Microphonic Tubes

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by CarlWerkmeister, Jun 22, 2003.


  1. CarlWerkmeister

    CarlWerkmeister Silver Supporting Member

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    How is the best way to check to see if a tube is microphonic?
    I have been using the "tap test", lightly tapping on the side of the tube and listening to whether I can hear it through the speaker. Is this a valid test? What is the best way?
    Thanks.
     
  2. Braciola

    Braciola Silver Supporting Member

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    That method works for me. (I like using a chop stick best)
    It is normal to tap the tube and hear it through your speaker, it is microphonic when you tap it and it rings like a bell.
     
  3. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    I agree. By design, all tubes are more or less "microphonic." Just as with PUPs, it's only a problem when the microphonics get in the way of the proper performance of the amp, e.g., if they squeal, oscillate out of control, yield harsh/audible/unusual overtones (at one or more frequencies), etc.

    Jim Collins always says he actually likes slightly microphonic PUPs, he feels they're more alive/sensitive. I've wondered if there is an analog to this effect with tubes, too.
     
  4. Jim Collins

    Jim Collins Member

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    Sometimes, the construction of the amp can get in the way of the time-honored tap test. I used to play a Mesa Maverick, 2x12 combo. This amp had board-mounted tube sockets. Furthermore, the preamp tubes were all mounted, very close together, on their own, small board. This board, and its sockets, were recessed in the chassis. At a gig, one of my preamp tubes suddenly went horribly microphonic. I mean, the amp was completely unusable -- horrible whistling at any level. I always brought spare tubes, so all I had to do was identify the tube, and replace it. (Actually replacing the preamp tubes in this amp is a lot harder than you might think.)

    I removed the bracket that clamped all the preamp tubes in place, and began lightly tapping. The vibration transfered from the tube being tapped, to the small PCB the preamp tube sockets were in, to all the other preamp tubes. The net result was that when you tapped any of the preamp tubes, no matter how lightly, the all vibrated, and you got the microphonic ringing.
     
  5. Ron

    Ron Supporting Member

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    The liveliness of PUs with slight microphonics is directly analogous to microphonic tubes. The microphonic nature of both can lead to positive electronic feedback in the form of increased sustain.

    If the feedback is not just right, it can get ugly - uncontrollable, loud, ear-splitting squeeling!
     
  6. Carstens

    Carstens Member

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    sometimes the microphonic tube can be coming from the tubes next to where you think it is. also, some times tubes just dont like where they are and if you move them to a different socket your problem could go away. just get a new tube and swap until u find the culprit.
     
  7. Champlifier

    Champlifier Supporting Member

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    I like to use the eraser end of a pencil as my tapping tool.:horse
     
  8. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I'm of the thought that one should never tap hot tubes. The more you provoke them, the more likely the damage (such as microphony) will become permanent. Vacuum tubes are more vulnerable to damage when they are hot.

    If you want to trouble-shoot to identify a microphonic tube, simply replace the tubes, one at a time, and listen for a change. Your microphonic preamp tube will likely be one closer to the input, so start with V1. Microphonic power tubes don't squeal as much as they rattle - like a tambourine sound.

    Again, my advise is to stop whacking your tubes :jo and get back to playing your guitar.

    - T
     
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  9. Rosewood

    Rosewood Member

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    My rule is if the tube stops playing it's song when I stop playing mine it's good for while longer. I've given up on any tube not being slightly microphonic, at least in v1.
     

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