SVT-CL won't bias properly

Discussion in 'Amps and Cabs [BG]' started by two fingers, May 13, 2018.

  1. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    So, I can't get the thing to bias. I'm trying to avoid cranking the screw left and right hard. Just minor tweeks and play for a while. But it just won't settle in properly biased.

    It's a late SLM era.

    I recently bought it used so I don't know the age of the tubes.

    What should I look for? What should I try?

    I'v done a little searching and several threads point to the 12au7 tubes. There seems to be a debate over "gain matched" or not.

    But that's as far as I got before getting lost in a sea of information that didn't help.
    Last edited: May 13, 2018
  2. Hey TFs.
    Do you have Ampeg's info on how to set the bias?
    Certainly they have a proceedure that involves setting a voltage at some test point.
    If you can hit the proper bias voltage and it still doesn't play right, then tubes, or something else could be amiss?
  3. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Hey thanks.

    Yeah this era has the bias lights that supposedly tell you when bias is set properly.

    Green is proper bias. I can't get them to stay green.

    It sounds a little "weak" when the lights turn red too.
    Old Garage-Bander likes this.
  4. Just so I understand the lights...
    Are they supposed to be green when the amp is at rest?
    Amp bias is normally set with the amp at a rest, not being played through.
    The lights may be your indicator for adjusting the bias in leu of not having a meter and accessing a test point.
    It might be normal that they change during play.

    See if the manuals have any info, or google, bias settings for [make model].
    If all else fails contact Ampegaha.
    two fingers likes this.
  5. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    Usually not always but

    Most likely a old tube went bad
    And it came close to taking out a screen resistor and they are open or really out of tolerance from the tube failure.

    Good time to check that board and likewise since its a pain in ass to remove it. Inspect the board and touch up solder connections and check large ceramic resistors for tolerance.

    And of course do voltage checks and make sure bias supply voltage is good. And if not check diodes and caps in bias supply

    More likely screen resistors. Easier replace all of them with factory rated flameproof. Dont want them haunting you.

    Could even be a bad tube in the bunch. Most likely be same socket with bad screen resistor
    two fingers likes this.
  6. The bias lights should be set to green when the amp is at rest. They will NOT stay green while you are playing. This is normal.
    two fingers likes this.
  7. beans-on-toast


    Aug 7, 2008
    The amp also has to be in playing mode and warmed up when setting the bias.

    Assuming the amp is fine, not being able to set the bias could be an indication that the power or small driver tubes are too worn down. If you are a gampler, try changing the 12AU7 and 12AX7 in the power amp chassis first. That’s a low cost option. Next step is to test or if that is not possible, change the power tubes.

    Of course, there are other possibilities if the amp circuits are not performing up to spec. Measuring the DC voltages and comparing them with the references on the schematic will help diagnose whatever is wrong.

    It wouldn’t hurt to have a tech look at it at this point.
    two fingers likes this.
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Proper step by step troubleshooting techniques will identify the actual problems, which once identified will make clear the necessary repairs.
    two fingers likes this.
  9. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Yes, Oby-Wan.

    agedhorse likes this.
  10. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

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