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    Mark III dual bias voltage using Mottram mod board

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    kardynal

    Posts : 2
    Join date : 2015-02-14
    Location : Taxachussetts

    Mark III dual bias voltage using Mottram mod board

    Post by kardynal on Sat Feb 14, 2015 3:55 pm

    New to this forum... new to tube amps... Just purchased a pair of Mark III's that seem to have the Mottram mod board that uses two 6sn7g tubes with two variable resistors which seems to be for dual bias adjustment. (My college basic electronics lab professor taught us to use the word "seems" when writing lab reports. Saves you on the "assume/ass" syndrome every time.) In any event I know where to check for bias voltage on each of the KT88's/6550's and how to adjust but what is the correct voltage is a mystery--can't seem to find a good answer anywhere on the web forums etc. It cant be 1.56 volts--seems way to high especially since the unadjusted voltage I'm getting now is .302 volts. My tube compliment is going to be two NOS year 1951 Sylvania/Baldwin 6sn7g tubes, two new Tung-Sol reissued 6550's being rectified with a new reissued Genalex gz34. Please help. I'd like to do this correctly with the tube investment I just made $$.

    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Re: Mark III dual bias voltage using Mottram mod board

    Post by Elrick on Sat Feb 14, 2015 5:42 pm

    I have the octal version of the Mark III as well. Board from Tubesfhifi. I set the bias anywhere between 0.500 to .525 as long as they are all set the same. Let amp warm up for about 20 minutes to properly set them. I'm also using Sylvania 6SN7's from a '59 Baldwin organ tone cabinet.

    kardynal

    Posts : 2
    Join date : 2015-02-14
    Location : Taxachussetts

    Re: Mark III dual bias voltage using Mottram mod board

    Post by kardynal on Sat Feb 14, 2015 9:42 pm

    Thank you Elrick.  Just curious about what you think about the Tubeshifi board and what are the other tubes you are using--6550/kt88 and rectifier?  I went with the reissued Tung-Sol per the El Paso test on youtube.

    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Re: Mark III dual bias voltage using Mottram mod board

    Post by Elrick on Sun Feb 15, 2015 9:36 am

    I'm very happy with my t4hifi board, sounds good and have been problem free. I'm using Russian KT88 Genalex Gold Lion's and Russian Tung-Sol 5AR4's with the brownish/reddish base. After a couple years of hard playing haven't lost a tube yet. Bias settings are stable for weeks at a time. Enjoy!

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Mark III dual bias voltage using Mottram mod board

    Post by MontanaWay on Sun Feb 15, 2015 12:46 pm

    all of Roy's T4HiFi preamp designs are excellent, head and shoulder above any original designs as well as alternative 'modern' designs out there!

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