Why need to separate cathode for RobRob's cascade switch mod?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 20, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    For RobRob's cascade switch mod:
    https://robrobinette.com/5e3_Modifications.htm#High_Gain_Channel

    He says a few times something like:
    "V1A and B's cathodes must be separated by installing a 1.5k ohm (1/2watt or higher) cathode resistor and one or two 22µF 25v cathode resistor bypass capacitors on V1A and V1B's cathodes (pins 3 & 8)."

    Why must the cathode be separated for this mod? Would it work if you kept the cathode bypass shared, or does it cause some kinda phase cancellation?
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Hmmmmmmm...that's a good question. Maybe there are phasing issues?
     
  3. itsGiusto

    itsGiusto Tele-Meister

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    Last edited: Aug 20, 2019
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    A common cathode resistor for two gain stages in series can cause interaction and oscillation. My "must be separated" statement is a little strong though.
     
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  5. shortcircuit

    shortcircuit Tele-Meister

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    A while back, I tried to cascade the two triodes in the 6J6 tube with a common cathode resistor, and that thing squealed like a stuck pig. Learned my lesson.
     
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  6. itsGiusto

    itsGiusto Tele-Meister

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    Cool, thanks for the clarification! So if I want to avoid higher chance of oscillation, I should separate then.

    When you separate the cathode, but want to keep the same exact bias and tone, you should use two resistors of double the value, right? And use the same value cap?
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Correct - since the cathodes are no longer in parallel you'd double the resistor value.
     
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  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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