Just how microphonic is TOO microphonic in a preamp tube? and related questions...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mohowski, Jan 18, 2008.

  1. mohowski

    mohowski Member

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    I've got a new Prosonic, and although I love the overall tone, I was trying to tame the drive channel by popping in some lower gain 12AX7WAs instead of the regular 12AX7s that were in it. I have about half a dozen Philips JAN 12ax7WAs that I got NIB several years ago, and they've been hanging out in a Fender Twin Reverb (nearly unused) since then.

    I tried putting the tubes in V1 and V2, and one tube was giving me an awful amount of hum. Just for kicks, I tried all of the Philips JAN tubes in V1 and V2, and when I tapped them with a pencil, you could definitely hear the sound coming through the amp. With the gains cranked up, 2 of them were so bad i got feedback when i tapped the tube with the volume higher than 3.

    What sucks is the tubes sounded GREAT and were a definite step in the direction I was going. When i ordered them years ago, they were not cheap tubes either. If they are no good in the drive channel of the prosonic, would they still be suitable in a clean amp (the fender Twin Reverb I originally intended the for)? What else could i use them for (reverb drivers, rackmounted preamps)?
     
  2. teleman1

    teleman1 Supporting Member

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    I could be wrong, but tappin them and hearing them should be expected. I think its what you hear. A humm, not good. A tap and a ringing, probably microphonic. But a tap and an amplified sound with no prolonged ringing or hum is probably good. They are pre-amp tubes and I wouldn't expect silence when tapped. I have no idea why this is typing bold print.
     
  3. rockon1

    rockon1 Member

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    I could be wrong but those JAN 12AX7WA's are probably higher gain than the Sovtek 12AX7WA's that came in the amp which are lower gain than original NOS 12AX7 specs.. As long as you can play the amp without it squealing on you than they arent too microphonic to you IMO. Hum is different than microphonic and another issue. The input V1 is usually most suseptuble to microphinc tubes. Try finding one out the the JAN 12AX7's that has the least microphonics for the V1. Bob
     
  4. Blue Strat

    Blue Strat Member

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    JAN Philips 12AX7WAs (not other brands) are typically higher gain, and more prone to microphonics than most any other tube.

    Humming is another problem unrelated to microphonics. If a tube hums noticeably more than all the others, it's not good. Hearing tapping amplified is only indicative of one thing: You can hear amplified tapping. Stop tapping and put both hands on the guitar. You'll play a lot better ;)

    Obviously, if you hear ringing or feedback, the tube is not usable in that particular amp or socket. Tubes like this almost always work ok as phase inverters, tremolo tubes or sometimes later gain stages than V1 or V2.

    Tubes that are too microphonic in high gain amps or overdrive sockets are often fine in vintage type (average gain) circuits.
     
  5. rooster

    rooster Member

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    It's quite simple. The bold print signifies that you dramatically removed your glasses, and used them to gesture, driving your point home. Then, to increase the impact of your statement, you leaned back in your chair, and put the end of one of the earpieces in your mouth and chewed on it reflectively.


    Sometimes ya gotta read between the lines.

    rooster.
     
  6. drfrankencopter

    drfrankencopter Member

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    How microhponic is too microphonic....simple test.

    1. Set up your amp how you typically play it. This means both physical layout (head/cab locations), and knob settings.
    2. Make sure the volume is set nice and loud...like you'd have it while jamming
    3. Set reverb to zero (or switch it off).
    4. Play some hard palm muted E power chords, and mute the strings after each stab....does the amp go 'pling, ting' following each chord. If so you've got microphonic tubes....now, does it bother you? (if yes, swap out some of your tubes),

    Cheers

    Kris
     
  7. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    :rollThat is a classic statement.



    Reminds me of a kind of conversation I've had, more than once, with my 5-year-old daughter:
    ...Her - "My arm hurts when I rub it"
    ...Me - "Then stop rubbing it"

    - Thom
     
  8. rockon1

    rockon1 Member

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    Another way I check a tubes "microphonic usability" level is to put it in the V1 of one of my high gain amps. with nothing plugged into the amp set the gain on full and slowly turn up the master. A tube on the "edge" will start to ring as the volume is turned up without anything plugged into the amp. If it doesnt start ringing its good to go. Sometimes they will start ringging at a volume level I'd never reach and thats usually good enough to use too. Bob
     

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