Using mains transformers as speaker transformers in valve (tube) amplifiers?

Posted by admin in Thursday, May 28th 2009

Topics: Engineering

Blue Rizla Girl asked:

I read something somewhere once about using small mains transformers as substitute speaker transformers in valve circuits.

Suppose I have a 6-0-6V transformer with a 240V primary. That gives a 20:1 turns ratio. So if the secondary is putting out Vs volts at Is amps into 8 ohms, then (neglecting losses) the primary will have 20*Vs volts across it and Is/20 amps through it. But we know that Is = Vs / 8; so the primary will appear to have an impedance of (20 * Vs) / (Vs / 160) = 3200 = 3.2k ohms.

Am I thinking right? Or will the presence of DC in the primary ruin everything?

I’m thinking of using an ECL82 (aka 6BM8) for my first project, as that has both a triode and a pentode with separate cathodes and seems to be available still. Is this a good choice?
I don’t need a split primary unless I’m building a push-pull amplifier, which is a bit ambitious; I want to build a single-ended one first!

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2 Comments

billrussell42 said in May 31st, 2009 at 10:23 pm

The primary is the one that needs the split winding, not the secondary. So you would need a 240-0-240 transformer. Possibly a transformer designed for 120 or 240 volt operation, so you can connect it as a 120-0-120 volt primary.

The secondary, where the speaker is connected, would be the 6v or 12v winding.

I lost my tube manual (tossed it) long ago. But from memory, I’d use 6L6s for the drivers, they should still be available, and have a lot of power.

How well will this work? Don’t know. give it a try.

tlbs101 said in May 31st, 2009 at 10:46 pm

Single-ended audio output transformers are specially designed with a small gap in the iron core material to prevent the DC plate current from saturating the core with magnetic flux. Power transformers do not have this gap (they would burn up if they did).

Push-pull audio output transformers do not have the gap, because the flux due to the DC current flows in opposite directions toward both plates, thus cancelling out their respective fluxes in the core.

It is technically possible to use a power transformer as a push-pull output transformer and your calculations are basically correct for impedance, voltage, etc. The HV output windings of most power transformers are usually split so, you could use the HV windings for the plates and the filament windings for the speaker. But, again, it cannot be used in a single-ended application.

Furthermore, power transformers are designed for efficient power transfer at 50/60 Hz without regard to frequency response. Audio transformers are designed specifically for a wide (audio) bandwidth. You might get good bass with a power transformer as an output transformer, but that’s about it.

Bottom line: you really need a single-ended audio output transformer designed for the tube you plan on using and the power level you plan on producing.