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    vta octal B+ voltage

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    kost

    Posts : 34
    Join date : 2009-05-12
    Age : 55
    Location : athens

    vta octal B+ voltage

    Post by kost on Tue Sep 25, 2018 8:30 am

    Hi
    I have a vta st 70 octal version and kt88 tubes.As i see the b+ has 350 volts instead of 400.Do you think that i can reduce the value of
    the 2k2 resistor between the two capacitors to adjust the voltage?
    thank you
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    Peter W.

    Posts : 1036
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: vta octal B+ voltage

    Post by Peter W. on Tue Sep 25, 2018 1:30 pm

    kost wrote:Hi
    I have a vta st 70 octal version and kt88 tubes.As i see the b+ has 350 volts instead of 400.Do you think that i can reduce the value of
    the 2k2 resistor between the two capacitors to adjust the voltage?
    thank you

    Two things:

    a) Reduced B+ will reduce the power-output of the amp somewhat, but it will lengthen the life of the output tubes as well.
    b) B+ is directly tied to input (wallplate) voltage. What is your present wallplate voltage?

    Put another way... if it ain't broke....
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    tubes4hifi
    Admin

    Posts : 1483
    Join date : 2008-11-30

    Re: vta octal B+ voltage

    Post by tubes4hifi on Fri Oct 05, 2018 2:40 pm

    B+ voltage will be inversely proportional to the amount of current running thru the output tubes.
    If you turn down the current, the B+ voltage will go up.
    The ST70 typically has around 410-420vdc if your AC input voltage is 120vac.
    That's running four output tubes at 40-45ma.
    If you are running KT88s at 50-60ma each, I can see B+ going down to around 380vdc.
    The VTA driver board is designed for 360-380vdc, the difference coming from the ( 2.2K ) dropping resistor.
    the board uses 25ma. Use ohms law. A 2.2K resistor will drop about 55v. A 1K resistor will drop around 25v.

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