[R-390] r-390A capacitor question?

[Charles Steinmetz]

Dave wrote:

>lets say that you are replacing all the capacitors in an r-390a for 
>example: and you were quite satisfied with the stock communications 
>grade audio.
>
>would it really make a discernible difference in performance 
>depending on what type of capacitor you used...metalized film, 
>foil/film, polystyrene,  disc ceramic?

As others have said, there is more than you would ever want to wade 
through in the archives.

Several short answers:

You mention audio, but 98% of the caps in a tube radio (like a 390A) 
are not in the audio path.  The vast majority of them are power 
supply or tube electrode bypass caps.  These days, all of these 
bypass capacitors should be replaced with 1kV ceramics (type X7R or 
X7U) because they perform better in that role.  They are also much 
smaller and they are less expensive.  Keep leads as short as possible 
and use teflon spaghetti tubing as necessary to insulate the leads.

There are a few coupling capacitors, both IF and audio.  Because the 
types of ceramic capacitors you would use to replace them (X7R or 
X7U) have high dielectric absorption and high voltage coefficients, 
they can cause distortion in these roles.  Accordingly, these 
capacitors should be replaced with plastic film capacitors.  For 
reasons I won't repeat here but you can read in the archives, I 
strongly recommend using only film-and-foil capacitors (not 
metallized film) wherever you use film capacitors in any tube 
equipment.  I recommend using 600 Vdc capacitors in the 390 and 390A.

The AGC filter capacitors should also be film capacitors.  100 Vdc is 
sufficient for these.

Repeating:  There are only a few coupling and AGC capacitors.  Most 
of the capacitors in the radio should be replaced with ceramics, as 
noted above.

All of that said, I will repeat something I've said before about 
replacing all of the capacitors in a radio:

>I am generally NOT a fan of replacing capacitors shotgun-style in a 
>390A.  I have looked after several hundred of them in my day, and am 
>still in touch with the owners of many of them.  With a few rare 
>exceptions (notably, radios that had been underwater for a time or 
>were otherwise badly abused, and a very few that seem to have been 
>built with a bad batch of caps), the 390A does not seem to suffer 
>from serial capacitor failure.  So, I believe the best approach is 
>to replace the few known troublesome caps and then just run the 
>radio.  If you have two or three cap failures within a few years, 
>then you might conclude that you have one of the rare, failure-prone 
>exceptions and THEN consider wholesale replacement.
>
>In brief response to those who may ask, "Why not shotgun the caps -- 
>there's nothing wrong with improving the radio, is there?":  To 
>change the caps in a 390A, you are working in very tight quarters, 
>and some of the caps are attached to standoff terminals that are 
>quite fragile and hard to obtain these days.  It is almost certain 
>that a person with average electronic construction/repair skills 
>will burn a bunch of wires with the soldering iron and break a few 
>standoffs in the process of replacing all of the caps in a 390A, as 
>well as knacker a few other things along the way.  "If it ain't 
>broke, don't fix it" is a valuable piece of advice taught by those 
>with lots of experience.


Best regards,

Charles