Posted by 1973shovel (A) on October 15, 2013 at 05:25:24

In Reply to: RE: PAS-3 filament voltage doubler capacitance posted by sbalfour@att.net on October 10, 2013 at 17:48:48

My experience has been that the more capacitance on the PAS filament supply, the better it sounds.

I've done quite a bit of experimenting with this in both my modded PAS (Vorhis Last PAS) and a stock (other than diodes replacing the selenium rectifier) PAS.

Forget the theory of whether or not the extra capacitance should make a difference. The experimenting is easy enough to do. Get yourself a couple of high capacitance caps of appropriate voltage, and temporarily clip them into the voltage doubler circuit, using test leads. Turn the PAS on, let the caps charge, and listen for a while.

Once you've got a handle on the sound, remove the alligator clip leads from the PAS circuit, and listen again. Leave the external caps charged, and after you've listened again, clip them back in. There's no issue disconnecting and re-connecting the external caps, as long as they're at the same voltage potential. Never try and clip in uncharged caps to a hot circuit, unless big sparks don't startle you. It's also bad for the caps!

If you don't hear an improvement over the stock 2000 µF caps, then there's no need to go further. But I'm betting you'll hear an improvement. I currently have two 120,000 µF (60,000 µF in series) external caps connected via an umbilical to my Vorhis PAS, in addition to two 4,700 µF on-board caps. I've tried adding two more 120,000 µF caps, and yes, I could hear further improvement, although to a lesser degree.

No need to get that crazy though. I have recently purchased a couple of Nichicon KW 22,000 µF, 25V caps from Mouser, for $7.44 each (link below), and they'll fit nicely inside the PAS chassis.

Try it for yourself. I'd like to read about your results.

PS: Something else you might want to consider are the diodes in the filament supply. I originally built my Vorhis PAS using 1N4007 diodes in place of the selenium rectifier. A few years ago I replaced the 1N4007s with two Schottky diodes (about 50¢ each), which further improved the sound. FYI, I don't consider myself a hopeless Tweaker, and I'm not shy about admitting when I don't hear any improvement in sound. Like raising wires off the floor, for example. Since it was free, I tried it, and didn't hear any difference.

But the Schottky diodes and extra capacitance? That worked for me.

• Nichicon KW   (Open in New Window)