[R-390] Paper Capacitor Replacement II

[2002tii]

Perry wrote:

>While any modern film capacitor will be decades better than the film 
>caps that they replace. It is still the wrong choice.
>      *     *     *
>When our BAs were designed, good ceramic bypass capacitors had not 
>been developed, so wax paper and foil capacitors were used. As a 
>result and urban myth of using ODs has developed.  ODs and their 
>film equivalents are fine for AUDIO only.
>      *     *     *
>Ceramic bypass capacitors have a much lower ESR at RF for any given 
>capacitance.  *     *     *   Additionally, they are far less 
>expensive and much smaller.

As I have repeated tirelessly over the years, ceramics are much 
superior as bypass caps and I strongly recommend using them when 
recapping any boatanchor.

However, it is an unwarranted overstatement to say that ODs are the 
"wrong choice" or that they are "for audio only."  ODs are much 
better than the original paper capacitors ever were, and the radios 
worked pretty OK with the paper caps when they were new.  One 
counter-example is the SP-600, which has problems with IF leakage 
when paper bypass caps are installed.  This improves a little bit 
with ODs, and the problem goes away with ceramics.  (BTW, Hammarlund 
officially adopted an engineering change switching from paper to 
ceramic bypass caps early in the SP-600 production cycle, then 
apparently continued making them with paper caps for years.  You do 
see a very few late examples with original ceramics, but IME they are rare.)

Ceramics should not be used for audio coupling, or in the detector circuit.

If one is using film caps in tube gear, the choice should always be 
limited to film-and-foil types to the exclusion of metallized film 
caps, to prevent damage from the clearing current when metallized 
caps "self heal" themselves -- and they all do self-heal at tube 
voltages, regardless of their voltage rating.  Unfortunately, I know 
of no true axial film-and-foil caps, so genuine ODs are the film caps 
of choice for tube gear (but do pay attention to which OD series you 
use -- Vishay/Sprague makes some metallized ODs as well as the 
film-and-foil types).

Best regards,

Don


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