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    ST-70 Series II Bias adj. fail

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    jacksonip

    Posts : 1
    Join date : 2011-10-20

    ST-70 Series II Bias adj. fail

    Post by jacksonip on Thu Oct 20, 2011 12:27 pm

    Hi, all

    I had one ST-70 series II, but cannot adjustment bias from LOW up to HIGH LED light on.
    Where / how to check which component(s) in circuit to solve it problem? Thanks!

    Best regards,
    Jackson

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Series II Bias adj. fail

    Post by Bob Latino on Thu Oct 20, 2011 3:32 pm

    jacksonip wrote:Hi, all

    I had one ST-70 series II, but cannot adjustment bias from LOW up to HIGH LED light on.
    Where / how to check which component(s) in circuit to solve it problem? Thanks!

    Best regards,
    Jackson

    Hi Jackson,

    On the Dynaco ST-70 Series II you adjust the bias control until the HIGH and LOW LED's are of equal brightness. Are you saying that you can't get the brightness of those two LED's about the same ?

    The ST-70 Series II also has a 15.6 ohm bias resistor for each pair of output tubes like the original ST-70. As an alternate way you could open the amp up and measure the DC voltage from pin 8 of one of the output tubes of the pair to chassis ground and set the bias control to 1.56 volts DC on your meter. This would give you 100 milliamps of bias current for each PAIR of output tubes OR 50 milliamps for each individual output tube.

    Bob

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: ST-70 Series II Bias adj. fail

    Post by tubes4hifi on Sat Oct 22, 2011 4:27 pm

    possibly one or both tubes on that side are so worn or out of spec (some new tubes aren't even in spec)
    that the adjustment range isn't wide enough

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