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Preamp tubes

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Jon, Jan 31, 2012.

  1. Jon

    Jon Member

    Oct 20, 2004
    Birmingham, UK
    Is there any difference between a 12AX7, a 12AX7A and a 7025?

    The tube diag on my 73 Twin shows a mixture of 12AT7s, 12AX7As and 7025s but every tube vendor seems to class 7025s and 12AX7As as exactly the same as 12AX7s - just wondered if this was the case and if so, why Fender bothered making the distinction in the diagram?
  2. Ronsonic

    Ronsonic Member

    May 14, 2006
    Sunny Tampa, Florida
    7025 is a low noise version. Saving money was the issue with those reserved for the higher gain positions.

    IIRC The "A" indicated a controlled warm up time tube. It won't have the flare on power up that the Euro versions and older numbers did.
  3. chervokas

    chervokas Member

    Feb 16, 2008
    7025 was, yes, a low-noise 12ax7....and 12ax7a could be used in circuits with parallel or series filament heaters, which used to matter but doesn't really matter today because nearly all audio circuits and guitar circuits you're going to come across -- unless you're using obscure, really old vintage stuff -- uses parallel filament wiring.

    But yes 7025 and 12ax7s are electrically identical and are direct substitutions, and for all intents and purposes today 12ax7s and 12ax7as will directly substitute for one another. Also, I'm not too sure there's all that much difference between modern production tubes labelled 7025 and 12ax7 today. It's not like the old days when millions and millions of tubes were sold for electronic gear used in everything from airplanes and medical equipment to ham radios and big bulk customers made sure when they wanted "low-noise" 7025, they got 'em.
  4. Rockledge

    Rockledge Member

    Jun 15, 2011
    In case you don't know this, be careful to not try to use a 12 AX7 in place of a 12 AT7.
    AT7s , if I remember correctly, have a lower gain level.
    Again, if I remember right the lower the letter, the lower the gain level. I have put AT7s in place of aX7s to make a preamp overdrive sooner, but I wouldn't ever consider doing it the other way around.
    I have read that putting an AX7 in an AT7 slot runs the risk of overdriving the circuit by trying to push too much current through it.

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