small-value poly cap in parallel with PS filter cap

[pvsage]

I'm in the midst of plans to build my first amp - a small, single-ended Class A amp similar to the GA-5 or Valve Junior (among others). I expect that I'll be modding, tweaking, and then modding the amp again, but that's a different issue, as is the question of whether the amp is an end in itself or a stepping stone to something like an 18watt. Right now my focus is on the power supply.

I've been researching power supplies for guitar amps and high-end audio, and I keep running across something that confuses me: a small poly cap, typically 100nF to 1uF, in parallel with a larger electrolytic cap, e.g. C11||C8 in the Valve Junior.

I've read of people actually replacing *all* the electrolytic caps with polypropylene caps in an effort to build an amplifier that will last until the End of Days, and that the poly caps tend to have lower resistance and inductance than electrolytics; essentially, they're less-imperfect than electros. Is this cap supposed to make that filter stage behave more like the electro/poly combo is actually a larger poly? And if this is the case, why not use similar poly caps in parallel with the other electrolytics in the power supply?

Peter

[xFallen]

Sounds like the smaller value cap across the filter cap is being used to reduce noise.


Barry

[Bruce / Mission Amps]

It's just an old Hi-Fi trick to lower the overall ESR (effective series resistance) of the filter bank circuit when used with a cheaper Electrolytic filter caps.
The idea being that you want to decouple audio freqs from the B+ rail anyhow so a low ESR cap should be better, right?
Supposedly, any decent plastic 100nF to 1uF cap will have much lower ESR then a cheaply made E-Cap and the plastic cap would be mounted in parallel with the E-cap.
Loosely... two ESRs (resistances) in parallel makes for less total ESR (resistance).
A cheap 450v-500v E-cap can cost only a few dollars and a plastic 630v cap isn't much $, so I think it's worth trying.

[pvsage]

Thanks Barry & Bruce!

The poly cap in this particular PS isn't at B+, it's at the preamp plate voltage. Would it be worth adding similar poly caps on B+ and the control grid voltage? I assume it couldn't hurt...I also assume it wouldn't make the tiniest difference if the caps are M150's, 715 OD's, hand-made boutique caps, or bargain bin box caps, so long as they're rated at a high enough voltage...

Peter