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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


    PAS power supply

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    PAS power supply Empty PAS power supply

    Post by Guest Thu Sep 23, 2010 4:04 pm

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    Last edited by PeterCapo on Tue Dec 01, 2020 12:51 pm; edited 1 time in total
    anbitet66
    anbitet66


    Posts : 143
    Join date : 2009-12-23
    Location : Valley Stream, NY

    PAS power supply Empty Re: PAS power supply

    Post by anbitet66 Thu Sep 23, 2010 8:57 pm

    The 12X4 has a limit of how much input capacitance it can tolerate before the possibility of flashover in the rectifier can happen. The quad capacitor has four sections that are all 20uF (if memory serves) and according to the 1959 Sylvania tube manual, the rectifier needs additional "impedance" if the CRC filter circuit involves more than 10uF of capacitance in the first stage due to hot switching transients (Someone help me here, what conditions can this happen?).

    The addition of the 4.75K resistor is adequate to protect the tube from destruction since the voltage on the first stage (405Volts) divided by the sum of the 4.75K resistor and the secondary winding resistance will serve to limit the maximum amount of current the tube can drive to the capacitor.

    I have a stock PAS3x and all the tubes ('cept one 12AX7) is original, and the rectifier seems to have no issue with the Dynaco stock circuit. Maybe the higher line voltage, with the reduction of load on the power transformer by removing the lamp and substituting an LED caused the higher than normal voltage you're seeing. Rest easy, the resistor was a good move, and just make sure it is a one watt type to handle the heaat needed to dissipate.

    Tony
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    PAS power supply Empty Re: PAS power supply

    Post by Guest Thu Sep 23, 2010 9:50 pm

    .


    Last edited by PeterCapo on Tue Dec 01, 2020 12:52 pm; edited 1 time in total
    anbitet66
    anbitet66


    Posts : 143
    Join date : 2009-12-23
    Location : Valley Stream, NY

    PAS power supply Empty Re: PAS power supply

    Post by anbitet66 Thu Sep 23, 2010 10:37 pm

    Peter,

    The time constant is determined by the CRC filter network. The resistor you added shouldn't cause any major change in the characteristics of the preamp power supply. Your best bet is to use it for a bit, see what you think, and then decide. I'm guessing it will work well and you'll leave it as-is.

    Tony

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    PAS power supply Empty Re: PAS power supply

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