HELP: 1 of 4 Power Tubes Died

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by NBlair930, Jan 21, 2012.

  1. NBlair930

    NBlair930 Silver Supporting Member

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    I just broke out an modern amp that had not been turned on for many months. the amp uses a quad of el84's and I had a nice set of NOS Mullards in there (thanks Mike/KCA many moons ago). While playing I did notice that the amp did not have the clean headroom that I always recalled it had in the past, but I thought nothing of it. When I was done playing I noticed that only 1 of the power tubes had a cloudy getter flashing -vaccume has been compromised. My questions, BEFORE I put another NOS Mullard in there are,
    1. Could playing with only 3 of 4 power tubes working done any harm to my amp?

    2. Is the fact that only 1 tube blew mean anything?

    3. Is this soemthing that I should have a amp tech look at?

    Thanks.
     
  2. davemccarthy707

    davemccarthy707 Member

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    1-maybe
    2-maybe
    3-yes

    your welcome
     
  3. Blue Strat

    Blue Strat Member

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    1: Probably no harm to the amp, possibly harm to the other EL84s as in any cathode biased amp running with less than the proper number of tubes.
    2: Try some cheap tubes in the amp and make sure they all work ok in all sockets. Short of doing that, you'd have to "go inside" and take measurements
    3: Couldn't hurt, but if you want to dodge that start with 1 and you might be able to slip by.
     
  4. NBlair930

    NBlair930 Silver Supporting Member

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    Mike (and Dave),
    Thanks for the quick replies. I will throw a quad of yugo's in there to test later tonight or tomorrow & let you know. I always figured that the amp would sound VERY VERY bad with 1 blown power tube. As i stated earlier, it did not sound like ii remembered, but it did not sound "horribly broken".

    With the replacement set of cheap tubes in place, if there is a major problem with the amp, will it present itself right away, when full power is applied to the tubes? Or after a few minutes of playing?

    Thanks again,
    Noel
     
  5. NBlair930

    NBlair930 Silver Supporting Member

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    Update. I pulled the Mullard el84's and replaced with a known-good quad of Yugo-EI's. All is good in the world -amp started up and no problems with the tubes & the amp once again sounds GREAT. I was wrong earlier because the amp did sound VERY VERY bad with the blown tube in, I just forgot how good the amp always sounded (b/t/w the amp in question is a Bruno Underground 30). This amp can do clean & loud nicely. I should note that the el84 that blew actually had a cracked base -could an amp problem have caused a tube base to crack?
    New Questions:
    1. Should I still make a visit to the amp tech?
    2. How can I salvage a perfectly good trio of once NOS Mullard el84's? I have a few other lightly used pair of QUALITY el84's. Can/Should I add a single to the Mullard trio? Cna/should I combine a used pair with a pair of the Mullards to make a quad? If "yes" does it matter what position the pairs tubes take in the amp (position 1/2 & 3/4) or (position 1/4 & 2/3)?
     
  6. davemccarthy707

    davemccarthy707 Member

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    Keep an eye out for red-plating. If it sounds right, and isn't showing any outward signs of problems, I would leave it as-is. As for the Mullards, I think you now have a matched pair with a spare ;)
     
  7. Blue Strat

    Blue Strat Member

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    +1. Play the amp for a few continuous hours with the Yugo's and keep checking for red plating. If it holds, you should be good.

    Anything that electrically matches the Mullards is fine to use to fill out the quad.
     
  8. NBlair930

    NBlair930 Silver Supporting Member

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    When trying to "match" the new 4th tube, is is still safe to use the Ratings figures from when the Mullard Quad was received & first installed a few years ago? These tubes were not heavily used by any means.
     
  9. davemccarthy707

    davemccarthy707 Member

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    Hmmm, a lot depends on who did it, and on what machine. They best way is if you know where you got it, then get one from the same source (providing they still use the same equipment). If not, you will have to do it yourself by trial and error in your amp by testing bias.
     
  10. Blue Strat

    Blue Strat Member

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    This^^^. Let me know if you're interested.
     
  11. NBlair930

    NBlair930 Silver Supporting Member

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    Mike,
    Phillips branded Mullards i got from you for a prior Underground 30 i owned many many years ago -so long ago that i thnik i had hair back then :)
    I will see if i can find the tube boxes as i usually keep them in a bin somewhere. You think you might have a single to match?
     
  12. Blue Strat

    Blue Strat Member

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    A single of SOME brand, probably not a Mullard.

    Please contact me directly. I don't wanna get banned for spamming the board (again).
     

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