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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 Tue Apr 05, 2011 12:55 pm

    Hi Bob

    Thanks for replying to my post.

    I replaced the 10K resistors for 5.1K, then I turned
    the amplifier on and after about 20 minutes later
    I could rebias the output tubes on both channels
    to 1.40 volts.

    Thank you for your help.

    "Best Dynaco Forum"

    albert.

    Bob Latino
    Admin

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

    Re: Dynaco ST-70 bias problem

    Post by Bob Latino on Tue Apr 05, 2011 3:47 pm

    Hi Albert,

    What happened to you when you replaced the selenium rectifier with a diode and found an improper bias range is a common occurance. Because a diode has more "forward voltage" than a selenium rectifier, those two 10K resistors on the 7 lug terminal strip have to be made smaller to get the bias into the proper range. You are also wise to bias the tubes a little lower than Dynaco recommended. Biasing at the "1.56 VDC" that Dynaco recommended back then is now considered a little on the high side. With a stock driver board, a 1.40 to 1.45 VDC bias point will get you more life out of your output tubes and your amp will sound just the same.

    Bob

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