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    MKIII - Why is the bias only OK with the driver tubes pulled

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    hansfob

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

    MKIII - Why is the bias only OK with the driver tubes pulled

    Post by hansfob on Thu Mar 24, 2011 8:42 pm

    Hi,

    Can anyone suggest why on my MKIII I can only just get the correct bias (pot fully CCW) with the driver tubes pulled out. When the driver tubes are in the bias measurement jumps right up to 2V+. I am getting -68V at the bias pot wiper and about the same on pin 5 of the output tube.

    I have a 25K linear bias pot for each output tube bias adjust. The resistor I am measuring the bias across is 10 Ohm.

    The coupling caps on the driver board are new as are the tubes so I am reluctant to suspect them.

    Many thanks for any help you may provide.


    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: MKIII - Why is the bias only OK with the driver tubes pulled

    Post by Bob Latino on Thu Mar 24, 2011 9:17 pm

    If you have a VTA driver board on your Mark III, the DC bias pots should be 50K - not 25K. A 25K pot in there will restrict the bias range.

    Bob

    hansfob

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

    Re: MKIII - Why is the bias only OK with the driver tubes pulled

    Post by hansfob on Fri Mar 25, 2011 6:19 pm

    Thanks Bob. I was worried that the bias supply voltage was not enough but -68V would seem OK according to what is documented for the MK3. The driver board I have is the Triode board and am trying to use dual bias so I believe 25K pots should be about right.

    Inserting the driver tubes seems to increase the current dramatically in the output tubes so I am unable to set the bias but I do not know why. Has anybody had this sort of symptom and found that it was a bad driver tube/s.

    Thanks for any help

    Brian

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: MKIII - Why is the bias only OK with the driver tubes pulled

    Post by Bob Latino on Fri Mar 25, 2011 6:45 pm

    If you have a Triode Electronics driver board and are having problems with bias and their driver board - maybe you should just Email Triode Electronics directly about this issue. How are you implementing this "dual bias" that you mentioned ? Do you mean instead of one bias pot for the pair of tubes, you are trying to implement separate bias for each of the two output tubes ? If you wanted to do that then you should have purchased the VTA Mark III driver board which is already set up for separate bias for each of the two output tubes on the Mark III.

    Bob

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