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    470k input resistor

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    wttaylor

    Posts : 3
    Join date : 2010-03-06

    470k input resistor

    Post by wttaylor on Mon Dec 31, 2012 2:03 pm

    What is the purpose of the 470k (Rp) resistor in the input of a stock (nearly) ST 70? How is the value calculated or derived? As I am using (LAST PAS) a cathode follower out of my preamp do I need this?

    Happy new year
    Regards
    Bill

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: 470k input resistor

    Post by Bob Latino on Mon Dec 31, 2012 4:24 pm

    Hi Bill,

    The 470K resistor on the input jacks sets the input impedance of a STOCK dynaco ST-70 or any Dynaco ST-70 with a stock replacement driver board. Other types of driver boards (like the VTA driver board) use an "on board" resistor to set the input impedance so these two 470K resistors (one for each channel) are not needed with the VTA driver board.

    The general rule is that the input impedance of a tube amp should be at least 10 times that of the output impedance of the preamp being used to drive the amp. The 470K input impedance of a stock ST-70 is sufficiently high to work with any preamp that I know of ..

    Bob

    wttaylor

    Posts : 3
    Join date : 2010-03-06

    Re: 470k input resistor

    Post by wttaylor on Mon Dec 31, 2012 7:01 pm

    Bob,

    Your suggesting it is sort of an arbitrary number based on not loading the preamp. I am sure someone has modeled the ST70 input in spice. Whether original 7199s, or, like mine an SV board with 6gh8s. Anybody ready to chime in with WAG of the impedance.

    Regards
    Bill

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