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    Quad Cap Resistor values

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    BCROW


    Posts : 10
    Join date : 2015-09-28

    Quad Cap Resistor values Empty Quad Cap Resistor values

    Post by BCROW Wed Feb 17, 2021 6:05 am

    Hi All,

    I've seen several values for this resistor, 2.2k, 4.7k, 6.8k, and 7.5k in various schematics all over the net.

    What is the correct value for a ST-70 and for a ST-120 using Bob's VTA driver board?

    Why the differences?  I'm guessing the upgraded caps, transformers and line voltages, but wanted to understand a bit better.

    I did do a search and found the post in the image below indicating that the goal is to have +- 380V to the VTA driver board.  I assume this applies to both ST-70 and ST-120?

    Thanks!

    Quad Cap Resistor values Screen12
    Bob Latino
    Bob Latino
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    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    Quad Cap Resistor values Empty Re: Quad Cap Resistor values

    Post by Bob Latino Wed Feb 17, 2021 9:23 am

    The correct value for a VTA ST-70 amp is 2.2K or 2200 ohms.

    The correct value for a VTA ST-120 amp is 4.7K or 4700 ohms.

    Both of these resistors should be 3 watt resistors.

    The difference is because this resistor "drops down" the B+ voltage that goes into the driver board. The VTA ST-70 runs a B+ voltage of about 440 - 450 volts DC. The VTA ST-120 runs a B+ voltage of about 490 - 500 VDC. The driver board on BOTH amps are supposed to run a B+ of 375 - 400 volts. You require a larger resistor on the VTA ST-120 to "knock down" the voltage to a value that is acceptable to run the driver board. (375 - 400 VDC).

    Bob
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    BCROW


    Posts : 10
    Join date : 2015-09-28

    Quad Cap Resistor values Empty Re: Quad Cap Resistor values

    Post by BCROW Wed Feb 17, 2021 9:39 am

    Thanks Bob!

    As always, very helpful and crystal clear!

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