RESOLVED - Fender DRRI blowing fuse

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 5E3, Jan 5, 2019.

  1. 5E3

    5E3 Member

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    My DRRI is blowing the fuse when powered on after about 2 seconds. I did a search and some trouble-shooting but still need advice. Amp worked fine about 9 months ago but has been off and unplugged from wall outlet since.

    My testing was performed with the standby switch in standby mode (not ON).

    • I removed the rectifier tube and powered on - fuse does not blow
    • With rectifier tube in removed power tubes and powered on - fuse does not blow
    • Swapped power tubes with original Fender GT 6V6, rectifier tube in, powered on - fuse blows after about 2 seconds

    I'm wondering if I need new tubes and if so which one(s) or should try some other test?

    Thanks for any suggestions.
     
  2. jthomas666

    jthomas666 Member

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    Well, if I understand what you have done so far... the fuse blew with the power tubes that had been in it for awhile (i.e., before it sat for 9 months) and the fuse blew when you put an older set of power tubes in. If the tubes were not blowing the fuse at the time that the older tubes were taken out, then new power tubes are unlikely to prevent the fuse from plowing.

    You should try the same kind of test with the preamp tubes. See if that keeps the fuse from blowing. If new preamp tubes do not fix the tube-blowing issue, then it time to take the amp to a tech.
     
  3. pdf64

    pdf64 Member

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    It seems strange that with it in standby mode, the fuse holds without the power tubes but blows when they’re fitted.
    Is that really the case or have I misunderstood?
    I think it’s a bad idea to repeatedly subject the amp to fault current, so plan any further tests carefully, and use a light bulb limiter or take it to a tech, rather than go through a pack of fuses.
     
    Steppin' Wolfe likes this.
  4. 5E3

    5E3 Member

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    Yes, that is correct.

    With the amp in Standby, power tubes removed, all other tubes in, the fuse does not blow when power switched on.

    Thanks for the replies.
     
  5. pdf64

    pdf64 Member

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    I guess that a power tube may have a severe cathode to heater short; I’m unsure how many pairs you’ve tried.
    So if you haven’t already, it’s worth trying a known good pair of power tubes in there.
     
  6. Steppin' Wolfe

    Steppin' Wolfe Member

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    IMHO, it is tech time. Even if new power tubes proves to solve the fusing problem, the amp needs to be rebiased. A full evaluation is in order. If the amp is under warranty, let Fender worry about it.
     
  7. 5E3

    5E3 Member

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    *** RESOLVED ***

    I replaced the rectifier and power tubes with new and the amp is working fine now. I checked the bias and it is also good.

    I don't know which tube or tubes were causing the issue, but I'm fine with replacing all three.

    Thanks to all who replied with thoughts and suggestions.
     
  8. jthomas666

    jthomas666 Member

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    congrats. rock on Garth.
     
  9. Blue Strat

    Blue Strat Member

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    Congrats. FWIW, when things like this happen install only ONE tube at a time to figure out which one is causing the problem.
     
    zenas, pdf64 and HotBluePlates like this.

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