1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

SVT-3 Pro: Retube and Caos

Discussion in 'Amps and Cabs [BG]' started by drews.anderson, Oct 20, 2013.


  1. I decided to retube my SVT-3 Pro with better quality tubes. Got some TungSol gold to replace the Groove Tubes 12AX7, but decided to keep the EHX 12AU7.
    Turned the Amp on.
    The Gain light (PEAK) didn't even flicker, despite any gain setting.
    Also, got a horrible amount of distortion, for ANY gain setting, from none to max.
    The Tubes are new, and worked ok on a friends Marshall guitar amp.
    Putting the Groove Tubes back got the SVT-3 Pro to work ok.
    Did I do something wrong?
    By Caos I meant Chaos.
     
  2. Foz

    Foz

    Jul 26, 2008
    Jax FL USA
    you got a bad tube.
     
  3. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    Yeah, possibly one or more of the tubes aren't up to spec, and also, the MOSFETS may need re-biasing.
    When all else fails, it's good to have a trusted amp tech.
     
  4. maybe I'll just drop it. Too much trouble to change an amp that already sounds good...
     
  5. I have the same head and have been thinking of changing up the pre tubes but am terrified of this bias thing, you know, if something goes wrong and I end up having to adjust the bias, I ain't so tech savy. Where's Teirs when you need him? GLWTT
     
  6. jeff62

    jeff62 Supporting Member

    Oct 24, 2005
    Orlando, FL
    Bad tube. The MOSFET bias is not affected by the preamp tubes. That said, these amps are notorious for needing that bias set. It's easy to do and there's a thread on the forum someplace about it. Set once and forget it.
     
  7. Lo-E

    Lo-E

    Dec 19, 2009
    Brooklyn, NY
    All mosfet biasing aside, try swapping those tubes one at a time instead of all at once. Then you'll know which tube is dead.

    As jeff62 pointed out, the biasing has nothing to do with the preamp tubes (although it is a worthwhile thing to set).
     
  8. I did that, already. Sounded bad with each one of the tubes. About the Bias, one of the 12AX7s goes on the power amp drive section...
     
  9. Kinda off the subject, but how much is a fair price to have a tech perform a MOSFET bias on the ampeg pro 3?
     
  10. jeff62

    jeff62 Supporting Member

    Oct 24, 2005
    Orlando, FL
    IMO, the fair price for the bias is probably the base charge the tech quotes you to open the amp. The actual time it takes is 5 min to undo the top, and about 10-15 min to set the bias. Less than that, really, but I'm being generous. While he/she is in there they should be blowing the extra dust out of the fan, looking for anything unusual, ensuring tubes are seated well, etc.
     
  11. "New" tubes these days does not mean GOOD tubes. Failures of new production tubes are common and things like grid leakage and inter-electrode capacitance are not held to the same strict tolerance as they were in "the old days". Tube operation in circuit is much more complex than most people realize.
     
  12. Psychobassguy

    Psychobassguy Banned

    Oct 14, 2013
    Sneedville
    Just because tubes have sockets and can be changed by someone without a soldering iron doesn't mean that they should be. The SVT3 Pro is a great sounding amp with a temperamental and overrated power section. If you want it to keep working, don't go messing with it. Others have said that its output stage bias is easy to adjust. In my experience, it is anything but. It is a wonderful recording and mid-level stage amp though.
     
  13. I'll never try to do it myself. even the size of the capacitor frightens me.
     
  14. svtb15

    svtb15 Banned

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig. Q+
    The 3 PRO has 5 tubes, 3 in the preamp.
    2 in the poweramp section... Those 2 handle the Inverter and driver of mosfets... TUBES drive the Mosfets...

    If the driver tube is different than the previous original tube, It will throw balance in the Bias off.. Put the old tube back it will be fine .. since that is what the amp was set up with originally..

    Changing the 3 preamp tubes will have no affect.. Only the the 12au or 12xy in the power amp section.

    Amp will need a Re Bias

    If you dont know what you are doing.. Leave it alone and take it to a tech...
    Bias to Cold and it sounds real distorted... Bias too HOT, and the fan will kick in , and the amp will get SUPER HOT and you could fry it in seconds..
    Take it to someone

     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.