PPIMV question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by silverjet89, Feb 6, 2012.

  1. silverjet89

    silverjet89 Member

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    Years ago I put a Fisher type 2 MV in my Vox Berkeley. In his schematic he shows a dual 100K Linear pot. I've noticed most of you on this forum use a dual 250K. What difference would this make to the tone & performance of the amp?
    Just wondering if I should change it out.

    Thanks
     
  2. Blue Strat

    Blue Strat Member

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    It would also depend on the caps before and after the pot as they figure into the frequency response. In general, a higher value pot will have less "loading effect" which can often rob highs and/or signal level depending on the application.
     
  3. Ronsonic

    Ronsonic Member

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    The pot in a PPIMV replaces the grid resistor and there is a published spec for the max value depending on the tube and the bias arrangement. IIRC, it is 100K for an EL34 in fixed bias.
     
  4. silverjet89

    silverjet89 Member

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    Thanks guys. The caps are the original value, .047.
    The original resistors were 220K so I guess a 250K pot would be closer to the original tone at full volume.
    Where's the best place to get a 250K dual pot?
     
  5. smolder

    smolder Gold Supporting Member

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    In the LawMar solution the 220k resistors are simply moved to the 250k dual pot. That provides an additional 250k resistance on the circuit that functions as the MV. When all the way open it's obviously as if that resistance is not there. The closer to the power tubes and caps in that circuit the better obviously. Short of that shielded wire can be used to help curb any noise that part of the circuit would introduce.
     

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