[R-390] Orange Drop vs ceramic disc

[2002tii]

Bob wrote:

>As to metallized fim vs foil and film.  Other than self healing and  lower
>pulse current capability,  I'd be surprised if one could measure the
>difference between the two in connection with distortion or power factor.

It's not (so much) a matter of film-and-foil being a more ideal 
capacitor (although the ESR is higher, if that matters in the 
application) -- the "self-healing" of metallized film caps is the 
problem.  Because of their construction, metallized film caps with 
high voltage across them WILL fail and self-heal, even in an 
application with no pulse current.  And the self-healing current 
(which is substantial) must flow in the circuit being coupled (i.e., 
the forward-biased grid of the following tube, or the mechanical 
filter the cap is supposed to keep DC off of) until the dielectric 
failure clears, generally in tens to hundreds of milliseconds.  Bad 
news for the following tube or mechanical filter.

There are several more involved discussions of this in the list archives.

Don't get me wrong -- I use metallized film caps all the time in 
solid-state designs.  But never in anything where they could see more 
than 100 V, no matter how high their voltage rating is.  This is the 
voice of extensive experience speaking....

Best regards,

Don


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