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Biasing SVT Classic

Discussion in 'Amps and Cabs [BG]' started by gutsofgold, Sep 18, 2008.

  1. gutsofgold


    Jul 15, 2007
    When biasing an SVT Classic, the manual says to power it on with out an input signal present. Do I want stand by OFF (as if I were playing the amp) or stand by ON (as if I were waiting for the amp to warm up) ???

  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    They mean don't plug a bass into it. I'm sure you'd have to have it out of standby to bias it, since they don't operate at the same temp in standby as out of it.
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    It has to be "on" as if you were playing it (not in standby). It also needs a load. Preferably not a speaker, but a speaker CAN be used.....
  4. Whoopysnorp


    Feb 5, 2006
    Why preferably not a speaker? I noticed the instructions on my SVT-II said to use a load resistor...
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Because with a speaker, noises around the area will show up in the readings.... they feed in through the speaker..... So the readings are not steady.

    Also a resistor is "known", and specifying it means you know what the conditions are. One less question.
  6. gutsofgold


    Jul 15, 2007
    I noticed that the bias is interactive with the GAIN knob. I biased it (using my speaker cab as a load) with the Gain set to where I usually play it. It sounds a lot better but I'm hoping I did this correct.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    There should be NO reason for any interaction...... the GAIN is early in the preamp, and tehre is a lot of circuitry between the GAIN and the poweramp bias.

    Either there is a goodly amount of hiss , or some other signal is coming through which could throw it off. It has to be from the first tube though, possibly hum from power supply? Signal will tend to throw off the bias reading.

    If gain does, I'd expect the ultra-low to affect it also.

    In any case, preamp controls should not affect bias. If they do, you can plug a shorting plug into the "power amp in " jack and eliminate that interference, assuming you don't want to fix it now.
  8. gutsofgold


    Jul 15, 2007
    Thanks I'll give this a try. It's an older SVT classic, has the blue writing on the front and the guy I bought it from was older and had been playing it for years. My guess is these tubes have just seen their day, he couldn't remember when he last replaced them and I've played it for about a year now.

    There was a loud hum when I had the amp on with nothing plugged in to the input. I assumed this was normal because plugging my bass amp stopped that and the amp worked just fine with no hiss or hum. I could even control the hum with the gain knob (making it louder or quieter) and the EQ knobs had an affect on it as well. Now that I read your post I assume this maybe the culprit.

    Will plugging any speaker cable into the "power amp in" jack work or do I need it to be shorted (the pos and neg ends togther)?
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis

    If that much hum, you may have more effects as well........ even with the plug.

    You also probably need some filter caps replaced........... or a jack fixed?????
  10. gutsofgold


    Jul 15, 2007
    My guess is power supply filter cap or grounding on the input jack. I'll have my tech guy look at it before I head out to some shows in a few weeks.
  11. JG WORLD


    Apr 1, 2009
    I am having a similar problem with my SVT Classic. It makes a hum with nothing plugged into the input, It gets louder when you turn the gain up and the eq changes the hum too. Is there a way a user can determine if this is a bad ground on the input jack or do I need to take it to a service technician to see weather this is a bad filter cap or input grounding problem.
  12. IMHO if you don't know that you need the amp ON to check the bias you should not be touching the amp. If you do something incorrectly you could well permanently damage the unit.

  13. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses

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