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Ampeg SVT-III Pro tubes

Discussion in 'Amps and Cabs [BG]' started by 5stringsofdeath, Mar 24, 2013.

  1. Question for anyone who knows these amps. I opened the top and turned the power on to see what kind of glow I get and how long it takes to warm up. It takes about 15 seconds to get a warm glow out of the 12AX7's but my main concern is on the initial power up. The one tube that's not an AX7 gets extremely bright as soon as I power up, then goes dim and warms up with the rest of them. It's the one second from the top. Is this common? I'm kind of new at the electrical side of things and am curious.
  2. I thought the SVT-III was solid state?

    Anyhow, certain tubes are well known to exhibit this initial brightening. Some Mullards (a very desirable tube) will even show a "flash" when they are powered up. (google "Mullard flash")

    So it's likely not anything to worry about.
  3. SVT III is tube pre-amp section, solid state power section
  4. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    +1 on the Mullards becoming a small light bulb for an instant on power up
    It is extremely unlikely that your amp has a Mullard in that slot; I never noticed on mine but if it is working OK it probably is OK.
    For piece of mind, put a new tube in your gig bag. A plastic medicine container with foam at the top and bottom works well.
  5. Yes the SVT is tubed pre-amp with 5 tubes. I was just wondering because 4 are the AX7s and the other is an unknown because I can't read it( an AR maybe). Thanks for the input!
  6. blackba


    Sep 7, 2008
    Michigan, USA
    As I remember when I retubed my svt3pro, it had 4 12ax7s and 1 12au7.

    Based on what you are saying, I think the amp is working fine.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    It's tube pre and hybrid power.....

    The last 12AX7 and the 12AU7 are in the power amp, they provide all the gain. The mosfet outputs just "follow" the tube output and act as a "transformer with current gain" to drive the speakers.

    Specifically, the 12AU7 drives the mosfet gates and provides "phase inversion" of a sort (push-pull drive, really) for the drive.
  8. goldenglory18

    goldenglory18 Supporting Member

    Nov 30, 2009
    Los Angeles
    Amateur. Take my advice for its resale value.
    I have one on the way in the next week. I can't wait to hear it as I haven't played a III pro in over 10 years...

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