Converting JMP back to 6550's

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JES1680, Jan 9, 2008.

  1. JES1680

    JES1680 Silver Supporting Member

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    I've got a 78 JMP MV 50 watt that was converted to EL34's. I'd like to go back to 6550's.
    Anyone aware of a schematic and or kit to do this? I know there are kits to go from 6550's to 34's, but how about the other way?

    Thanks
     
  2. John Phillips

    John Phillips Member

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    How well was it converted?

    It's possible that all that was done was to bridge the range resistor next to the bias trimmer with another resistor (this is the crude 'quick fix' method of converting to EL34s). If so, all you have to do is remove it and you're back to stock.

    But if it was done properly - replacing the limiting resistor in the bias supply and the two bias feed resistors - you'll have to redo these the other way.

    The resistors are:

    The resistor next to the bias diode should be 150K (220K in EL34 amps, and if converted correctly).

    The resistor next to the trimmer should be 47K (for both tube types - this is the one that will be bridged with another if the bias conversion has been done crudely).

    The two resistors that go to the orange and green wires to the power tube sockets should be 150K (220K in EL34 amps, and if converted correctly).
     
  3. JES1680

    JES1680 Silver Supporting Member

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    I assume it was done correctly as it was done by a highly regarded amp builder.

    I haven't been under the hood yet.

    Thank you for the info.
     
  4. Swarty

    Swarty Member

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    The NFB also comes off a different tap on the OT.
     
  5. John Phillips

    John Phillips Member

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    8 ohms for 6550s? 70s and later EL34 Marshalls usually use the 4-ohm tap. Personally I think they sound better with it on the 8-ohm anyway.
     

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