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Discussion in 'Amps and Cabs [BG]' started by Groover, Mar 8, 2010.

  1. Groover


    Jun 28, 2005
    Ohio, USA
    Sorry... I looked in the DIY sticky, but didn't see it and some other threads I found, didn't really break it down enought for me to get it... :ninja:

    If there is a step-by-step on how to BIAS an SVT somewhere?

    I know I can take it to a tech, but I like to tinker....
  2. DaveMcLain


    Jun 19, 2005
    Cuba MO
    Check out the schematic for the amp. On there you'll find a step by step of how to do the job. It tells you what test points to use etc. If you follow the directions the SVT is easy to adjust.
  3. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    the procedure should be written right on the front panel of the lower chassis near the bias pots.
  4. Groover


    Jun 28, 2005
    Ohio, USA
    Ok, thanks. After looking at the front chassis again, it does make sense.

    The Hum Control (V4) is not mentioned though. It that independent? Meaning, turn it until no hum? At any volume or load?

    I guess I need to understand a little more about this...

    What are you guys finding to be the best way to set the Bias? Run the amp for a little, as in play it, then set it, then check again?

    Sort of like check the tuning and intonation on strings? Set them up as best as you can when first installed, then keep checking them until settled?
  5. Hi.

    If You look at the schematic, you'll find the hum-balance-pot on the heater circuit. It's there to fine tune the hum component that's present in the AC. You just turn it to a position where the hum is least noticeable. You may need to re-adjust it as the location changes.

    If the tubes are new, I usually bias them as soon as I turn the HT voltage on. Before that I usually let the tubes to warm up for 10 minutes or so. Then I let the tubes "settle" for 10 minutes or so and re-check the bias. If You don't have an oscilloscope, that's about it.

    I recheck the bias periodically for new tubes for the first about 20 or so hours of operation. Better tubes show no or just a little bias drift, the "not-so-good" ones quite a bit.

  6. Groover


    Jun 28, 2005
    Ohio, USA
    Great, thanks for extra details. The tubes will be new, so I'm gearing up on what I need to get this thing tweaked. Will a decent Voltmeter do? (You mentioned Oscilloscope...)
  7. Hi.

    Any decent voltmeter will do, no need for an oscilloscope for a simple biasing with the test points.

    When a tech biases an amp, he/she will bench the amp, ie. measure the power before clipping to a specific resistive load, determine the general shape of the power tubes etc. That's done with an oscilloscope.

  8. Primary

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    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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