[R-390] Blocking capacitor question

[2002tii]

Perry wrote:

> It is almost universally accepted that the original C553 0.01mf 300
> WVDC 20% paper should be replaced with a 600 volt "orange drop" type
> of film cap.

> My question is: can one use a .01 1,000 volt disc ceramic as well or
> is there a "gotcha" that makes using ceramics an un-good idea?
> Thoughts appreciated.

It'll work, but for three reasons the film capacitor is better. First,
the leakage of ceramic caps is higher than film caps, so film caps are
better at blocking DC. Second, and related, ceramics are more likely to
fail shorted than modern film-and-foil caps (though not more likely
than aging paper caps), and a short at C553 is particularly
unfortunate. And third, ceramic caps distort through-signals much more
than film caps do because of the much greater nonlinearities in the
dielectric. While a ceramic IF blocking cap won't "sound bad" in the
same direct way that a ceramic audio coupling cap can, this distortion
acts as a mixer and can (at least theoretically) raise the spurious
response of the receiver.

I always use a modern, high-quality film-and-foil (NOT metallized
film!) capacitor for C553. Metallized film capacitors use very thin
sheets of plastic dielectric coated with metal. The thin dielectric is
prone to failure, not least because the vapor-deposition of the metal
tends to damage the plastic. This dielectric failure causes a shorted
condition. However, the metallization is also very, very thin, and the
resulting arc can actually vaporize the fault area and the cap will
continue to work (assuming that the surrounding circuitry is not
damaged by the momentary arc current). Not exactly what you want at
C553. Capacitor manufacturers call this "self- healing." Film-and-foil
caps are not self-healing -- but they don't need to be, because they
are made with thicker dielectric sheets that haven't been subjected to
vapor deposition, and therefore aren't generally self-destructing to
begin with.

On this topic -- my computer monitor almost certainly has a failing
metallized film cap somewhere inside. Every week or two, the display
starts to look fuzzy and then there is a good loud SNAP inside. The
screen flashes bright white and then returns to normal. An Audio
Research tube audio amplifier I fixed for a guy years ago had one that
zapped every 10 minutes or so. You could actually see the flash inside
the cap under subdued lighting. So, I never trust the voltage ratings
of metallized film caps. However, 200v or higher rated MF caps seem to
work perfectly reliably in SS gear where the total DC supply voltage is
no more than 100v. Basically, I'd never use them in tube equipment.

Best regards,

Don