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    Biasing a VTA modified ST70

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    breading

    Posts : 24
    Join date : 2010-01-08

    Biasing a VTA modified ST70

    Post by breading on Tue Apr 17, 2018 10:02 pm

    What is the correct bias voltage for a ST70 with Roy Mottram's VTA octal drive upgrade? Is it .45VDC or .40VDC? I am running Genalex KT77s

    Thanks.

    chefothefuture

    Posts : 26
    Join date : 2018-03-28

    Re: Biasing a VTA modified ST70

    Post by chefothefuture on Tue Apr 17, 2018 11:45 pm

    The instructions for the octal VTA board in an ST70 say 0.40VDC.
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    peterh

    Posts : 985
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Biasing a VTA modified ST70

    Post by peterh on Wed Apr 18, 2018 1:34 am

    breading wrote:What is the correct bias voltage for a ST70 with Roy Mottram's VTA octal drive upgrade?  Is it .45VDC or .40VDC? I am running Genalex KT77s

    Thanks.
    The voltage depends on what cathode resistors are installed. Original st70 has 15.6 ohm
    shared between both tubes in each channel, the "new" st70 has one 10ohm resistor
    per tube.
    The goal is in all cases which current to draw in each tube. Original suggestion is 50mA
    per tube, some suggests 0.40 to 0.45mA per tube.
    Figure out what resistors you have, then what voltage it should be to get 0.4mA per tube.

    chefothefuture

    Posts : 26
    Join date : 2018-03-28

    Re: Biasing a VTA modified ST70

    Post by chefothefuture on Wed Apr 18, 2018 11:40 am

    From my recent discussions concerning biasing other amps, the general opinion I get is that bias adjustments are specific to the tube’s current draw(among other things) so a “blanket” adjustment number can lead to problems. A case point for me is my recent experience with a quad of Psvane KT120s in a VTA ST120. Biasing to the suggested voltage of .50v caused the tubes to red plate. Actually voltages above .20v caused problems. So, if I understand correctly, if you know the tube’s current draw in Ma, a corresponding voltage taken at pin 8 could be determined. Help me if I’m wrong please!
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    tubes4hifi
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    Join date : 2008-11-30

    Re: Biasing a VTA modified ST70

    Post by tubes4hifi on Sat Apr 21, 2018 2:43 pm

    Chef,
    correct, it's not the VOLTAGE that matters, it's the current thru the tube. Current across a resistor gives a voltage. 40ma across a 10 ohm resistor gives 0.40vdc
    Current setting (and thus bias voltage setting) depends on the tube used. Generally, EL34s around 40ma. KT88s around 55ma. Other tubes as per spec.

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