Marshall JMP MV 2203 -77 problem

Discussion in 'The Workbench' started by evilrocker, Oct 28, 2015.

  1. evilrocker

    evilrocker New Member

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    So I recently had a tech who replaced powertubes and checked bias + checked the whole amp. Today I fired it up and there is a loud hum and crackling noise. Tried different guitar cables, different speaker cables, tried different preamp tubes. And besides this noice and hum issue, two of four power tubes glows like their about to set on fire. Any ideas? Check my video to see and hear it: https://youtu.be/JcC4EzhJOSU

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    Last edited: Oct 28, 2015
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The bias is failing on one side.
    Don't play the amp until you get it fixed right.

    What will also cause this:
    1 bad power tube...
    High frequency oscillation.
    Bad speaker cables...
    But oscillation is less common than just a broken bias path.
     
  3. mott555

    mott555 Well-Known Member

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    Redplating is BAD. Turn your amp off immediately if you see that, like the tubes on the right.
     

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