Silvertone Twin Twelve 1484 - Intermittant Reverb Squeal if Rev Depth Knob Turned On

[Jecarroll7]

This is my first twin twelve.

Amp had multiple problems when I got it, but appears to be original including the tubes.

No output turned out to be an output tube socket retention issue. Corrected and replaced output tubes with a new matched set, thorough cleaning of all connections and pots.

Also did a 3 prong conversion on it, and lifted the heaters with 100 ohm resistors.

Amp seems to work great, both channels - no hum. Tremolo is working.

About 3vac ripple on the original filter caps, the can cap measures in the mV range. Could change them but haven't seen a reason to as yet.

The last issue is the reverb. When I switch the reverb on I either get a soft feedback sounding squeal that quickly builds, or a loud squeal off the bat.

I can bump the tank and the squeal will go away until the tank settles down (you hear the tank rattling through the speakers).

Dressed the wiring, checked the bypass cap on V4, and have taken the ohm meter out and checked the screen/grid/plat resistors around V4 and V5.

All voltages seem spot on.

I've read here this amp uses a piezo style tank, I've never worked with one. Seems unlikely but could the tank itself be the issue?

I've attached a schematic. Thanks in advance.

[drewl]

You're lucky it still has the original crappy reverb.
Most of the ones I see the reverb is gone.

Good call on the tube sockets, I've also had to replace many screwed up preamp sockets.

These reverb units are very sensitive, most of them squeal.
You can "carefully" take it apart and try and retape and remount the insides if they are loose.
That should help.
I have retrofitted regular teverb tanks on these which is another alternative.

Others I've used the reverb circuit for an additional gain stage for some people.

[52 Bill]

Have you tried dampening the reverb tank with anything? Try and cover it with a rag and see if the feedback stops. Sometimes adding a strip of foam tape is enough to change the resonant frequency of the metal tank cover.

[Jecarroll7]

Can't dampen

Haven't found a way to dampen it. I can push down on the tank and hold it, that somewhat works.

Attached a reverb mod for this amp I found over on EL34world that looks interesting. Contains a picture of a spring reverb tank mounted in the head cab.

[Leo_Gnardo]

Yes I'd recommend steering away from the Sears Silvertone reverb run with piezo transducers, no wonder there's lots of treble trouble, enough to feed back. They sounded rather rancid anyway, weak and crashy, maybe good for a punk band, not much finesse there. I was about to recommend you have a look at some 60's Ampeg reverbs as in the Reverberocket, Gemini I, II, and VI for instance. You already have a 6CG7 to drive it as Ampeg does, and I'd recommend swapping in a new 600V rated film cap to pass the signal thru on the drive side. Though I haven't done it myself I think you can adapt the insides of a short reverb can as a replacement for the piezo-driven original. You'll want to select a tank that has the highest possible input impedance too since you'll be driving it without a transformer. Going to do it soon though, I have similar Silvertone that's been beckoning me for attention a while now. "Fix me fix me ! ! !" Then up for sale you go... luv ya but I need the money.

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edit after looking at the Hoffman site mod:

Interesting to see the recommended mod drives the tank from 6CG7 cathode not plate. If you try the Ampeg style plate drive instead, use a 600 or 630V rated cap, 0.1 to 0.47 uF should be plenty.