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Post PI MV on cathode biased amp

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  • Post PI MV on cathode biased amp

    I built a 5F6-A circuit with cathode instead of fixed bias. I now would like to put in a post PI master volume (dual pot) and am unsure how to deal with the legs of the pots that went to the bias suppy that is now grounded. I realize that I need caps after the pots and before the grids of the 6L6s, but I don't know what to do about the common legs on the pots. Thanks...


  • #2
    Post PI MV

    A better way to ask the question is what has to happen to the Ken Fischer dual pot master volume mod as in Gerald Weber's book to use it in a cathode biased amp....



    • #3
      You don't need those extra caps imho.

      In a cathode-biased 5F6A:
      dual 250k (log) pot
      and the non-signal-lugs to ground
      Love, peace & loudness,


      • #4
        You might wanna have a look at the Fender Tonemaster schematic. There you got a post PI MV with half the value caps before and after the pot(s). Works for fixed and cathode bias.
        You can find the schematic at


        • #5
          The lugs that went to the bias supply in a fixed bias amp just go to ground now. You don't need the caps after the PPIMV now since the grids of the power tubes are ground referenced thru the pot. You can also take out any grid to ground resistors in the amp that may be there.


          • #6
            Yes, exactly. You just treat the ground reference as if it were the bias supply. Because thats what it is. It's just a 0V reference instead of a -V reference. So, obviously, you would replace the grid load resistors with the dual ganged pot in the same way that you would replace the bias feed resistors in a fixed bias amp.

            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A