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Last edited by PeterCapo on Tue Dec 01, 2020 12:51 pm; edited 1 time in total
PAS power supply
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Last edited by PeterCapo on Tue Dec 01, 2020 12:51 pm; edited 1 time in total
anbitet66- Posts : 143
Join date : 2009-12-23
Location : Valley Stream, NY
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
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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Last edited by PeterCapo on Tue Dec 01, 2020 12:52 pm; edited 1 time in total
anbitet66- Posts : 143
Join date : 2009-12-23
Location : Valley Stream, NY
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
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