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wrenchone 22nd January 2013 05:04 AM

Contemporary Transformer for Doubler
I don't seem to have a whole lot of difficulty scoring OOS transformers configured for HV doubler output, like in the old Scott/Fisher amplifiers/receivers. However, it'd be nice to have a source for contemporary iron that would work the same way. Specifically, I'm looking for a 140-150V winding specified for doubler operation, a couple of 6.3V windings with respectable output, and a bias winding capable of at least 30V. Has anyone encountered a source of contemporary power iron with similar specs?

Eli Duttman 22nd January 2013 06:34 AM

The B+ part is very easy. The other secondaries on the same lamination stack is, AFAIK, anything but easy.

Triad offers 2 inexpensive isolation trafos with dual primaries that fill the bill. The N-68X can very comfortably supply 100 mA. of Greinacher ("full wave") doubled B+. The N-77U is good for twice as much.

Mouser and Allied carry Triad magnetics. I find myself calling for the N-68X and N-77U in my designs, if for no reason other than the small dent they put into the bank balance.

wrenchone 22nd January 2013 06:45 AM

I have a fair number of isolation/step-up transformers, a couple of which I'm planning to try in doubler mode, with supplementary iron for filament/bias. The name of the game in that case is an 6HV5 SE amp, as doubled 230V is a little high for anything else I'm wanting to do just now.

What I'm looking for at present is someone who did a rough copy of one of the old Scott/Fisher XFMRs. I have a preliminary design on hand for a partial feedback screen driven SE amp using cheap tubes that wants a 400V and ~ 200V supply, plus a fair amount of filament current. I can find my own iron, but I'm looking for readily available solutions that I can suggest for newbie types.

Tom Bavis 22nd January 2013 12:51 PM

Antek has a few choices at 150 and 175V, but without a bias winding... Antek - Transformers - Grid View[]=0

MelB 22nd January 2013 01:08 PM

These all have a bias tap. B+ could be a bit high...they are center tapped.
Hammond Mfg. - Universal Primary - "Classic" Power Transformers
Digikey will get them for you.

kevinkr 22nd January 2013 04:19 PM

Edcor will build almost anything you want for a not insane price particularly if you want at least two. I have not checked lately to see whether or not they have standard offerings in the voltage range and combination you want.

FWIW I wouldn't expect such a transformer to offer much economic advantage in hobbyist volumes.. Why not use a winding specified for the target voltage with a bridge? Regulation might be a bit better, and it is certainly less demanding on capacitor choice particularly at higher load currents.

Tom Bavis 22nd January 2013 06:10 PM

A full-wave bridge and center-tapped transformer is another way to get the 200 / 400V, but still, I can't think of any but the Anteks that would give the right voltage. As mentioned, Edcor has a one-time $40 setup charge for a new power design...

Tubelab_com 22nd January 2013 06:15 PM


Why not use a winding specified for the target voltage with a bridge?
I agree. I have been working on sweep tube amps where you need two HV supplies that can be roughly related by a factor of two. The easiest way to get this is an Antek toroid with two identical HV secondaries. Wire them in series just like a normal tube transformer with a CT. Connect a FWB across the entire secondary and use two filter caps, one from each DC terminal of the bridge to the CT. So far this looks just like the split power supply in most solid state amps. Instead of grounding the CT, we ground the negative terminal of the bridge. The CT is the B+ connection and the bridge positive is the B++ connection.

I have used an Antek AS-1T150 to get 200 volts and 400 volts and 6 amps of 6.3 volts. It is sufficient for a 30 WPC amp that is used for HiFi and costs $35. I have tried the AS-2T230. It gives just over 300 volts about 620 volts and 8 amps of 6.3 volts.

I am using an Antek AS-1236 to make negative bias and light up a bunch of 35LR6's too.

I suppose that you could use this method to figure out exactly what you want, then have Edcor make them.

wrenchone 22nd January 2013 10:12 PM

I am using the doubler iron for this project because that's what I have. I see no reason to go out and buy yet another piece of iron when the one I have on hand will do the job just fine. The rub is telling a newbie where to find a transformer that will do something similar.

I used two stacked supplies with the "Miz Piggy" screen driven SE amp becasue I happened to have a transformer with two appropriate windings. That transformer was a one-off find in a surplus shop.

I'll build and prove the circuit with the stuff I have, than try again to figure out how to tell someone else how to do the power supply.

wrenchone 23rd January 2013 05:33 AM

Today I received yet another piece of doubler iron, from a Griefkit RF power amp. I has a 260V/0.7A wdg, a 110V bias wdg, and a hefty 6.3V winding. It originally powered a HA-20 RF power amplifier. I have other plans for it.

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