Saturday, June 25, 2016

Repairing a Silvertone Attack From Mars Reverb

If you're a surf bunny the "reverb" that was installed on Silvertone Twin Twelve and Danelectro amps is not for you, but if you're a fan of fifties science fiction horror movie sound effects you might like having it available from time to time.

It's usually found on a spring mounted piece of masonite with an aluminum cover, but some of the early ones were mounted inside a chunk of PVC pipe with a twenty foot drop cord so you could put it outside the room and keep it from feeding back too much.

The reverb unit was actually patented by Nat Daniel, the Godfather of Cheap, in 1963, and if you care to look it up the patent number is 3,092,792. I can tell you this device was in use before then because my early 1961 Silvertone combo model 1474 had the reverb in a PVC pipe.

But anyway-back to the model 1484 Twin Twelve and its onboard reverb. It's parked in the head along side the preamp tubes and is sort of shock mounted on a chunk of spring steel.

The working parts consist of two piezoelectric wafers wrapped in masking tape one of which functions as a driver for the reverb spring and one of which serves to recover the signal and pass it on to a recovery circuit.

I suspect that many of these units may work OK but most people do not care for the tone, as it is something of an acquired taste like Greek olives-not for everyone.

You can test the piezos with an insulation tester, and on the higher voltage settings it'll make audible noises in the tank. It's better for testing to remove the tank and reconnect it with test leads so as to test it.

In my particular case I acquired a Silvertone Twin Twelve recently and once it was up and running some contact cleaner in the reverb switch and attached pot produced lots of booming when the tank was tapped on. Reversing it produced the same effect so I knew both piezos were working properly.

Next to check was the voltages which are provided in the schematic diagram.  In my case the plate voltages on V5 were low to nonexistent. Replacing R45 and R46 got me within range and the reverb worked well enough within its limitations. The moral of the story is never think of an excuse not to test something.

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