The AM Forum

I can't find a discussion of this.

Are modern metal film type Run Capacitors, suitable for choke input filters?  Or do I need to shell out big bucks for computer grade electrolytics?

I'm working on the main ps for my CB-80 which has a plate voltage circa 1000vdc for the Eimac 35T (about 125w load), and the Oscillator-Buffer stage with a 6L6 and Type 42 (about 25W load) circa 250v.

bill

CDE Metallized film SF type are reasonably priced, generally under $25 each.
http://www.cde.com/catalogs/SF.pdf

Just for grins, here is the schematic as passed down thru the ages.... and the PS photos.  

The last documented work on the radio is from 1985.  A hodge podge of caps, internal and even external were kluged in.  Some under-rated by hundreds of volts and all of dubious ancestry.  The bias resistors were reversed which resulted in the undersized one being scorched, no surprise.

I've read many accounts about using motor caps, and the consensus is run caps are OK, start caps are a no-no.  

Some have expressed concern about ESR and such, but my test is "do they get hot?"  Usually no, but I've had them go leaky before my use, even new ones.

The voltage ratings also go up some 25% for DC service, an added bonus.  480AC rated run just fine @ 600DC.

I've built several HV supplies with run caps (up to 100uF) over the years and not had one failure.  I've also used a combo bleeder-divider if having to use some in series.

AND...anytime I can use a non-pol cap in place of some short-lived paste electrolytic from China it a good thing!

So, one vote yes. :D

73DG

I've done this a few times. 660VAC ones on a 600VDC line in an audio amp for a decade and never had a problem.

No experience pushing them to 'full' voltage for such long periods, only briefly as a laser flash lamp cap at 900VDC or so. -but never blew one. had a 100uF bank made of 8x 12uF ones, with a SSY-1 laser head. Their internal inductance plus the external inductor made a much longer pulse than the smaller plastic cap. time on target!!! I think they'll take a lot of abuse considering what they are designed for.

CDE's got a long history. Is it worthwhile to call an engineer there and just ask them? When I was a kid I used to do that kind of thing when there was an 800 number. Usually got good advice, sometimes got told new things I didn't understand with an explanation. In those days, phones were answered, company people returned calls, receptionists would connect you to an engineer and engineers seemed to take pleasure in answering a young experimenter's questions. Different times.

Thanks for asking as I often wondered
don