The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


2 posters

    Quad Cap Resistor values

    BCROW
    avatar
    BCROW


    Posts : 10
    Join date : 2015-09-28
    • Post n°1

    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
    • Back to top
    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3277
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    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
    • Back to top
    BCROW
    avatar
    BCROW


    Posts : 10
    Join date : 2015-09-28
    • Post n°3

    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!
    • Back to top
    Sponsored content

    Sponsored content


    • Post n°4

    Quad Cap Resistor values Empty Re: Quad Cap Resistor values

    Post by Sponsored content

    • Back to top
      Current date/time is Mon Nov 25, 2024 10:13 pm