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Question for the tube gurus/amp techs

Discussion in 'Amps [BG]' started by prd004, Feb 17, 2014.

  1. prd004

    prd004

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    I'm putting new preamp tubes in one of my amps, SVT-III Pro, three 12AX7's.

    Is there any advantage to matching preamp tubes? It's a few extra bucks to match them, which I don't mind, I just don't want to waste money.
  2. Tim1

    Tim1

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    I have only ever worried about matching power tubes. I have never been aware of any need to match preamp tubes, although at times there can be an advantage in getting a balanced driver tube. The experts will no doubt explain this.
  3. nashvillebill

    nashvillebill

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    No need to match preamp tubes. Even a phase inverter tube isn't terribly critical, the phase inverter circuitry itself isn't going to be perfectly matched...and that's where some of the harmonics of the "tube sound" are generated.
  4. Interceptor

    Interceptor Supporting Member

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    Oh boy.

    I think it is time for pitch forks and torches. Snake oil alert!!!!!

    Matched is kind of handy for a pair of tubes working in tandem in a class B circuit. Preamp stages are typically Class A, and tube matching from stage to stage isn't at all a design thought.

    Irritating to me. They cast a shadow of fear and tap unsuspecting wallets.
  5. BurningSkies

    BurningSkies

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    Nope. No real advantage to matched preamp tubes.
  6. beans-on-toast

    beans-on-toast

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    Just to be clear, a 12AX7 is two triodes in one tube. They are talking about matching the two halves inside a tube. Not matching one 12AX7 against the other.

    As others have said, no need to match the triodes, although it wouldn't hurt. If you wanted to spend more money, an alternative out be to buy a low noise version of a 12AX7, such as a 7025, for use in the input stage.
  7. Jim C

    Jim C Supporting Member

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    Isn't a 7027A a power tube as used in a V4B?
  8. BassmanPaul

    BassmanPaul Gold Supporting Member

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    Errr

    David, I think you mean 7025 do you not? :D
  9. BassmanPaul

    BassmanPaul Gold Supporting Member

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    Why are you replacing the pre tubes? They last for decades.

    As for matching, pre tube sections are usually close enough as to not matter. The tolerances of the other components in the circuit would throw even matched sections off. Don't waste your money.
  10. beans-on-toast

    beans-on-toast

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    A dyslexic guy walks into a bra....

    Yes I got my power and pre-amp tubes backwards. That should have been 7025, not 7027 (I'll fix it). I was only two numbers off but it makes a big difference. :p
  11. prd004

    prd004

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    I'm replacing them because the amp is 20 years old and I was cleaning all the pots, switches, faders, inputs, and outputs and changing the tubes.
  12. prd004

    prd004

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    Thanks all got it!
  13. beans-on-toast

    beans-on-toast

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    If they be original, you might want to have the electrolytic capacitors checked out.
  14. P Town

    P Town Guest

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    Dyslexics of the world. Untie!
  15. Jim C

    Jim C Supporting Member

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    Didn't mean this as some big deal correction but rather making sure I was on the right track :hyper:
  16. beans-on-toast

    beans-on-toast

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    I need all the help I can get! :p
  17. prd004

    prd004

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    It's all stock, I am the original owner
  18. BassmanPaul

    BassmanPaul Gold Supporting Member

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    I'd still be hesitant to replace pre-amp tubes that may well be better than the new ones you install.
  19. BurningSkies

    BurningSkies

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    Yes. I'd agree. I'd also be more concerned to take out any components that are past their useful life for 'end tone concerns'...aka the caps.

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