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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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    Feedback capacitors C11, C12

    colin86325
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    colin86325


    Posts : 61
    Join date : 2014-04-01
    • Post n°1

    Feedback capacitors C11, C12 Empty Feedback capacitors C11, C12

    Post by colin86325 Sun Jan 29, 2023 1:25 pm

    Hi everyone,
    Question on C11 and C12 of the VTA board--these are specified at 500V. However, it appears (to a novice) on the schematic that much lower voltage exists here. Could one safely use a 250V cap here? I'd like to use a C0G cap.
    I'm thinking that the 500V comes from the fact that silver-mica caps in this size (220pf) are commonly rated at 500V?

    Thanks a bunch!
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    Bob Latino
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    Feedback capacitors C11, C12 Empty Re: Feedback capacitors C11, C12

    Post by Bob Latino Sun Jan 29, 2023 9:41 pm

    colin86325 wrote:Hi everyone,
    Question on C11 and C12 of the VTA board--these are specified at 500V.  However, it appears (to a novice) on the schematic that much lower voltage exists here.  Could one safely use a 250V cap here? I'd like to use a C0G cap.
    I'm thinking that the 500V comes from the fact that silver-mica caps in this size (220pf) are commonly rated at 500V?  

    Thanks a bunch!

    A silver mica cap is very resistant to changing its value over the years. If the value were to change on this cap, it would alter the amount of feedback for each channel. Some of the original Dynaco ST-70 amps are now over 60 years old, yet the silver mica caps on the original amp (the caps that look like dominos) are usually still remarkably close to their original value while the other caps on the driver board have usually drifted from what they should be. Yes .. You can probably put in a 250 volt silver mica cap and it will be just fine but the difference in price would be minimal. Don't use a film cap here .. Long term it will probably change its value slightly.

    Bob
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    colin86325
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    colin86325


    Posts : 61
    Join date : 2014-04-01
    • Post n°3

    Feedback capacitors C11, C12 Empty Re: Feedback capacitors C11, C12

    Post by colin86325 Mon Jan 30, 2023 4:08 pm

    Thank you Bob. Yes, I found some ceramic C0G/NP0 capacitors that should do the trick and remain stable for along time.
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    • Post n°4

    Feedback capacitors C11, C12 Empty Re: Feedback capacitors C11, C12

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