Bias and channel switching supply on Fender red knob PT replacement

[Ptron]

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.

[hiwattdoctor]

look at the schematic of the marshall jcm 900 or ampeg v4 , if your bias is on the same design : it never work well , i had several problem with this "capacitor" bias , i mod the circuit to standard bias with another transformer ( 48 vac 25 ma work well) and sometime need to have doubling voltage , you can repare amp with bias winding dead with this method , look at the bias circuit of a jmp marshall and put 100 µf x2 capacitor and 10 turn trimmer and you have a very good bias circuit ( excuse for my bad english , i m in france) question why do you have not put a standard twin transformer in your red knob amp ??HAMMOND EDB 290FEX for europe 240v but the 120 volt model exist, i can t remenber the ref.