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fuses keep blowing in guitar player's Marshall JCM 900.... any ideas?

Discussion in 'Amps and Cabs [BG]' started by ::::BASSIST::::, Mar 15, 2006.

  1. ::::BASSIST::::

    ::::BASSIST:::: Progress Not Perfection.

    Sep 2, 2004
    Vancouver, BC Canada
    my guitarist's amp has had the same fuse, no problem, for 13 years. then it blew. he put another one in. 20 min later it blows.

    he has an older Marshall JCM 900 tube amp... sounds great.

    could it be that one of his tubes is malfunctioning in some way causing the fuse to blow?

    any ideas?
  2. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    It could be a lot of things. Take it to a qualified technician. Obviously something else is wrong which is causing the fuse to blow.
  3. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    I heard that with older Marshalls: blowing fuses = bad tubes.
    Don't know if that applies to the new models too.
  4. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Moved to amps.
  5. I'm not an amp expert, but it seems to me if he's one of those "it goes to 11! so i beter put it there!" type people, he could just need to turn down the volume. but i dont know much so please dont flame me
  6. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    The only user service possible for most people is to replace all the tubes. If there's a different problem, the amp could conceivably eat the new tubes, so it's a bit of a crapshoot. Those amps have a lot of solid state circuitry that could fail in various ways and blow fuses too though.

    The key thing here is what NOT to do: put a bigger value fuse in there. I had a completely cooked but otherwise nice old Fender tube amp brought to me recently that would've been fine if the guy had just copped to getting the needed repairs rather than throwing a 20 amp fuse in a 6 amp slot.

    Best of luck!
  7. nt40lanman


    Jul 25, 2005
    It's likely a bad tube. Is it blowing one of the "tube fuses" or the main fuse?

    If it's one of the tube fuses, open the back, and pull out the center 2 tubes. put new fuses in and test it. If it doesn't blow, one of the tubes you have out is bad. If it does, swap the tubes, fix the fuse and try again.
  8. It could be a bad tube, it could be a poorly seated tube, try reseating them all, or i dare say it could even be one of the caps?
  9. ::::BASSIST::::

    ::::BASSIST:::: Progress Not Perfection.

    Sep 2, 2004
    Vancouver, BC Canada
    thanks guys.

    it was funny just watching him stand there cursing under his breath for literally 5 minutes. then it got frustrating.
  10. My daughter's guitarist had the same problem with her Mesa rig. New tubes was the cure. Her Mesa is self-biasing, which I find truly amazing, and a nice touch.

    My daughter has an old JCM 800 and it requires manual biasing, due to its age.
  11. by self biasing, it has a fixed bias, which will probably be colder than it could be

    Manual bias is the best way to get tubes running optimally
  12. Actually, it's not, and Mesa is full of it about that, but the poster here very well have tube problems...or about a thousand other things. The amp needs to go to someone who can repair it; it's broken.

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