[Racal] RE: [R-390] Source for axial caps

Sun Aug 1 11:29:58 EDT 2004

Hi

Some of the places that the good old BBOD's show up in are a bit 
surprising based on series resonance effects. The missing element in 
the analysis usually is that the impedances on a tube circuit are so 
high that you can get away with operating well past series resonance 
without any real harm. Without data on the old paper parts it would be 
a bit tough to know just how much we are shifting things around with 
the newer capacitors. There certainly aren't many posts indicating any 
trouble in this area though.

I would attribute the popularity of the Orange drops to two things:

1) The epoxy coating on the cap is fairly forgiving when you bump it 
with a soldering iron.

2) Good marketing by Sprague over the years combined with a distinctive 
look to the parts.

There aren't a lot of variables in a plastic capacitor. The dielectric 
material pretty much determines the leakage regardless of who made the 
part. Dielectric also is a big factor in the loss versus frequency 
curves provided you compare parts with similar construction. Even self 
resonance characteristics are fairly similar for parts of the same 
construction and same physical size.

There are a fairly small number of people making the film for 
capacitors so that's not as much a variable these days as it may have 
been in the past. There are a few interesting dual layer films today 
that didn't exist a couple of decades back but i don't think they do 
much for boat anchors.   The availability of some of the good high 
frequency dielectrics has dried up in the past few years.

Fortunately we don't seem to need good Q at IF frequencies in our 
parts. The tuned circuits in an R390 all seem to be set up with ceramic 
and silver mica caps.

Construction wise you have two ways to do the plates, metal foil and 
vapor deposited film. The film gives you higher ESR and smaller volume. 
Most of what we look at are wound parts rather than stacked foil so 
usually that is not a variable.

I like the Illinois Capacitor parts. They seem to work perfectly well 
to replace the old paper and foil parts. I tend to go for two voltage 
ratings. Something low for tight spaces and the 630 V's for the rest of 
the stuff. One side advantage of the higher voltage parts is that they 
generally have a bit lower leakage. Of course the best paper cap in the 
world leaks more than the worst plastic cap I have ever seen ....

Provided the voltage ratings are adequate and you don't get parts that 
are only rated to 85C (like polystyrene) just about any modern plastic 
part will do a lot better than the paper parts it's replacing. I 
certainly would not pay a premium price for any special capacitors.  
It's probably a good idea to avoid the ones that the audio guys are 
after because they will drive up the prices ...

	Take Care!

		Bob Camp
		KB8TQ

On Jul 27, 2004, at 1:08 PM, Christopher J Galbraith wrote:

> Hi Jordan and gang,
>
> I like the 630V metallized polypropylene films made by Illinois
> Capacitor (IC), available through AES (www.tubesandmore.com).  Pretty
> cheap, under a buck for 0.1uF/630V.
>
> You can get to their performance specs at this link (select the "MPW"
> line):
>
> http://www.illcap.com/Film.asp
>
> Pretty low dissipation factor, rated to 105 deg C, and relatively small
> in size compared to SBE 715P ("orange drops") (but still an *axial* 
> that
> fits all BA applications I've tried).  If you use 630V rated ones, you
> may get a longer life in BA rx duty (200-350V typically), as capacitor
> lifetimes increase as you "de-rate" temperature and applied voltage.
> Good for bypass and coupling.
>
> In fact, 400V polyester units should be plenty of capacitor for bypass
> applications...and polyesters are rated to 125 deg F.  They have a
> higher (worse) dissipation factor (i.e. lower Q/higher E.S.R.), but I'm
> not certain that would matter much.  But, for not much more money
> (cents), I just buy the 630V MPWs for everything and simplify my life a
> little bit.
>
> As an aside, I'm curious why "orange drops" are so popular in BA
> restorations.  They look like they're better suited (and are
> recommended) for audio coupling or high current/pulse bypass or
> filtering.  I recall looking at their impedance vs. frequency once and
> noting that 0.1uF units were getting rather close to their
> self-resonance for 455 kHz applications.
>
> My thinking is that I'd like to replace these capacitors once, at least
> in my lifetime.  We're lucky to have such nice materials and
> manufacturing these days.
>
> 73, Chris KA8WFC
>
>
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Jordan Arndt
> Sent: Monday, July 26, 2004 7:07 PM
> Cc: boatanchors; RACAL; Collins Reflector; R-390 Group
> Subject: [R-390] Source for axial caps
>
>
>
>
> Does anyone know a good source for axial caps ie .0047, .047 ,.022
> etc.... @ 250-400V..? Preferably film type caps.. Poly tubulars etc...
>
> 73 de Jordan...
>
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