Microphonic 6V6s in a Blues Junior

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Yoda, Apr 14, 2015.

  1. Yoda

    Yoda Member

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    Hello all,

    I just got my Blues Junior back after having it converted to octal power tube sockets. The power tubes are NOS Sylvania 6V6GTs. The tubes are a little bit microphonic when playing the lower strings but if I touch the tubes the glassy ringing sound goes away.

    Are there tube dampers available to reduce the microphonics?

    [​IMG]
     
  2. billyguitar

    billyguitar Member

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    Get some red O rings that fit from a hardware store. That should stop it.
     
  3. Yoda

    Yoda Member

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    I was thinking of something that puts spring tension on the tubes. I don't think rubber rings will be effective.
     
  4. Blue Strat

    Blue Strat Member

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    Retainer springs are so often problematic, in themselves, that half the time THEY're causing the rattling that's being blamed on the tubes.

    Dampers DO work in some scenarios. They change the frequency that the tube will vibrate at which is at the core of the issue.

    What do you have the bias of the 6V6s set to?
     
  5. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Some combo's can and do rattle the crap out of tubes. EL-84's are quite durable in that regard
     
  6. TimmyP

    TimmyP Member

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    I dunno. In AC30/6TBXs, I think I've replaced more microphonic EL84s than 12A*7s.
     
  7. Yoda

    Yoda Member

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    They are biased at 27mA.
     
  8. Trout

    Trout Member

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    All the tube dampeners in the world cant fix a tube from being microphonic once the internal parts begin the rattle.
    Combo amps just do this, they rattle the heck out of them, they get loose on the supports & mica spacers and game over.

    At best they can change the frequency of the audible noise being emitted from the tube/glass, but the microphonics?
    Nope. Not going to happen.

    Those parts inside do not stabilize just because of a rubber band on the bottle.

    YMMV but that's my experience.
     
  9. Blue Strat

    Blue Strat Member

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    If the plate voltage is near 400 you should be closer to 20mA.
     
  10. Blue Strat

    Blue Strat Member

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    Sometimes the issue is the base of the tubes rattling against the face of the sockets, not a tube problem at all. A customer solved this, at least temporarily, with non conductive duct tape. Gasket material would be a good choice.
     
  11. kbgear

    kbgear Member

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    This. I've had modern tube sockets which fit new make tubes very snugly, but NOS RCA (for example) are less firm and don't fully seat to the socket, like the guide pin is a tiny bit too long.

    I was kinda surprised no one makes something like this -- like a 1 to 2/64" thick disc of high-temperature silicone, which would also help absorb vibration.

    Hmm, looks like sheets of similar stuff are available, I'll have to poke around.
     
  12. DeaconBlues

    DeaconBlues Member

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    Your bias setting seems to be too high. As per Blue Strat, check your plate voltage.
     
  13. billyguitar

    billyguitar Member

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    I've used red O rings on KT88s. They work fine. BTW: the KT88s made a tinkling glass kind of sound.
     
  14. Blue Strat

    Blue Strat Member

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    I must say that I particularly enjoy people who come here with problems who suddenly know what WON'T work, without trying, better than the people they're asking advice of...yet continue to have the problem. Gotta love web forums. ;)
     
  15. Yoda

    Yoda Member

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    Hey now, I said I didn't think they would work, not that I know they won't! I'll look into them since multiple people said to try the rings.

    I'll check the plate voltage tonight when I have time.
     
  16. pdf64

    pdf64 Member

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  17. schmidlin

    schmidlin Member

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    Definitely liking this thread :)
     
  18. Yoda

    Yoda Member

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    I measured the plate voltage at 335v. According to Weber's bias calculator the bias should be 25mA.

    These tubes are not matched so one consistently measures 5mA higher than the other. How should I set the bias in this case?
     
  19. Yoda

    Yoda Member

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