Jvm215c only works on red channels

Discussion in 'The Workbench' started by stephs, Dec 13, 2019.

  1. stephs

    stephs New Member

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    My Jvm215c is only loud on both red channels, the green and orange have almost no volume, like on and off.
    I have new powertubes, and its pretty loud, but only with the red channels.
    Anyone with a suggestion/opinion?
    Bad preamp tube maybe???
     
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  2. stephs

    stephs New Member

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  3. SkyMonkey

    SkyMonkey Well-Known Member

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    Could be preamp tubes. That is usually what people suggest.
    I don't have a JVM, and have little tube swapping experience, but hang on in there and someone will reply with better advice.
     
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  4. SkyMonkey

    SkyMonkey Well-Known Member

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    This partial Marshall schematic shows what tube is part of which gain stage. It is taken from the JVM 215 manual.

    JVM 215 gain stages schematic.PNG
     
    Last edited: Dec 14, 2019
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  5. stephs

    stephs New Member

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    Thanks!
    It could be v6 and or v7 correct?
    Monday I'm goin to buy spare preamp tubes, I trew them away several years ago:erk:
    You know, digital, in-ears, backnto real amps...
     
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  6. SkyMonkey

    SkyMonkey Well-Known Member

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    V6 & V7 seem to be the tubes in all gain stages.
    12AX7 preamp tubes are double triodes. That means they are 2 triodes in one tube, hence V6A, V6B, V7A, etc.
    I am not sure if only one triode in a tube can fail.
    They appear to have separate heaters (to get the tube to operating temp) in each triode, so maybe one has failed.
    Have you managed to look at the tubes when they are hot, preferably in a darkened room?

    I don't know your understanding of tube electronics (mine is pretty poor) but here is a basic explanation:
    https://valvetubeguitaramps.com/what-makes-a-valve-tube-fail/

    It is a good idea to have a few spare tubes, that are known to be good, hanging around to test with.
    I only have one I changed out of my VS65R, but it is good enough to test with.
     
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