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In Reply to: Sonic Observations - Replaced Selenium on PAS posted by crooner on September 16, 2004 at 22:57:13:
Well, when I replaced the filament voltage doubler with 6A2 6-amp 200 PIV diodes. I also use two 4700UF caps with 1.5 ohms resistance @ 10-watts installed in the transformer lead that goes to the center position of the two caps in series.
This resistance lowers the voltage to 12.6 volts as solid-state replacement rectifiers over-voltaged the tubes with my modification. Another benefit is lowering the high peak to average current. This is easier on the transformer & reduces noise as well. My preamp produced no audio hum after modifications.
Follow Ups:
So, what's the practical benefit of using larger caps? further reduction in hum? I am using 2,200 uF replacements. I was thinking of sticking to those, as long as the heater voltages are okay.
Well, I had the caps in my parts bin. The caps have a little better filtering, but most likely unnecessary.
Make that resistor .5 ohms. I may add all series caps were replaced with AuriCap. Huge improvement, it is a new preamp.Stock PAS? Not a chance of having a place in my tube audio gear inventory.
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