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  1. #1
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    Default Cybertwin (orig) which 12ax7 is bad?

    I've got an old Cybertwin that has served me quite nicely for many years.

    Recently, the sound became intermittent, low level and distorted on some of the sims.

    It is mostly solid state but uses two 12ax7 pre-amp tubes (sometimes just one and other times both depending upon the sim).

    Apparently, several of the sims work perfectly but others consistently fail so, I surmise that only one of the tubes is bad.

    Does anyone know how to tell which is the bad tube, by knowing which sims work versus those that don't.

    I've got a collection of various 12ax7 tubes from several manufacturers. The tubes inside are Sovtec, overprinted with Fender in red ink, but I've also tried to vary the sound quality using other brands. Tube selection guidance would be appreciated!

    One reason I'd like to know which sims use which tube(s) is so that I can select different tubes based upon their sonic characteristics.

    Thanks in advance!

    BTW I will post what I learn from the results of my own experimentation.

  2. #2
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    Default

    Couple of options, assuming you don't have a tube tester. Fire up the amp and tap both with the eraser end of a pencil. If either makes a weird noise, there you go. Or you can just switch each one out for another 12ax7 tube (ideally one that you know is good, or with a few different ones if you aren't sure). Switch out one and try the amp, and if there is no difference, switch out the other. If the problem persists, that would suggest a different culprit.
    The Human Torch was denied a bank loan.

  3. #3
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    Default

    My thought would be to just replace them both with tubes of your own choice. It sounds as if they're either original tubes or at least have not been changed since you've had the amp. Put 'em both in an move forward...
    Bottle Rocket Scientist.....

  4. #4
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    Default

    I'd buy two new tubes and swap them out one at a time to determine which is the bad tube. I'd then put both new tubes in the amp and keep the one good original as a spare.
    I'm a riddle, wrapped in a mystery, inside an enigma.

  5. #5
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    What the good Cap'n said.
    Not to dis the Bad man, but after all, he IS a badxmpl...
    The Human Torch was denied a bank loan.

  6. #6
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    No dis noted. The Cap'n just added a step to my suggestion (which had stopped short of diagnosing). I personally wouldn't keep a 10 year old tube once it had been removed. Just sayin'. I suppose it's possible that a SIMM is Bad? in which case the "diagnostic procedure" of swapping one (possibly good tube for another) wouldn't determine much except to aim suspicion in another direction....

    Good morning, All!
    Bottle Rocket Scientist.....

 

 

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