[R-390] The OD discussion

[2002tii]

Ken wrote:

>Having said that though...do you guys think I have gone overboard 
>with my choices (716 series Orange Drops and Silvered Mica) as I've 
>read that Ceramics seem to be good enough to use almost everywhere. 
>The reason that I didn't choose Ceramics is that I read somewhere 
>that the Silvered Mica were a step up from the Ceramics.

Use dipped silver micas for everything under 10,000 pF.  These will 
all be part of tuned circuits or RF coupling caps, and ceramics are 
generally not as good in this application.

Disk ceramics are actually better than film caps in bypass 
applications.  Film-and-foil ODs (or other film-and-foil caps of 
equal quality) are fine -- they work at least as well as the original 
paper caps, and will outlast you and your heirs.  If you want the 
best film caps for bypass applications, those would be the OD 779P 
series.  But for the BEST performance, use disk ceramics in all 
bypass applications.

Use film-and-foil ODs (or other film-and-foil caps of equal quality) 
in all IF and audio coupling applications.  Note that ODs are made 
with both polyester and polypropylene film.  The PP film-and-foil 
types (715P/716P series) are the best, but the PE film-and-foil types 
(225P) are better than the original paper caps ever were.

If you don't know which caps are bypass caps and which are coupling 
caps, you may want to let someone who does work on your radio.

Finally, avoid metallized film caps of any sort in tube equipment -- 
use only film-and-foil types.  If a capacitor vendor won't show you 
the manufacturer's data sheet stating that the caps are 
film-and-foil, they are metallized film.  Any cap that is 
significantly smaller than the equivalent film-and-foil OD is 
metallized film.  NOTE that ODs are made in both metallized film and 
film-and-foil versions, as well as a "hybrid" version for AC 
applications, so you have to pay attention to the series numbers.

Best regards,

Don