65 Twin Reverb RI, blown fuse question.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by willatkinson, Mar 13, 2015.

  1. willatkinson

    willatkinson Member

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    Okay, so the amp was blowing a fuse (the small one on the board)every time you turn it on. I replaced all the tubes, assuming one was bad... But they all needed replaced anyway.

    That fixed the fuse blowing instantly.

    Now I can turn the amp on, play it for a while, and when I shut it off, it will blow the fuse if I turn it back on.

    Is this most likely a bad filter cap? They all look okay, but I guess that doesn't mean anything.

    If replacing those doesn't fix it, would that mean it is probably the output transformer?
     
  2. pdf64

    pdf64 Member

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    Do you mean if the amp is turned back on straight away?
    If so, don't do that; give the in-rush current thermistor chance to cool down, eg 1 min between switching off and then back on.
     
  3. willatkinson

    willatkinson Member

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    It's actually back to blowing instantly upon turning it on. I replaced the tubes and replaced all the large filter caps.
     
  4. willatkinson

    willatkinson Member

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    It doesn't blow if I take any one power tube out. But I can visually see the fuse is stressed. And the fuse crackles.
     
  5. swiveltung

    swiveltung Member

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    Are there 470 ohm power tube screen resistors in that amp? if so, check those out.
     
  6. willatkinson

    willatkinson Member

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    Does a resistor have visual signs of failure? That's the resistor that crosses the middle of the tube socket, right? And thank you so much for the assist!
     
  7. willatkinson

    willatkinson Member

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    I stuck my multimeter on each leg of them and each measured 460-470.
     
  8. Blue Strat

    Blue Strat Member

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    Is that fuse supposed to be a slow blow fuse? If so, is it?

    Did you rebias the amp for the new power tubes?
     
  9. willatkinson

    willatkinson Member

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    It is. And it is. I haven't rebiased yet. Would too high bias blow a fuse?
     
  10. Blue Strat

    Blue Strat Member

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    Shouldn't, but why guess about things you can be sure about. There's no good reason for not biasing new power tubes so...
     
  11. willatkinson

    willatkinson Member

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    Thanks. Unfortunately, I don't have a bias probe. I just, however, turned the bias all the way down (cold) and it isn't blowing the fuse. Does that help, diagnostically speaking?
     
  12. Blue Strat

    Blue Strat Member

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    Somewhat...but it's kinda like sticking your hand in the air and guessing the wind speed. ;)
     
  13. willatkinson

    willatkinson Member

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    Ha! I'll bias it properly and see where that leaves me I guess. Radioahack is out of fuses and I only have one good one left!
     
  14. willatkinson

    willatkinson Member

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    Eh, it was a fluke. I turned down the bias, played it for twenty minutes. Turned it on again about twenty minutes after that, and the fuse immediately blew. I'm down to my last fuse!
     
  15. Diablo1

    Diablo1 Member

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    Time for the "light bulb limter". Google it.
     
  16. willatkinson

    willatkinson Member

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    So I did these tests on the transformer.

    It passed all accept for:
    Does that mean it needs replaced. To be clear, I connected my multimeter (black lead) to the B+ lead from the transformer, and touched the red led from my multimeter to the chassis and got 0 reading.
     
  17. Diablo1

    Diablo1 Member

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    Did you disconnect your 3 primary transformer leads from the amp before testing? Do that first, and then measure resistance between the primary center wire and the chassis. Also measure resistance between the primary center wire and the blue primary wire, then between the primary center wire and brown primary wire. You should have infinite resistance between the primary center wire and chassis.
     
  18. willatkinson

    willatkinson Member

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    Ah. I didn't disconnect the leads to the tubes, just the board.... Will try that. Thanks.
     
  19. Diablo1

    Diablo1 Member

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  20. willatkinson

    willatkinson Member

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