Reverb oscillation fixed

[pdf64]

How about screening the pan's open face off with some foil?
I think that pans are intended for a specified mounting orientation WRT vertical, denoted by the part id code.

[Chuck H]

How about screening the pan's open face off with some foil?
I think that pans are intended for a specified mounting orientation WRT vertical, denoted by the part id code.

I've tried that. No difference. I paid no attention to the proper mounting plane while experimenting, but it's final orientation is "correct".

[g1]

Out of curiosity, does flipping the pan over move the springs further from the magnets?

[Chuck H]

Out of curiosity, does flipping the pan over move the springs further from the magnets?

Not much at all. With the pan in correct orientation the whine persists anywhere near either speaker, but is reduced when between them (where the speaker cable is, incidentally) And this is the same no matter which way it's turned end to end. With the pan upside down there is no whine anywhere in the cabinet. Also regardless of end to end orientation.

[defaced]

How about screening the pan's open face off with some foil?
I think that pans are intended for a specified mounting orientation WRT vertical, denoted by the part id code.

I've tried that. No difference. I paid no attention to the proper mounting plane while experimenting, but it's final orientation is "correct".

Thinking out loud here. To me, foil suggests something that's not ferromagnetic, like aluminum foil. The problem being that it's invisible to magnetic fields. If your pan is steel, then it would shield against magnetic fields. I'm thinking magnetic field interaction between the reverb pan and the speaker, like mentioned in the first reply, but this being a different way to solve it. So maybe change the piece of foil for a sheet of steel?

[Chuck H]

The pan is magnetic. It's grounded too. In it's correct orientation the transducers and springs are behind the pan WRT the speaker magnets. It's when I flip the pan and the transducers and springs are exposed to the magnets that the oscillation stops. I don't think that the springs and transducers being more exposed to the speakers magnets is related to the oscillation though.

[The Dude]

Can you post a schematic of the amp or the reverb circuit in the amp? Possibly someone will have thoughts on the circuit. I fully realize that change in tank orientation, proximity, etc. provide a change in symptoms. That doesn't mean some small circuit change might not fix the problem all together. You've got me a bit curious about this myself, as I've run into it several times. Each time the fix seems to be different. Maybe we can all learn something from this using you as a guinea pig.

[Chuck H]

Hmmm... That leaves my design open to criticism from my peers!?! I'm typically shy about such things. Not for any secretive reasons or anything. So, ok then. It's drawn pretty small but it should be readable.

Some things may look funny. The amp is a Fender/Vox thing. Everything on "SW1" is on a single rotary switch. The weird tone stacks actually emulate the originals pretty close using different circuit values so that I could arrange them on dual ganged pots as there is only one tone set on the amp face. Ok. enough excuses. Here you go.

[g1]

Thanks Chuck. By tomorrow I should have it up on ebay .

[Tom Phillips]

I like the "Boing" component name

[52 Bill]

Does the amp do this in a different location? Does your bench have a metal top?

I'm just wondering if the oscillation is from some outside source.

[Drewline]

I had the same problem with a Gibson GA-15 a while ago with a larger speaker stuffed into it. Take the tank out of the amp & the oscillation would stop. I ended up reducing the reverb output to stop the oscillation. It turned out that it was the speaker causing the problem. My work around was to reduce the gain of the reverb recovery by replacing R26 (220K) with a 47K & adding a 82K between C13 & V1B pin 8. Still had plenty of reverb.
I really wish I'd known to try grounding the speaker basket. Thanks for the info.

[The Dude]

Chuck,

First off, nice work on the schemo and agreed the "BOING" component name is a nice touch. Can you say more precisely at what frequency the oscillation is/was?

[TimmyP1955]

Double check all the grounds, and that the caps are good on nodes C and F?

[Chuck H]

Does the amp do this in a different location? Does your bench have a metal top?

I'm just wondering if the oscillation is from some outside source.

No metal top on my bench. It doesn't happen with the pan out of the amp. Absolutely, only when the pan is near one of the two speakers. The amp had always done this. Even in the hands of the customer at another location. He was anxious to have it and told me he never likes the reverb above five anyway. I released the amp and made him promise that when he went back out to gig I could get it back for final tweaking and adjustments. That's why this is coming up now. I added a safety feature to the bias supply via the 1M resistor on the "intensity" pot in the schem. I never liked the idea of the bias voltage being entirely dependent on a pot wiper and the trem was a bit whompy anyhow. I slowed the rem down a bit for that sleazy throb and I fixed the reverb trouble the amp was having. All good now. I'd buy that amp in a second for what the customer paid Nicer than anything I personally own. Not my usual type amp, but it does what it was meant to really well with one exception. The bias wiggle trem isn't really a good choice for an amp with 465Vp! A little too much crossover garble with most power tubes. Ruby EL34BSTR's make it acceptable though. I'm considering moving the trem to a preamp tube cathode, but that's on the back burner for now.

EDIT: The pan now is a 8AB2A1B. The first pan was a two spring long tank. The problem occurred with both pans. I changed pans because of an acoustic stability issue that was solved with the three spring short tank.