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Ampeg svt3 tubes

Discussion in 'Amps and Cabs [BG]' started by shatner, Dec 19, 2004.

  1. shatner


    Sep 22, 2004
    Isle Of Wight, UK
    Just about to replace the tubes in my SVT3pro and was wondering what the 12AU7 does. All the others are 12AX7's. Can I replace the 12AU7 with a 12AX7?
    I have ordered a 12AU7 anyway but read on the forum somewhere that somebody had replaced them all with 12AX7's.
    I'm buying Electro Harmonix ones. Anybody got any tips on replacement brands?

  2. No, there's a difference in gain.
  3. notanaggie

    notanaggie Guest

    Sep 30, 2003
    And a difference in drive capability.

    The 3PRo is supposed to be "tube driven mosfet", so presumably the 12AU7 is used to get some more current drive than the 12AX7 would provide.

    Mosfets do take current to "charge up" the gates, even though they don't actually "draw" current. They look like a good-sized capacitor from the driving side.

    That's more important to the people doing switch mode supplies than amplifiers, but there should be some of the effect in an amplifier.
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The 12AU7 does drive the mosfets. I don't know what effect using a 12AX7 would give, never tried it.

    I would expect it not to work as well, since the circuit is designed to use the 12AU7.
  5. Gonzo64

    Gonzo64 Bass Ackwards

    Apr 7, 2012
    Towson, MD
    I just purchased a second SLM-era SVT3Pro, and after a very positive experience (Thank you, Jerrold!) tweaking the MOSFET bias, I took the lid off this one to perform the same miracle on my new(er) amp. The first thing I noticed was a complete set of brand-new Groove Tube 12AX7s. ( No 12AU7 to be found!). The initial measurements gave me an average of 2.4 mV n both banks. After adjustment, I landed on approx. 23.9mV, and decided to call it a day. I used the amp for about 4 hours practicing that day, and decided it would be ok for band rehearsal the following day. Living in a townhouse, I wasn't able to really push it, but I cranked it up as soon as I got to our rehearsal space. The amp performed beautifully all day. Should I bother replacing the one 12AX7 with the proper 12AU7?, and will I need to pre-emptively reduce the bias current before powering-up since this tube drives the mosfets? Jerrold, I never spoke with you directly, but t
    he wealth of info you've posted on here qualifies you for a big "thank you".