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Re: Sears Silvertone Twin Twelve

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10/27/1999 1:41 AM

Re: Sears Silvertone Twin Twelve

I've got a head but no cab, and I'm sure there's others -- Maybe you'd be willing to split up the pair?

Here's what you can do that makes them worth keeping, FWIW.

First, get the schematic. It's in the Weber book, or I can email you a copy. Start with the power supply:
PSU uses a low-voltage (TV?) power transformer, maybe 100VAC across the secondary. It seends B+ thru a voltage doubler chain to get the HV on the tube plates. Ditch the PT and put in a Hammond or other similar iron. Cut out the voltage doubler and replace with a few 1n4007s in a full-wave rectifier. New filter caps, of course.

Preamp, mostly fine. Cut out the reverb tank, and mount a 47ohm resistor across the terminal strips where the reverb tank was originally connected. This turns the reverb control into a footswitchable gain boost, very tasty. The 6CG7s sound awful nice used like this. PERHAPS you should rewire the 6CG7 socket for the phase inverter to take a 12AX7. I think you only have to cut the wire off pin 5 and move it to pin 9. This gives you a bit more preamp gain.

Power amp - I like to rework the power amp to take 6V6s. I cut out the bias supply and put in cathode resistors and a cathode bypass cap. Value depends on plate voltage. I like to run them at ~350V for a nice blues tone. Other than that, it's mostly okay. You COULD replace the OT, but with 6V6s it's more than adequate. OTOH I also like EL34s in this amp, in which case you'd want a beefier OT.

Keep in mind that this amp is NOT NOT NOT a real good mod platform. The chassis is small and all the controls are mounted to the folded-up "front panel." Also it's flimsy and old. You might be better off selling this thing on ebay since the cab is in good shape (#1 casualty of the vintage amp world is missing Twin Twelve cabinets

and buying a Traynor

Alternatively, I have a junker sitting in my shop now - PT's fine, no OT, no reverb tank, no tubes, that I'd sell for $50. This would let you play around with the amp and you'd still have yours in good shape if you get strapped 4 cash or whatever.

Farrow
http://surf.to/pharaohamps

10/27/1999 5:09 PM
pumpstein	Why change the PS (voltage doubler) setup? Why waste the PS (voltage doubler) setup? Not that I disagree, but can you make a case for why this is necessary/better? Thanks, P

10/27/1999 7:09 PM
Mike	Instead of the doubler setup, will it work if I tie 2 legs of both outputs from the main power supply together to simulate a 240V center tapped transformer? I'd then like to use a 5AR4 rectifier to get a B+ around 314V. Of course this would be for a different amp altogether.

10/28/1999 12:12 AM

If the PT doesn't have a CT then I wouldn't try to "fake it." It's much easier and safer to make a full wave bridge rectifier setup. Use the 2 diodes in th GZ34 for the "forward" diodes in the bridge and 4 1N4007s for the reverse diodes. (2 in series x 2) This way you still get the sag of a rectifier tube (tho not all that much from a GZ34) but you don't need the CT for the full-wave rectifier. This also works GREAT in 80's Rose-Morris AC30s, BTW. Although there I'd use a GZ32 (5U4) which is a little easier on pwoer tubes.

Pumpstein, I usually remove the doubler because it takes up more than half the real estate inside the amp, involves lots of components and can be unreliable and dangerous. YMMV, of course, but simply replacing the PT with one that can deliver the needed B+ voltage in the first place is MUCH more elegant. The thread was originally "mods for the Twin Twelve."

BTW if you want to see some amazing amp design, check out the old Wards Airline amps with 4x6L6s and 2 OTs! They use 12AX7 heaters as cathode resistors -- you get free DC heaters on your preamp tubes. Plus they sound AWESOME esp. into a 6x10 cab. Eat your heart out!

Farrow
http://surf.to/pharaohamps

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