[R-390] C553

[David Wise]

A fuse is way too slow; the filter will go first.

Not to steal Charles' thunder, but a while ago I researched 1kV film/foil .01's stocked at Mouser.
At the present time, they have exactly one honest-to-god film-foil at 1kV or up:

CDE
  715P103516LD3 1.6KV $4.04

The Kemet PHE448 series is also true film/foil but they're out and not ordering.

Everything else is either 630V or less, or contains metalized film.  If the 1.6kV 715p's run out, you can go two ways with film: either settle for 600V, or settle for metalized and run up the voltage to reduce the odds of a self-heal event.  There are in-stock hits for either approach.  Here are the film/foils:

CDE
  715P10356KD3 600V $2.34 
  DMT6S1K 630V $1.55
  WMF6S1K-F 630V $2.24 >>> AXIAL LEADS

All the over-1kV metalized caps are multi-section design, essentially two or more caps in series.  A few of these are hybrids, with foil main electrodes sandwiched around floating metalized film(s).  I don't know what that does for the statistical self-heal rate, but here they are, since they're also a minority:

CDE
  942C20S1K-F 2KV $3.26 >>> AXIAL LEADS
  DPPM20S1K-F 2KV $3.09
EPCOS
  B32633A2103J10 2KV $2.17
KEMET
  R73TN2100SE00J 1.6KV $0.98
WIMA
  FKP1U021006B00JYSD 2KV $2.69
  FKP4U021005G00KYSD 2KV $1.91
  FKP1Y021006F00KYSD 6KV $2.84 >>> THIS THING IS HUGE

Finally, there's the single C0G ceramic:

AVX
  SV14AA103KAR 1KV $6.69

Failure rate takes off as you approach rated voltage.
Although they used the best grade of part, MIL-HDBK-217 didn't exist, and Collins only derated 30%, where today 50% is understood to be the minimum.  That 300V cap should have been 400 or maybe even 600.  We're using 1000 - 80% derating - so we can sleep extra easy.

Myself, I'm using an El-Menco .01/1600 dipped radial.  I tore one open to confirm it's film/foil.
 
Dave Wise

From: Larry H [mailto:dinlarh at att.net] 
Sent: Wednesday, August 06, 2014 12:40 AM
To: David Wise; Charles Steinmetz; 390 list
Subject: Re: [R-390] C553

Charles, I've known about the killer C553 cap for a long time and have installed high voltage reliable caps in there to hopefully save my filters from destruction.  What cap would you recommend for C553?

David,  your idea about a current limiting device in that circuit got me wondering - what about the most basic one we use all the time?  A tiny little solder in fuse of some reasonable value.  Any drawback to using a fuse?

Regards, Larry