[R-390] R-390A cap replacement

[2002tii]

Roy wrote:

>"Orange Drops" by Sprague, Inc. are not needed.

True, although there is good reason for using them wherever you use 
film capacitors (on another thread I suggested that all bypass caps 
should be replaced with .01 or .02 uF ceramics for optimum 
performance, so IMO there is little need for film caps in boatanchors 
at all -- only for audio coupling and some IF coupling applications, 
one of the latter being the DC blocking cap for the mechanical 
filters in the 390A).

Anyway, as I have said before -- but it is worth repeating -- there 
is a world of difference between film-and-foil caps and 
metallized-film caps.  Besides the performance differences 
(metallized film caps generally have significantly greater ESR, as 
well as exhibiting greater nonlinearity due to the higher field 
gradient across the thinner dielectric), film-and-foil types are 
many, many times more reliable than metallized film types.  Further, 
metallized film caps are designed to be "self-healing" in the event 
of a dielectric breakdown.  Sounds good, right?  Unfortunately, it's 
only good marketing.  What this means is that if the dielectric 
breaks down, the capacitor will arc until it vaporizes enough of the 
metallization to clear the short.  That means whatever the cap was 
intended to keep DC off of -- this could be your mechanical filter, 
or the grid of the next stage -- has to to tolerate the "healing" 
current while this vaporization happens.  One prominent manufacturer 
of tube-type hi-fi equipment in the '70s and '80s used big, honking 
metallized film coupling capacitors from the drivers to the output 
tube grids of audio amplifiers.  You could actually see flashes of 
light from inside the caps, often several times a minute, as they 
drew their "healing" current through the grids of the output 
tubes.  Needless to say, output tube life was seriously compromised.

So, my advice is wherever you use a film cap in tube equipment, make 
sure it is a film-and-foil type, NOT metallized film.  The Orange 
Drop film-and-foil caps have decades of demonstrated reliability, so 
although they are not the only choice, they are a good one.

Best regards,

Don