I’m replacing the power tranny on a fender red knob “The Twin” with a kit/design put together by a well known amp guy. The original one got the B+ of a voltage doubler circuit. It also had a separate winding which provided the bias voltage and the supply for the channel switching. The replacement kit uses a Hammond 360J (325-0-325), eliminating the need for the voltage doubling for the B+. The new bias supply taps off one side of the secondary. Instead of a regular resistive voltage divider to bring the raw AC into the neighborhood required for the bias supply, it taps off a .047uF/1000V (56k impedance) and a 27k resistor.
Any idea why the cap is used? A simple resistor there would only dissipate a watt or so. I don’t see how it’s blocking any DC.
For the channel switching supply, he uses a separate 25V transformer with a voltage doubling rectifier circuit. The caps in the doubler are 100uf/350V.
Any Idea why he choose such a high voltage rating? His kit includes a 100uF/150V for bias supply filtering so this doesn’t seem to be a matter of what he had on hand.
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