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    6 adjustments 4 tubes ?

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    scottlowe76

    Posts : 13
    Join date : 2017-06-15

    6 adjustments 4 tubes ?

    Post by scottlowe76 on Wed Jul 19, 2017 4:31 pm

    Hi I love my amp but Know little . I have a VTA ST- 120 I know where and how to adjust the bias for output tubes . My question is there are two additional pots on the board that look the same ( trans side of board ) what do they do ? Thank you for your patience ,knowledge ,wisdom and willingness to share .
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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: 6 adjustments 4 tubes ?

    Post by Bob Latino on Wed Jul 19, 2017 9:31 pm

    scottlowe76 wrote:Hi I love my amp but Know little  . I have a VTA  ST- 120   I know where and how to adjust the bias  for output tubes . My question is there are two additional pots on the board that look the same  ( trans side of board ) what do they do ?  Thank you for your patience ,knowledge ,wisdom and  willingness to share .

    The two extra pots on the bottom (or top) of the VTA ST-120 driver board are the 10K AC balance pots. On all the newer VTA ST-120 amps these 10K pots are not present. The newer VTA ST-120 boards (since April of 2012) now have CCS which is a constant current source in the tail of the phase splitter part of the circuit. The CCS is handle by a small LM334 integrated circuit and requires no adjustment.

    Bob
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    tubes4hifi
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    Posts : 1374
    Join date : 2008-11-30

    Re: 6 adjustments 4 tubes ?

    Post by tubes4hifi on Thu Jul 20, 2017 11:41 am

    the AC balance pots were there to correct for minor differences in the sections of the dual triode tubes, but 99% of the time there is not enough difference to make any adjustment worthwhile,
    so just leave them set at mid-point. Newer boards (past 5 years or so) have no AC balance, that is automatically corrected by the CCS circuit.

    scottlowe76

    Posts : 13
    Join date : 2017-06-15

    Re: 6 adjustments 4 tubes ?

    Post by scottlowe76 on Thu Jul 20, 2017 2:27 pm

    So now that I have been biasing at .50 All power tubes stay right there. ( McShane tubes) . What and where would one tap to read what numbers? I'm not going to touch it , but I'm just curious. Thx so much for your help.

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