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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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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.