Unloaded 18W PT Voltage

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turtle441
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Unloaded 18W PT Voltage

Post by turtle441 »

Somewhat random question, but does anyone know what the typical PT secondary voltages are for an 18W PT when fully unloaded? I usually associate 18W PT's with a 290-0-290 secondary, but I'm assuming that's when under load.

As a backstory, I'm working on building a clone of a Friedman mini. Some documentation on another forum reported that the stock PT was 310-0-310 unloaded. I'm trying to reverse engineer to sort out what PT to order for the project. Loaded B+ was 376 VDC, which appears awfully close to to what we could get with a 290-0-290 PT and solid state rectifier.

Figured, who knows random 18W specs better than you folks?
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katopan
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Re: Unloaded 18W PT Voltage

Post by katopan »

Regulation on typical amp PTs of this sort of size is around 8%. Theory would make that unloaded voltage somewhere around 313Vac. I've measured my 18 Watt before but so long ago I couldn't tell you. But 8% is the number I use for transformer design when I've wound my own. Some lower quality ones go up to 10%.

The rule of thumb I use for loaded AC to DC with a solid state rectifier is 1.32x. That accounts for the typical average first filter cap discharge between peaks taken away from the textbook rectifier conversion of 1.414x.

Using all that for a target loaded B+ of 376VDC means you would be looking for a HT secondary of 285VAC which would measure around 308VAC unloaded. You're right that a 290VAC PT would be pretty close.
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turtle441
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Re: Unloaded 18W PT Voltage

Post by turtle441 »

Thanks. Always helps to get an extra mind working on these things. I'm decent at the paint-by-numbers stuff, but trying to get smarter in understanding the theory/math behind these amps.
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