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    Gain of a VTA ST70

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    Zimmer64

    Posts : 113
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Gain of a VTA ST70

    Post by Zimmer64 on Thu Jul 25, 2013 10:34 am

    Hi,

    Does anyone know how many db gain the VTA ST70 has (I have Bob's kit)?

    Thanks

    Michael

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Gain of a VTA ST70

    Post by tubes4hifi on Sat Jul 27, 2013 11:29 pm

    it's probably been 25 years since I ever remember trying to measure the GAIN of an ST70 amp. Most amps don't publish a GAIN spec.
    They publish a sensitivity spec (input voltage required to produce full output power).
    But if memory serves me well (not usually!) a stock ST70 amp has a gain of around 20 db and about 20db of negative feedback.
    With the VTA driver board, the gain is around 30-35db and about 12-15db of negative feedback. This is all BEFORE the output stage.
    Let's check around and see if anyone has tried to measure this in recent years.
    Are you asking about the gain of the driver board, or the gain of the entire amp from input to speaker outputs?
    In general terms, it takes about 1/2v rms to drive the amp to full output of around 40 watts.
    Don't forget you are going thru an output transformer,
    that couples the output of the power tubes thru a transformer of around 4500 ohms down to 8 ohms.
    I know that the driver board is swinging nearly 200v p-p into the output tubes after negative feedback is applied.
    So your question doesn't have a simple answer, unless you want to get very much more specific.

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