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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


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    ST-70 Coupling Capacitor Question

    davidmac5
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    davidmac5


    Posts : 12
    Join date : 2020-04-25
    • Post n°1

    ST-70 Coupling Capacitor Question Empty ST-70 Coupling Capacitor Question

    Post by davidmac5 Tue Mar 02, 2021 5:32 am

    Hi All,

    I recently replaced my defective PC-3 driver board with another one I had on hand. It still has the original .1uF 400V coupling capacitors on it. I want to replace them all, I have a few .47uF 500V Russian K42Y-2 on hand, my question is can I use them in place of the .1uF caps or is .47uF to much?
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    peterh
    peterh
    peterh


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    Join date : 2012-12-25
    Location : gothenburg, sweden
    • Post n°2

    ST-70 Coupling Capacitor Question Empty Re: ST-70 Coupling Capacitor Question

    Post by peterh Tue Mar 02, 2021 7:04 am

    davidmac5 wrote:Hi All,

    I recently replaced my defective PC-3 driver board with another one I had on hand. It still has the original .1uF 400V coupling capacitors on it. I want to replace them all, I have a few .47uF 500V Russian K42Y-2 on hand, my question is can I use them in place of the .1uF caps or is .47uF to much?
    The recommendation is to stay with 0.1 uF.
    Increasing coupling caps will reduce lowfreq stability and might cause "motorboating".
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    Bob Latino
    Bob Latino
    Bob Latino
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    Location : Massachusetts
    • Post n°3

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    Post by Bob Latino Tue Mar 02, 2021 5:18 pm

    peterh wrote:
    davidmac5 wrote:Hi All,

    I recently replaced my defective PC-3 driver board with another one I had on hand. It still has the original .1uF 400V coupling capacitors on it. I want to replace them all, I have a few .47uF 500V Russian K42Y-2 on hand, my question is can I use them in place of the .1uF caps or is .47uF to much?
    The recommendation is to stay with 0.1 uF.
    Increasing coupling caps will reduce lowfreq stability and might cause "motorboating".

    I agree with Peter here .. I have tried .10, .22 and .33 uF as coupling caps in the VTA amps and they all work fine. Theoretically the larger cap will have slightly more bass response at 20 Hz but since most speakers have very little response in that range, I can't detect any difference in low end response on any of my speakers here. I had a discussion with Roy Mottram one time about this very subject and his idea was the same as mine. If the cap used is above .33 uF, the more likely the cap "could" pass some DC and maybe cause motorboating. Also > Whatever cap you do use, it should be rated at 400 volts or higher.

    Bob
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