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    ST70 Bias too hot / high



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    Join date : 2014-11-15

    ST70 Bias too hot / high

    Post by chet6120 on Fri Jul 10, 2015 11:15 am

    This must be a common problem but I did a Google search and a search here but couldn’t find a good answer on how to lower the Voltage coming out of the Bias Circuit. A couple of places said add resistors in parallel to the 10k resistors in the 7 lug bias circuit, but each had different values.

    Is this the correct way to solve the problem? If so what value resistors should be used?

    I just rebuilt my ST70. Replaced - the Quad cap (with upgrade), Bias circuit (including diode), all octal tube sockets, and added a Bias Balance Control - all from
    With the Bias pots all the way counter clockwise I get 1.25 Vdc at each of the four test points. (Using a new set of matched tubes.) The readings should be set to 0.40 to 0.50 with 10 ohm bias resisters.

    Thanks for any advice.


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    Join date : 2008-11-30

    Bias to hot

    Post by DynakitParts on Fri Jul 10, 2015 2:14 pm

    Hi Chet..
    Contact me at:

    I will help you through this problem.

    Kevin @ Dynakit


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    Re: ST70 Bias too hot / high

    Post by GP49 on Fri Jul 10, 2015 4:27 pm

    What you really want is a HIGHER output voltage from the bias circuitry, not lower...hence paralleling that resistor, which REDUCES the resistance. What you are measuring isn't the bias, it is the IDLE CURRENT on the output tubes, which you read as voltage at the top of the cathode resistor.

    This confuses the neophyte, so do as Kevin says...consult with him.

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