JCM 900 5881 to EL34's

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guitarhurricane, Nov 17, 2005.

  1. Nov 17, 2005 #1
    guitarhurricane

    guitarhurricane Member

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    I have a friend who wants to change the tubes in his jcm 900 with 5881's to el34's. It should be as simple as switching out the tubes and rebiasing right? Anybody have any good info on doing just that with this type amp? There really aren't any amp techs without having to drive 2 1/2 to 3 hrs and I am hoping I can help him do the switch. Any info or thoughts/tips would be greatly appreciatted.
     
    guitarhurricane, Nov 17, 2005
    guitarhurricane, Nov 17, 2005
    #1
  2. Nov 18, 2005 #2
    rockon1

    rockon1 Member

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    Its more complicated than just a rebiasing.I think they use the pins slightly differently.
     
    rockon1, Nov 18, 2005
    rockon1, Nov 18, 2005
    #2
  3. Nov 18, 2005 #3
    VintageJon

    VintageJon Member

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    To do this you MUST connect pins 1 and 8 together and raise the screen resistors to 1K MIMIMUM, (I'd go 1.5K or 2K...)

    In addition a re-bias is required...

    -Jon
     
    VintageJon, Nov 18, 2005
    VintageJon, Nov 18, 2005
    #3
  4. Nov 19, 2005 #4
    guitarhurricane

    guitarhurricane Member

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    Thanks guys. I appreciatte the help.
     
    guitarhurricane, Nov 19, 2005
    guitarhurricane, Nov 19, 2005
    #4
  5. Nov 20, 2005 #5
    Wakarusa

    Wakarusa Member

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    This change also usually requires a modification to the bias circuit. The range of the bias circuit for 6L6GC (those Sovtek 5881 it came with aren't really 5881's) generally leaves an EL34 biased cold cold cold.
     
    Wakarusa, Nov 20, 2005
    Wakarusa, Nov 20, 2005
    #5

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