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    R29 - 32 ST-120 driver board

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    Dale Stevens

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    Post by Dale Stevens on Wed Mar 18, 2020 4:54 pm

    Has anyone changed these from 150K to 100K? Bob had recommended that on a previous post. I'm running KT88.
    If needed, then what type and wattage?  Dale
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Wed Mar 18, 2020 6:11 pm

    Dale Stevens wrote:Has anyone changed these from 150K to 100K? Bob had recommended that on a previous post. I'm running KT88.
    If needed, then what type and wattage?  Dale  

    Hi Dale,

    It won't really make any difference in the amp if you switch from 150K to 100K as long as you can bias the amp properly. The VTA M-125's always had 100K in R30 and R32. On a recommendation from Roy the newer ST-120's changed to 100K for these 4 spots about 5 or 6 years ago. 100K will allow a little more current available to bias the four output tubes in the ST-120. In my own personal ST-120 from 2008 I still have the 150K resistors in there. My amp biases fine .. Never had a problem ..

    Bob

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    Dale Stevens

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    Post by Dale Stevens on Thu Mar 19, 2020 6:34 pm

    Thx, Mr. Bob. My bias is spot on; Variac @ 117vac. Amp is August, 2014. Great product. Dale
    tubes4hifi
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    Post by tubes4hifi on Thu Mar 26, 2020 8:07 pm

    certain modern brands of KT88s just don't want to hold a steady bias with 150K resistors, and 100K fixes that problem.
    Similar to the fact that you need to use 50K resistors with KT120/150 tubes.

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