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    STA70 Bias Tolerence

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    Bugs762
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    STA70 Bias Tolerence

    Post by Bugs762 on Fri Apr 24, 2009 10:31 am

    What is an acceptable tollerence when you set tube bias? When I set it I can get it down to tenths of a milivolt and then it wanders a little and drives me nuts. I'm setting the bias on an ST70 VTA board to .4 V.

    Thanks

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: STA70 Bias Tolerence

    Post by Bob Latino on Fri Apr 24, 2009 12:58 pm

    Bugs762 wrote:What is an acceptable tollerence when you set tube bias? When I set it I can get it down to tenths of a milivolt and then it wanders a little and drives me nuts. I'm setting the bias on an ST70 VTA board to .4 V.

    Thanks

    Bugs,

    The bias will go up and down slightly as your line voltage goes up and down. If you have it set at .40 volts it may wander a little between .39 and .41 - this is nothing to worry about. Sometimes a tube that is worn will not hold a steady bias. If only one tube wanders it is usually a problem with the tube so replace it. If all tubes wander then your rectifier may not be good. Understand also that bias will not stabilize for at least 15 minutes after you turn the amp on after a cold start. Always set the bias after the amp has been fully warmed up. You should also have your speakers connected. Some amps won't bias accurately without a "load" (the speakers) on the output transformers. Also realize that when you turn the bias UP on one tube the bias will go DOWN very slightly on the other three tubes since all four output tubes "share" the available bias current.

    Bob

    Bugs762
    Guest

    Re: STA70 Bias Tolerence

    Post by Bugs762 on Fri Apr 24, 2009 3:50 pm

    My rectifier must be going bad, all four power tube biases fluctuate +- .03V in unison.

    Thanks,
    Bugs

    Sal

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    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: STA70 Bias Tolerence

    Post by Sal on Sat Apr 25, 2009 9:04 am

    Bugs, also make sure you are not playing any music when setting the bias and use RCA shorting plugs if you have them.

    Sal

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: STA70 Bias Tolerence

    Post by Bugs on Sat Apr 25, 2009 12:54 pm

    Okay, so maybe I've been doing the bias check all wrong. I notice the drift when music is playing. I've set the bias with the amp hooked to speakers and hooked to the preamp. The preamp is on, but all input devices are off.

    Should I disconnect the amp from the preamp and use one side of an RCA cable to connect the left and right inputs to the amp?

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: STA70 Bias Tolerence

    Post by Bob Latino on Sat Apr 25, 2009 1:02 pm

    The bias will drift a little if the music is playing - that's normal. Check the bias with no signal going through the amp. The preamp may be connected and may be on or off but if the preamp is ON the volume should be turned all the way down. When you are setting the bias you are setting the "idle" current so the amp should be at idle (on but no signal passing through the amp)

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: STA70 Bias Tolerence

    Post by Bugs on Sat Apr 25, 2009 1:27 pm

    Okay, got it.

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