In light of the 100 dollar amp challenge, I want to try to squeeze a watt per dollar out of a scrap built amp. So a 100$ 100w amp it is.
I had the idea of using 2 12v halogen light transformers, back to back,using the secondary of the first transformer as a heater supply.
I've done this before for small-ish amps (up to 10 watt) , but at 100 watts, would there be risk of hum and oscillations? I could Alsways add a third transformer, as weight is obviously going to be a lot already, one more or less isn't going to make the difference 😉
I had the idea of using 2 12v halogen light transformers, back to back,using the secondary of the first transformer as a heater supply.
I've done this before for small-ish amps (up to 10 watt) , but at 100 watts, would there be risk of hum and oscillations? I could Alsways add a third transformer, as weight is obviously going to be a lot already, one more or less isn't going to make the difference 😉
At that power level, two back to back 200VA or larger halogen transformers are going to cost more than a proper Tube amp power transformer, and will weigh twice as much.
To play it safe you might need up to 300VA transformers.
besides, you´d get 300V +V, 100W tube amps are usually around 430/450V for a classic 4 x 6L6/EL34 one. Unless you want to use 12 x EL84 that is.
To play it safe you might need up to 300VA transformers.
besides, you´d get 300V +V, 100W tube amps are usually around 430/450V for a classic 4 x 6L6/EL34 one. Unless you want to use 12 x EL84 that is.
This topology, although may work, has some drawbacks I'll try to explain. Let's suppose that you have in your inlet, 240VAC. Then, a transformer from 240V to 12V, gave a transformer turns (voltage) ratio of 20. But any transformer has internal resistances and inductances, so, under load, output voltage drops. As an example, let me assume a drop of 1V (Because of the heaters plus the second trafo load over the first). Then, this 11V again multiplied by 20 (supposing again two identical transformers) will give 220V, not 240 as it may in first instance, supposed. But the second trafo also has its own leackages and drops, so the secondary voltage under load, will be still less. Also, the second transformer load over the first will make this 11V perhaps too little to 12.6VAC heaters voltage.
A unique transformer surely will have much less drops, because of tightly coupling between the primary and secondaries.
I expect to be sufficiently clear, and to to confuse you still more.
Osvaldo.
A unique transformer surely will have much less drops, because of tightly coupling between the primary and secondaries.
I expect to be sufficiently clear, and to to confuse you still more.
Osvaldo.
I already have a couple of these transformers sitting around, might as well use 'em. They're all rated at 12v 15amps (105 vac). I might just go for 2 6L6's, aim for 40 watts.
I was indeed wondering about voltage drops in the secondary of the first transformer. I have another 13v 8A transformer from a simple car battery charger, might use that for heaters.
I was indeed wondering about voltage drops in the secondary of the first transformer. I have another 13v 8A transformer from a simple car battery charger, might use that for heaters.
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On a $/W basis, b2b PTs mean about twice the iron and twice the cost.
> transformers sitting around
Ah, well, in my prime I might have "all" parts for 100W sitting around, $0/W. Unfair? (I do not know the rules, or really care.)
> transformers sitting around
Ah, well, in my prime I might have "all" parts for 100W sitting around, $0/W. Unfair? (I do not know the rules, or really care.)
I'm a serial user of B2B transformers of various flavours. One also has the option of wiring multiple transformers up in series. Or of using a voltage multiplier to get a higher plate voltage. Or of doing the same on the 12V to get a bias voltage. A bloke called Grant Wills has been mucking around with these ideas with much success. His Lamington III gets 40W without a single "valve" transformer to be found. You can see his website Valve Heaven and related discussions on GuitarGearNet.au DIY forum
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