SVT3Pro Bias Setting

Discussion in 'Amps and Cabs [BG]' started by arnocl, May 25, 2008.

  1. Foz


    Jul 26, 2008
    Jax FL USA
    How I did it...

    Check em cold - fire it up with no speaker connected - take your measurements and right em down - turn it off

    Calc your current averages per bank and from that estimate to what mv reading your hottest one needs to move to achieve the 25mv average. hook your clips to that one - fire it back up and twiddle the bias pot [gently gently catchee monkey] till you have that mv on the hot one - verify you have about 25mv back checking the other again - then plug in the speaker and sample.

    I think I gave up twiddling when I got the average per bank +/- .5 mv of 25 - couldn't get it spot on.

    FWIW I probably wouldn't raise it much higher than 150% of spec [37.5mv] across any single resistor even if that left me a bit low on average - YMMV.

    Good Luck
  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    What Foz suggests should work. There is a small tendency for the differences to even out at higher currents, so check to make sure you really have the average correct after you think you nailed it.

    BTW, the tubes do NOT affect the bias. if they do, there may be a leaky capacitor.
  3. Foz


    Jul 26, 2008
    Jax FL USA
    Thanks for your help JT
  4. fisi_badtoworse


    Aug 13, 2009
    While you are doing bias ajustment, What would be the amp potenciometers possition?
    What tube trademark it´s right for the driver?
    What´s the diferent between using12au7A or 12au7wa?
    What´s better high gain or low gain for HI-FI (without microphonies and noise)?
  5. Foz


    Jul 26, 2008
    Jax FL USA
    doesn't matter

    i don't understand - I used current manufacture Tesla 12ax7's and a JJ 12au7 if that helps

    the 12au7 is a basically a low power version of the 12ax7 class - you need to use one of these in this amp

    probably a normal power rating versus high power would be what you want
  6. TomChumpell


    Nov 1, 2007
    Hi. Does the mV multimeter/multitester have to be set on AC or DC voltage for the mesurement through the source resistors ? Thanks a lot for your help, JT and you all.
  7. BassmanPaul

    BassmanPaul Inactive

    The 12AU7 has less GAIN than a 12AX7 but it has a higher plate dissipation so therefor is higher powered.

  8. Foz


    Jul 26, 2008
    Jax FL USA
    and provides less power to the gain structure...

    What's your game?
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    its a DC measurement. No signal, no speaker plugged in. Be careful, the adjustment is touchy, goes from not much to red hot bias in a small amount of turning.
  10. TomChumpell


    Nov 1, 2007
    Hi Jerrold. Thanks again for your help. This amp sounds so much better when the bias is setup properly. Have a nice day.
  11. matkeller


    Apr 20, 2004
    +1M on this assistance from Jerrold Tiers!!!

    I purchased a brand new SVT-3pro and when I received it sounded ok cranked up at gig volume but at low to medium volume it had an unpleasant distortion, especially on the end of notes. I put it in the box to return it.... Then I decided to google the issue.... and boom! Talkbass came through for me.

    Thanks to the help here I measured the mosfet bias and the average on the 2 banks of mosfets was about 1.2 mV instead of 25 mV! No wonder it sounded bad.

    As noted the trim pot is very touchy. Knowing this I moved it the most imperceptible amount I could. It went up to about 9mV on the first device. I nudged it a tad more and I was at 44mV! My next try put me back at 1mV, next try 15'ish, then I got roughly a 23 mV average across the devices:

    Blue dots            Red Dots
    -----------          -----------
      23.7                 18.5
      20.9                 24.6
      22.1                 26.2
      24.2                 24.8
     ---------            ----------
      22.7                 23.5
    I'm VERY happy with this amp now, especially after popping some JJ tubes from Eurotubes in there. Jerrold Tiers is the real deal. Thank you sir!
  12. I really want to try this bias adjustment, but I'm going crazy looking for these resistor banks. How do I find the resistors that I'm supposed to measure? I don't see any sort of red or blue dots anywhere inside this thing. My amp was made by SLM in September 2002.
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The resistors in question are the large ceramic rectangular items located in the middle of the heatsink.....

    For the record, I actually don't recommend people do this adjustment..... people who can will know they can, others should probably not........ if you don't watch out, you can do expensive damage.

    Without being too picky, may I suggest that people who can go ahead and adjust the bias are folks who would instantly identify and locate the resistors.......... A word to the wise.......
  14. Hey thanks, I actually realized which resistors they are right after I posted and felt like a real dumb dumb. Then I felt even more dumb when I realized that my multimeter is really cheap and doesn't measure mV. :facepalm:

    Anyway I'm going to hold off on this adjustment until I get new tubes and have my good meter from work.
  15. matkeller


    Apr 20, 2004

    Agreed! Get a tech to do it if you don't have electronics/amp experience under your belt. I know my way around schematics and amps. But working on a live amp and being forced to use both hands at once (!!!) to take readings in the output section.... Man if this doesn't make you nervous as hell, then you shouldn't be in there poking! You can easily short out components in the amp or yourself! Either way you're putting a lot at risk to avoid a minimum shop fee.
  16. Well I do have some electronics experience, although only with low-voltage circuits. This is why I didn't recognize the high power resistors at first. Actually, that's not totally true. I've done a good amount of high voltage work installing breakers and running power in venues/studios. This isn't as complex as working on an amp, but if working around busbars with 220V across them isn't scary, I don't know what is.

    As far as putting two hands in the amp, the tip I've heard is to clip one side of your meter with an alligator clip (w/o power connected), and then you only have to focus on one side, plus you only use hand at a time.

    Anyway, I do appreciate your concern, and I'm open to any tips you have about moving into working with high voltage/ AC circuits. It's not about avoiding a minimum shop fee. I simply like doing things myself.

    Thanks again guys.
  17. cellarghost


    Feb 10, 2011
    Hi All

    I know this thread is old - but it's still relevent!
    Just a quick note to say THANKS! Jerrold and others for the biasing info. I too bought a 1995 SVT 3 Pro off ebay and it had the fuzzy sound associated with the low bias voltage. Using the info here I was able to adjust it myself last night - and what a difference! They measured 1mV to 2mV when I first measured it. Now they're around 24mV averaged. I had even bought all new tubes, but now I don't think I even need them. I will change them out though when I get the new fan I ordered - also from info I got here. You guys are the best. I was so happy, I became a supporting member!

  18. BassmanPaul

    BassmanPaul Inactive

    Sorry - just saw this and it makes no sense to me.
  19. Grif


    Jun 23, 2009
    Green Bay, WI
    Awesome thread! Much needed info not found elsewhere.

    I recently had my Pro 3 in for service. They supposedly went over everything and couldn't find anything wrong or needing replacement. I got home and it still had no thump and the fuzz was still there. I popped the top and the bias was 0-1mV average. I cranked it up to 20 mV average to be conservative and BAM! Back in buisness.

    I was so displeased with it until I adjusted the bias. Since I purchased it, I was never quite satisfied with it. It just kept going downhill. I was going to sell it off cheap figuring it was burned up. Now it's loud and has the tone!

    This info saved my amp! Now it's back in the running with my Pro 4. I now feel comfortable taking either one to a show. It may take over as my main rig!
  20. cowpoke


    Jul 23, 2009
    Hey guys, I just set mine, and the big resistors in the front of the power amp board, left of the bias pot (R21 and R63) are hot as hell. I measured them with my temp gun and they are 170 degrees F. Everything else is running ok. Heatsink temp is around 95F. Is this normal or do I have another problem. I am afraid to let this thing run very long with those ceramic resistors so hot.
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