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Cathode Bypass Cap = No Effect

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by HaroldBrooks, Apr 16, 2018.

  1. HaroldBrooks

    HaroldBrooks Member

    Jan 18, 2015
    Just for the heck of it, I tried adding a cathode resistor Bypass cap around the 200ohm 10watt cathode resistor used for the two 6V6 tubes in this push pull circuit, and it doesn't seem to do a thing, not at 22uf or 100uf. Doesn't matter either way in the least, no addition to gain or Bass...


    I have a couple of amps with this basic circuit, and they all seem to have the same 'issue'.

    There must be something important I am missing. I hooked two separate caps from pin #8 on both 6V6 tube, nada...

    Any help is greatly appreciated, and thanks in advance !
  2. oldhousescott

    oldhousescott Silver Supporting Member

    Feb 21, 2008
    There's a couple of things that may influence that behavior. First, the field coil is in series ahead of the plate supply and it's working to maintain a constant current to some degree. Second, even with the push-pull stage, a reasonably warm-biased AB stage will see both tubes conducting for a good bit of the excursion so that the sum of the cathode currents into the cathode resistor stays fairly consistent. Combine those two and the dV/dt across the cathode resistor gets pretty small. That would be my guess, anyway.
    Steppin' Wolfe and HotBluePlates like this.
  3. pdf64

    pdf64 Member

    Aug 9, 2008
    Staffordshire, UK.
    Yes, in a p-p shared cathode bias amp, a bypass cap only takes effect at high signal levels, especially overdriven.

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