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    ST70 Bias Question

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    G Heiman

    Posts : 3
    Join date : 2013-04-30

    ST70 Bias Question

    Post by G Heiman on Sat Jul 06, 2013 9:17 am

    Hello to all,
    I have an ST70 that was built from a Dynakitparts kit with the BBCU bias kit upgrade.  I can bias my Russian Mullard EL34 tubes correctly, but if I try to bias a set of Gold Lion KT66 tubes, the lowest I can get the bias is .63 volts.  Is there a modification I can do so I can use these tubes?

    Bob Latino
    Admin

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

    Re: ST70 Bias Question

    Post by Bob Latino on Sat Jul 06, 2013 9:29 am

    G Heiman wrote:Hello to all,
    I have an ST70 that was built from a Dynakitparts kit with the BBCU bias kit upgrade.  I can bias my Russian Mullard EL34 tubes correctly, but if I try to bias a set of Gold Lion KT66 tubes, the lowest I can get the bias is .63 volts.  Is there a modification I can do so I can use these tubes?

    Hi,

     You may have to modify the bias range on a stock type Dynaco ST-70 to bias certain tubes other than EL34's. To do so you have to lower the value of the two 10K resistors between lugs 1 and 2 and between lugs 3 and 4 on the 7 lug terminal strip. Right now they are 10K resistors. Try two 7.5K or two 5K (1/2 watt is fine) resistors. This should allow you to properly bias KT66 output tubes in your stock type ST-70.

    Bob

    G Heiman

    Posts : 3
    Join date : 2013-04-30

    Re: ST70 Bias Question

    Post by G Heiman on Sat Jul 06, 2013 12:04 pm

    Thanks Bob!  I'll get some resistors and make the change.
    George

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: ST70 Bias Question

    Post by PeterCapo on Thu Jul 11, 2013 1:07 am

    George,

    How did you make out with this?

    Thanks.
    Peter

    G Heiman

    Posts : 3
    Join date : 2013-04-30

    Re: ST70 Bias Question

    Post by G Heiman on Fri Jul 12, 2013 11:01 am

    I spoke with Kevin at Dynakitparts who sent me a diagram for ST-70 bias control expansion mod. which is very close to what Bob recommended. It allowed you the option of piggy backing a 10K and a 15K resistor to the existing resistors. I ordered the parts from Kevin, and after I made this change the bias went down from .63 to .59v. Now I'm wondering if it would be safe to let these tubes run for awhile to see if the bias comes down, or reduce the resistance further. I would like to be able to use either set of tubes.
    George

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