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Thread: Marshall JVM205H blows HT fuse

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    Supporting Member Randall's Avatar
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    Marshall JVM205H blows HT fuse

    This amp came in 3 months ago with blown 500mA HT fuse F6. I found nothing wrong with it, put in a new fuse, ran it for a couple of days on and off, and gave it back. Guy has gigged and practiced with it regularly until last night when at practice he switched it on, and no sound. F6 is blown again. I don't even know where to begin with this. Something seems to be on the edge of fault, but what? I'd hate to guess at replacing stuff and give it back hoping maybe I got it.

    Is there a more practical way to shoot this thing? BTW, it's a 205H, but it has the 410-60-00 board.

    marshall_jvm410.pdf

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by Randall View Post
    This amp came in 3 months ago with blown 500mA HT fuse F6. I found nothing wrong with it, put in a new fuse, ran it for a couple of days on and off, and gave it back. Guy has gigged and practiced with it regularly until last night when at practice he switched it on, and no sound. F6 is blown again. I don't even know where to begin with this. Something seems to be on the edge of fault, but what? I'd hate to guess at replacing stuff and give it back hoping maybe I got it.

    Is there a more practical way to shoot this thing? BTW, it's a 205H, but it has the 410-60-00 board.

    marshall_jvm410.pdf
    How old are power tubes?

    I'd slap new power tubes in, bias and then play for a while.

    I had tubes that would blow HT fuse on power up here and there, although they would bias and sound fine. Eventually I'd just swap em.

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    Lifetime Member Enzo's Avatar
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    Agree, he has been using the amp for some time, why assume it is the same old repair? Might be. but might not too. Certainly tubes can fail at any time, so I'd sure swap in new power tubes.

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    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Randall's Avatar
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    They are the original Marshall branded tubes, the amp is less than 2 years old. But, I did just put them on my bias probe, and the first one jumped up to about 130mA before settling down to 35mA. I powered down and tested it a second time, and it didn't flare up like the first time, but it did steadily and slowly rise to 45mA. A third time it only rose to 35mA. The other tube sits at 33mA. So I think we have the culprit. I'll put in a fresh set.

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    It's weird, because it WAS working fine.....

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    Senior Member TimmyP1955's Avatar
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    Give the power tubes a little vibration - you may find that one arcs under the right circumstances. (Remove your bias probe first - I recently forgot ;-(

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