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    MKIII W/VTA high current thru output tubes?



    Posts : 37
    Join date : 2009-01-21

    MKIII W/VTA high current thru output tubes?

    Post by scott6058 on Sun Apr 19, 2009 6:31 pm

    I was curious my current thru the output tubes across the 10 ohm resistor at pins 1 & 8 is a bit high 0.58+ volts even with the pots trimmed down. I swapped some rectifier tubes and it made some difference but not much. I just purchased a new set of output tubes and they are the Russian Gold Lion type. Thanks for any help.
    Regards Scott
    Bob Latino

    Posts : 2639
    Join date : 2008-11-26
    Location : Massachusetts

    Re: MKIII W/VTA high current thru output tubes?

    Post by Bob Latino on Sun Apr 19, 2009 6:46 pm


    What you need to do is to pull the main bias resistor - the one right next to the silicon diode and replace it with a resistor of a LOWER value. I am not as familiar with the Mark III version of the VTA driver board as I am with the ST-70 version so I don't know what value the resistor is in the Mark III version and I don't know the resistor number? The resistor is right next to the 100 Mfd @ 100 volt capacitor on the bottom of the driver board

    In the meantime running those tubes at .58 volts bias voltage per each KT88 is OK.


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