[R-390] C553

[Charles Steinmetz]

Larry wrote:

>Charles, I've known about the killer C553 cap for a long time and 
>have installed high voltage reliable caps in there to hopefully save 
>my filters from destruction.  What cap would you recommend for C553?

I have always used 600v SBE Series 225P, 418P, or 715P Orange Drops, 
but any 600v film-and-foil (NOT metallized film) cap should do 
fine.  (If you use a Series 715P polypropylene, you can get 800v or 
even 1200v if you feel the need.  Note that the 715P Series are 
larger, because the dielectric constant of PP is lower than that of polyester.)

I'll take this opportunity to say once again that the vast majority 
of the paper caps in a 390A (or any tube boatanchor) are RF bypass 
caps on power supply lines and tube electrodes.  These bypass 
capacitors should be replaced with ceramic caps, because they do the 
job better than plastic or paper caps.  1kV Class II (Z5U or, 
preferably, X7R) caps are fine in these applications (but not for 
C553, which is a signal coupling cap).

Best regards,

Charles