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Old 10th December 2001, 12:11 AM   #1
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Power transformers can be very expensive, but some of the biggest, baddest transformers that you can easily obtain can usually be gotten for free, from old microwave ovens. Microwave oven power transformers (MOTs). They put out 2kVAC, 0.5A, rated about 1kVA, so are by nature very dangerous, quite lethal if mistreated, in fact. But for tube audio supplies, they might be useful. It would take two of them, with both the primaries and secondaries in series. This would give 2kV with a center tap. Resistive current limiting, like some 100W light bulbs, would be put in series with the primaries, to lower the voltage across the transformers to a voltage useful for valve audio amps. The primaries would see about 40VAC (20VAC each) for 350V CT. If more current, but not more voltage is needed, just add inductive ballasts into the primary circuit, because this form of current limiting does not cause large voltage drop. You know all this stuff if you're into tesla coil building or other HV fun, or if you just know all about electrical principles. The current and voltage limiting would also reduce power to a level that's not as dangerous (a non-limited MOT WILL KILL you-INTANTLY, if you get a shock across the chest from one).

These transformers would also be good for amps because, like all tube supplies, they have a filament winding, but that would probably need to be re-wound since the primary voltage would be lowered.

Keep in mind that if you use resistive current limiting to lower the voltage, you should keep a load on the transformer at all times. In fact, I don't know if it would even work. I set up one MOT with the primary coil and 2 100W light bulbs in series. I measured primary and secondary voltage as I struck an arc across the secondary. At no load, secondary voltage was a little over 1000V, shorted, it was nearly 0V (duh). It varied anywhere in between as I varied the arc length. I guess this happens because as the load increases, the voltage lowers to keep the current the same. Because this is how resistive current limiting works, I don't know if MOTs would work for tube amps unless you rewind the primary (that wouldn't be worth it) or run it with low voltage. If the amp tried to draw enough from the power supply, the voltage might drop so much that the amp would not work, and the filament voltage would drop to, which I understand can contaminate the plate. so, would my idea work? Any ideas on making it work?
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Old 10th December 2001, 12:23 AM   #2
Helix is offline Helix  United Kingdom
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or, you could just buy the correct transformer
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Old 10th December 2001, 12:28 AM   #3
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That's not the point, I'll only do that if I can't or shouldn't use MOTs.

Hey, ppl asked about using horiz. outputs for audio output tubes, so why shouldn't I be able to ask about using MOTs for audio power transformers? Is it just because everyone is sick of me because of the 4 x 1800W amp thread?

[Edited by Kilowatt on 12-09-2001 at 06:34 PM]
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Old 10th December 2001, 12:39 AM   #4
Helix is offline Helix  United Kingdom
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don't think it'll work
a) you will have these two bloody lamps on, all the time
b) there resistance of the bulbs changes when the get hot, hence the secondary voltage will be all over-the-place as the bulb/valves warm up.

But to give it a go, you will have to measure there 'hot' resistence when they are running at the power you expect to use them at.

Could give you a bonus softstart circuit, but i don't think valve amps suffer from this anyway to any degree (dunno never owned one)

Didn't mean to make too much of a negative comment, it's just that it sounded like too much hard work to save the cost of new parts (which are always fantastic to recieve in the post, ahh the excitement! )

[Edited by Helix on 12-09-2001 at 06:49 PM]
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Old 10th December 2001, 12:45 AM   #5
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I don't think I'll try it. It wouln't be very efficient, the PS voltage would swing all over the place, which would cause the amp to cut out, and varying heater voltage would ruin the tubes. There's too much to worry about for that idea, and I've got too many threads going.
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Old 10th December 2001, 05:41 AM   #6
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If, as you say, these transformers have 2000V secondaries that would give you a little over 2800V on your rail (assuming a cap input filter). The first problem that you come to is that there aren't any tubes rated for that kind of voltage. In the usual pentode tubes, I think the 6CA7 is about as far as you're going to get, and it's only good for something like 800V. Some of the power triode tubes take fairly high voltages, but again, I think they're in the upper hundreds. I haven't built any SET amps, so I don't claim detailed knowledge of those tubes.
Tube power amplifiers don't, as a rule, use bipolar power supplies. There are no P-channel tubes in existance, only Ns. Yes, there are strategies where you can stack two power supplies on top of each other, and play a few tricks here and there, but it's not for a really high voltage power stage, it's for direct coupling the other stages--that sort of thing.
Using light bulbs to drop the rail would work, yes, but besides the waste, would require a whole lot more than just 1 or 2 bulbs per channel (Ohm's Law). They'd also load the output transformer pretty badly. Frequency response, damping, etc., would suffer.

Grey
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Old 10th December 2001, 06:13 AM   #7
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That just further re-enforces that I won't use MOTs. Also, most (sane) people find the little glow of vacuum tubes quite nice. Who wants all those lams glaring at them?
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