Silvertone 4732

[RWood]

This is an organ amp from a '63 Silvertone, similar to the 4733 and built by the Thomas organ company (model AR-1?) It's a push-pull 6GK6 power amp with two small signal tubes and a 6CA4 rectifier.

I am planning to build either a SuperLite or an 18 watt Stout, since the power tubes are very similar to 6BQ5s, and just a volume and tone. Both of these amps use cascaded triodes for the input, and then the phase inverter.

My question today is about grid bias. This PT has a low voltage, center-tapped secondary, around 25-0-25 VAC, in addition to the high voltage set. I would like to use this to provide an adjustable negative voltage to the grids. I'm having trouble finding a good circuit to copy. Most of the ones I see (classic Fender and Marshall) use one leg off the rectifier plate. Mine is much lower voltage than that, and is centertapped.

The closest I've seen is actually an SVT () which has not one but two bias secondaries. Naturally, mine won't be nearly as complex.

Any ideas on some good resources for me to learn and design this bias network? Most of the amps I've done so far have been cathode biased and it is time to learn some new stuff, and listen to the results.

Thanks,

RWood

[otto pärt]

Maybe you could remove the 25-0-25 VAC winding's center-tap to ground connection and hook up a bridge rectifier. That should give you enough voltage.

Just an idea.

[RWood]

Thanks for the reply, Otto. I'll give it a try.

Rwood