the preamp level & patching on a bandaid at the power amp doesn't make a lot of sense. Yet, it's cheap & fast & worth a try.This isn't the future I signed up for. -
I've had a bad cathode bypass cap cause that. It was in the reverb recovery, but I don't see why it wouldn't be possible elsewhere. (Removing the cap made it worse - ?!)Comment
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I figured it out. It was due to my test rig. I had my scope ground tied to my signal generator. Oh well, live and learn. Thanks for the assistance.Comment
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