[R-390] Observation on the Cap Questions and Comments

[chuck.rippel at cox.net]

Some years ago, I did a good bit of research on the question while I was 
rebuilding my 75A-4 and the first SP-600.  The take-away IRT tube radios 
was GENERALLY as follows:


Electrolytic's are generally used as filters in power supplies; you may 
see the odd application as a cathode bypass in an audio circuit.  The 
voltage rating of the replacement cap should be such that the applied 
voltage, be it P-P or DC should not exceed 80% of the rated voltage of 
the cap.

In circuits in which audio or RF up to but not exceeding 455KC in 
frequency, Sprague Orangedrops work wonderfully and Orangedrops with 
Polypropylene media (the more modern 716P Series) in particular, excell 
in audio circuits.

Circuits which have RF whose frequency exceeds 455KC in benefit from 
Ceramic Discs.  1KV rated caps, in my book, are considered a minimum.


As an example, the average SP-600JXxx will need something on the order 
of 38 .01 ceramics, 16 .022 Orangedrops a couple .5's and one Multi 
Section Electrolytic filter cap.

I use 600V, 715P Orangedrops to replace the dreaded C-553.  I've not had 
one fail (that I know of) in 30 years.  Because of space considerations 
I have started using polypropylene media, 630V tubular metal film caps 
(http://www.tubesandmore.com/products/C-U-630V) in R390A audio deck 
boards.  The .01 audio input coupling cap, buried in that same module 
will get a .01 Orangedrop or maybe even a .022 if I can squeeze it in. 
Of course, there is that 8ufd electrolytic which always seems to be 
leaking acid.  A easy to find, 10ufd, 35V axial works just fine there 
with the (+) lead soldered to the inboard solder pad.

That application will yield about the best >audio detail< and overall 
sound the radio is capable of giving.  In this case, the best possible 
audio detail is the goal.


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Chuck Rippel