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    PAS power supply

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    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    PAS power supply

    Post by PeterCapo on Thu Sep 23, 2010 4:04 pm


    PAS with phono board disconnected and incandescent pilot lamp replaced with an LED.

    Voltage off of pin 7 of the 12X4 was a little high, so I put an additional 4.75K resistor in series with it, and now the voltages line up just right with the chart in the original manual.

    BUT, it occurred to me that I am not sure I’ve ever seen an RCR power supply filter. I seem to recall that the output from the rectifier tube goes straight to the first capacitor in a CRC or CLC filter network.

    Is there any reason why a resistor should not be connected in series between 12X4 pin 7 and the first filter capacitor in the power supply?

    Thank you.
    Peter

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: PAS power supply

    Post by anbitet66 on 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

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: PAS power supply

    Post by PeterCapo on Thu Sep 23, 2010 9:50 pm

    Tony,

    Thank you for your response. What about any effects on the time constant of the filter network by adding the resistor before the first capacitor? Could it affect the smoothing characteristics or is it a non-issue?

    Thanks.
    Peter

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: PAS power supply

    Post by anbitet66 on 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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