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    B+ on VTA Board in MKIII

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    hansfob

    Posts : 18
    Join date : 2010-10-24
    Age : 52
    Location : Rickmansworth, England

    B+ on VTA Board in MKIII

    Post by hansfob on Fri May 04, 2012 1:51 pm

    Hi,

    I have a working MKIII amp fitted with a low gain VTA board that is all biased up nicely. I am concerned because the B+ on the VTA board is reading 500V DC and all I have read says that it should be 400-440V DC.

    I have since changed the resistor on the quad cap from 6.8K to 10K as I believed this should drop the voltage but it has made no difference. In fact there does not seem to be any voltage drop at any stage of the quad cap. Should I expect a voltage drop and if so could the quad cap be the problem? If not what can be used to drop the B+ to be in range? The transformer vibrates slightly if that is any clue.

    Many thanks for any help you may provide.


    tubes4hifi
    Admin

    Posts : 1291
    Join date : 2008-11-30

    Re: B+ on VTA Board in MKIII

    Post by tubes4hifi on Fri May 04, 2012 2:21 pm

    is the amp actually working?? Seems you have zero current draw.

    hansfob

    Posts : 18
    Join date : 2010-10-24
    Age : 52
    Location : Rickmansworth, England

    Re: B+ on VTA Board in MKIII

    Post by hansfob on Sat May 05, 2012 6:03 pm

    Many thanks,

    Yes lack of current draw.

    Realised that when I measured 500V I did not have the output tubes in. Checked the voltage again, with them in, and I get 380V. Is this a little low? The amplifier biases and sounds great.

    Regards

    tubes4hifi
    Admin

    Posts : 1291
    Join date : 2008-11-30

    Re: B+ on VTA Board in MKIII

    Post by tubes4hifi on Sun May 06, 2012 12:29 pm

    perfect!

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