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Biasing a tube amp to run "hot"

Discussion in 'Amps and Cabs [BG]' started by lo-end, Feb 7, 2006.


  1. lo-end

    lo-end

    Jun 15, 2001
    PA
    I want to bias my SVT-II (non-pro) so the power tubes run a little bit hotter. Im looking to get my power tubes to break up at lower volume levels. How much is too much? What should the multimeter read?

    According to my common sense, I think doing this would shorten the life of the tubes so I'm not going to go totally crazy, I just want to coax a little more dirt out of this thing

    Anyone done this before with a tube amp? SVT or other?
     
  2. I'm no expert, but I think the opposite is true. If you want more distortion you need to bias to the cold side, and I think that whether you bias your tubes cold or hot it would shorten the tube life...if you want more power tube distorion you can try putting a higher gain tube in the driver position. Many amps use a 12au7, depending on what yours is you can try a 12ay7 or a 12at7 instead. This would normally hit your power tubes with a bigger signal and push them into overdrive sooner than with the lower gain 12au7.
     
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Not the amp to do that on......... You may shorten the life a little too much.

    If you want more dirt, hit it with a larger signal, turn it up more...

    Biasing the tubes hotter I suppose may pull the supply down in some amps, and that may "dirty it up" some with them, but I wouldn't do it with an SVT. It's already pushing the tubes pretty hard.
     
  4. lo-end

    lo-end

    Jun 15, 2001
    PA

    is this correct?
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    It's going to depend on the circuit....... I don't know if there is a good "general" answer to that.

    The 12AU7 is a low gain higher current tube. The 12AT7 for instance, is a bit lower current, higher gain tube, somewhat intermediate between a 12AX7 and a 12AU7.

    But if the stage in question is a follower, it might make nearly no difference.... so......
     
  6. jz0h4d

    jz0h4d

    Apr 26, 2005
    This is just a bad ideal from the word go. In the first place it won't help you to get distortion at a lower volume, and it will kill your expensive power tubes . Colder bias will give you more crossover distortion, but crossover distortion sounds like ****.

    Changing tube types will change the gain structure of the amp, but that just means you'll hit your max output wattage with the pre amp gain at 4 instead of 5.
    Try a V4B.