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Using mains transformers as speaker transformers in valve (tube) amplifiers?
June 2, 2009 | 2 Comments
Blue Rizla Girl asked:
I once read something somewhere about using small processors in the pipeline as a replacement transformer in the speaker circuit of the valve. Assume that the processor has a 6-0-6V with a 240V primary. What makes a turn ratio of 20:1. Thus if the secondary is putting out against a volt is the amps into 8 ohms, then (neglecting losses) in the primary will have 20 * Vs volts through it Is/20 and amps with it. But we know that it is = v / 8, and so appear to have primary impedance (20 * v) / (contro/160) = 3200 = 3.2k Ohm. I think that right? Or the presence of DC in the primary ruin everything? I 'M who thinks of using a ECL82 (aka 6BM8) for my first project, like that has both a pentodo a triode with separate cathodes and appears to be available yet. This is a good choice? I don 't need a split primary unless I' m building an amplifier in opposition, which is a po'ambizioso; I want to build a first asymmetrical!
I once read something somewhere about using small processors in the pipeline as a replacement transformer in the speaker circuit of the valve. Assume that the processor has a 6-0-6V with a 240V primary. What makes a turn ratio of 20:1. Thus if the secondary is putting out against a volt is the amps into 8 ohms, then (neglecting losses) in the primary will have 20 * Vs volts through it Is/20 and amps with it. But we know that it is = v / 8, and so appear to have primary impedance (20 * v) / (contro/160) = 3200 = 3.2k Ohm. I think that right? Or the presence of DC in the primary ruin everything? I 'M who thinks of using a ECL82 (aka 6BM8) for my first project, like that has both a pentodo a triode with separate cathodes and appears to be available yet. This is a good choice? I don 't need a split primary unless I' m building an amplifier in opposition, which is a po'ambizioso; I want to build a first asymmetrical!
Comments
2 Comments so far
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.
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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.
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