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After scouring the web I cannot find, nor figure out the proper way to wire a fixed/adjustable bias switch for these two amps. Can someone either explain how to do this for each amp in detail, or show me a layout/schematic? Thank you very much! -Casey
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Go to Londonpower.com, scroll through the FAQ and you will find a rough schem.
Switch will be identical in both cases, a DPDT in fixed mode the cathodes are grounded (can be via a 1ohm test resistor) and the power tube grid load (220K) connects to the c- supply. In cathode mode, the 220K grid load resistors are grounded & disconnected from c-, the power tube cathodes are connected to the power tube cathode resistor. For typical 5F6A voltages you will need a 470-680ohm resistor (50mA per tube +/- 5mA depending on B+). -
so how do i figure these cathode resistors for both amplifiers?Comment
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Here's an excellent guide on finding the right cathode resistor value: Designing Common-Cathode AmplifiersOriginally posted by duffman0733 View PostComment
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What's your B+ going to be, what transformers are you using?
If you are using a typical Champ style 125P1B transformer you can use anything from 470ohms (5W) for a plate to ground voltage of <410vdc, 680ohms will work for 400+, you might even try 800ohms if you want to ensure a dissipation nearer to 14W (I have never seen a stock Fender built champ that did dissipate <14W, SF Champs run at 20W-ish).
A 5F6A runs 470-480vdc with a GZ34 & a decent current load (RIs can run well over 500). 330ohms for amps with a plate to ground voltage o <460vdc, 470ohms for 460-490volts, 680ohms for anything higher. 10W resistor. 100v bypass cap.
You won't really know your B+ until you have everything together and a current load on the PT, with the rectifier that you want to use, so buy all the resistors mentioned and try them one by one & stick with the value that gives the best tone and doesn't dissipate the 5F6A tubes much above 22W.Comment
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