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Crackle, crackle, hummmmHUMMMMMM! (Bad output tube?)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by justnick, May 16, 2011.

  1. justnick

    justnick Gold Supporting Member

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    So my beloved Carr Rambler went down on a gig last night.

    First couple songs it seemed to breakup early and there was a fairly loud hum. I assumed bad lights in the place and kept playing.

    Eventually it was distorting horribly and had a REALLY loud hum.

    I shut it down and borrowed an amp.

    At home I powered it up and same loud hum, unaffected by volume or reverb controls. Both tubes were glowing normally on standby and when powered up. I confirmed the tubes were seated well.

    I swapped out the power tubes (quite new) and everything now seems normal.

    I'm guessing that was the issue but I have not had tube failure manifest in this way. Do you think I'm in the clear with new tubes?

    n
     
  2. Blue Strat

    Blue Strat Member

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    Impossible to say for sure, but the symptoms are consistent with one of your old tubes taking a dump.

    The best way to check the stability now would be to get a bias probe on your new power tubes, monitor them to make sure they're running in spec and stay running in spec for at least a half hour.

    Sometimes you can take out an associated resistor or cathode capacitor (if the amp is cathode biased) when a power tube shorts out. This will lead to a never ending cycle of blowing power tubes and not necessarily right away.
     
  3. XmasTree

    XmasTree Member

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    this happened to me the other day
    i feel accomplished having my first power tube go bad.....haha
     
  4. Blue Strat

    Blue Strat Member

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    It's a milestone in any tube amp owners life. :)
     
  5. justnick

    justnick Gold Supporting Member

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    Thanks Blue Strat, much appreciated.

    So, if I run the new tubes with a bias probe, for a couple hours, and detect no major changes in current draw, probably all is well?

    n
     
  6. Blue Strat

    Blue Strat Member

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    I'd be convinced in 30 minutes or less. Note that when bias probes get hot they can start acting funny, so you might have to take them out and cool them off while continuing to run the amp.
     
  7. justnick

    justnick Gold Supporting Member

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    Great, thanks very much, most helpful.

    n
     
  8. ripoffriffs

    ripoffriffs Supporting Member

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    I'm just surprised the fuse didn't blow. That's been my experience with power tube failures.
     

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