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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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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