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    Dynaco ST-70 Bias problem

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    Albert Isaza

    Posts : 2
    Join date : 2011-03-10

    Dynaco ST-70 Bias problem

    Post by Albert Isaza on Fri Apr 01, 2011 12:12 pm

    Hi to all, I'm new to this forum.

    Some years ago I purchased an used Dynaco ST-70 kit.
    A few month ago I replaced the old selenium rectifier
    for 1N4007,470uF.100V,10K resistors.

    On the PC-3 installed new resistors,new orange drop 715P/600V capacitors,new sovtek 7199,the EL-34's and the rectifier tube GZ-34 are good.


    The problem I have now is that I can not adjust the
    bias on the left output, the digital meter reads 2.5V
    with the pot turned counterclockwise,if I turn it clockwise the meter gives me more than 3 volts.

    The right output is adjustable to 1.56 or less.

    Voltage test points on can cap A=278V, B=348V, C=418V and
    D=418V.
    I appreciate your help and thanks in advance.
    .
    albert.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco ST-70 Bias problem

    Post by Bob Latino on Fri Apr 01, 2011 6:06 pm

    Hi Albert,

    A diode has more forward flow voltage than a selenium rectifier so the two 10K resistors on the 7 lug terminal strip must be made a smaller value. What is commonly done is to parallel each of the two 10K resistors with two more 10K resistors effectively giving you two 5K resistors. This should allow you to properly bias your amp again.

    Bob

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