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Preamp tube matching and SPax7??

Discussion in 'Amps and Cabs [BG]' started by FireBug, Apr 9, 2006.

  1. FireBug


    Sep 18, 2005
    Well, the first thing I did when I got my Bass 400+ a while back was open it up and clean it out. Dusted all the crap off of the amp and the Mesa tubes and then plugged them back in. I must say that this amp is badly in need of a retube. Five of the power tubes date back to 5-17-97:rollno:.

    I also noticed that the preamp tube (Mesa) that is in the furthest left position is called a SPax7. Any clues as to the difference between this and a regular 12ax7?

    Also, I read somewhere that 12ax7s self bias to the circuitry they are in. Is this true? Why would Bob at Eurotubes be offering matched 12ax7 sets then?

  2. Why are you thinking you need a retube? Just needed to ask the obvious 1st question......

  3. yeah, why? my SVT-II needed retubing last year, and it was the original power tubes that were taken out (ie from 89), and it had been a well used amp (touring band and a rental, and one of the tubes was totally dead the other 5 on the way out)

    So, just because they are from 97 doesnt mean they are crap, if it still works fine then you dont need a retube

    Also, i think SPax7's are just 12AX7's, aslo, i think you are mixed up, you do not need to bias 12ax7's or any other preamp tube as far as i am aware, ever, where are you seeing bob selling matched 12AX7's? you can get balanced 12AX7's and triode matched 12AX7's, but that just means the two halfs have the same voltage draw (IIRC), because each pre tube is two circuits, IE 12DW7's which are infact one half 12AU7 and one half 12AX7

    I'd recommend you find out at least a little about tubes and stuff before you go about pulling things out and putting things in

    - Will
  4. Bias a preamp tube? Never seen that done. Nor encountered matched sets of preamp tubes. Don't know what that's about, someone else probably will. The SP12ax7 is IMO Boogie's best preamp tube, quiter than many 212Ax7s. I've had guitar amps that screamed feedback helplessly wiht Rubys quiet right down with SP12AX7s. I like them better than JJs, but they're expensive. Mine had 4.
  5. They are expensive? your gonna hate this, they are just rebranded sovteks or something (probably not sovteks, but something of that ilk), they are not really anything special other than Mesa putting thier name on them and selling them for more IIRC

    - Will
  6. I'm aware they'r erebranded, as are most tubes rebranded. Often Sovteks. They are tested to Mesa's specs, just like Groove Tubes, EH, or anyone else tests tubes to their specs. I like them, and that's all that matters to me.
  7. FireBug


    Sep 18, 2005
    I'm getting microphonics and wooshing noises.
  8. Oh Heaven forbid!!! Ten years old! I guess I'd better not tell the 35+ year old tubes in my SVT that they're too old, then. :rollno:

    Seriously; the best way to tell the health of tubes is to measure the amp's output with them in it. Even tube testers are just educated guesses in most cases.

    Yeah. Mesa charged more for what everyone else calls a 7025 / 12AX7A. You don't HAVE to use one. Any 12AX7 will work and sound just fine.

    That's called cathode bias, and yes, it's true.

    Because lots of people with more money than sense insist on "matched" preamp tubes, even though in most cases, each section (12AX7's are dual triodes) is already performing a dissimilar gain function. Bob offers them because it's easier than explaining to people that they don't need them

    Mesa's "specs" are just a scam to sell tubes under their name. Very often, their "bias matched" power tubes don't even hold to their own 'ratings,' and I've personally had BRAND NEW Mesa branded power tubes fail straight out of the box. So much more "specs."

    That could be tubes or it could be caps or resistors or a multitude of other things. 90% of problems with tubes amps usually ARE NOT the tubes.

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