Help: M/V on a '75 JMP 100

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gkelm, Feb 7, 2006.


  1. gkelm

    gkelm Supporting Member

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    Hey fellas-
    Just picked up this head, really sounds great (and it was at a nice price)...trannys look orig, set of winged-C 6550s. Not floored with the m/v, like it better dimed and attenuated.

    Anyway, no holes drilled, the pot is in the bottom input of ch. 2. Just curious as to what type of m/v circuit this is, if you can tell by looking at it....seems neatly done. Any insights appreciated. :)
    TIA,
    Greg

    It's the one on top...already relic'd. :AOK

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  2. hasserl

    hasserl Member

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    That there is a post PI MV, not a Rich mod. The stock coupling caps from the PI were removed and four .1uf caps were installed to isolate the dual ganged pot from the high voltage. It should work just fine.
     
  3. gkelm

    gkelm Supporting Member

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    Thanks Hasserl...I've heard that the PPI is a pretty good m/v mod, and seems to work fine. With the m/v dimed is the tone supposed to be more or less what it would be w/out the mod?
     
  4. hasserl

    hasserl Member

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    Yes, with it dimed it is basically removed from the circuit. The value of the four caps has an effect on the tone though. Depending on what the value of the two original caps was this might have made a change. But this would effect the overall tone, and would not be dependant on whether the MV was dimed or turned down.

    With the MV turned down the power tubes will not be hit with as much signal from the PI, so the volume is reduced, but so is the tonal effect of the power tubes and their influence on the overall sound. However, in these old Marshalls, most of the overdrive is developed right there in the preamp & phase inverter, so this style of MV usually retains most of the character of the amp when turned down. I like 'em. Nothing wrong with cranking it and using an attenuator though. You could have it removed and returned to stock if it bothers you having it in there. I have to say it's not a real attractive addition to the chassis and board.
     
  5. John_M

    John_M Supporting Member

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    Not to hijack - but I've got a '76 JMP which was the first year for the stock MV. Is this a PPI MV or another beast?

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  6. hasserl

    hasserl Member

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    That is a pre PI MV.
     
  7. gkelm

    gkelm Supporting Member

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    Thanks again...yes, I agree, not as lovely as just the regular caps there...at least no holes were drilled! If I keep it I may have the knob moved around to the back. Looks like the filter caps need replacin'...better get that done.
    Greg
     

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