[R-390] Urban Legend Cap

[Charles P. Steinmetz]

Wes wrote:

>I am a little confused after reading all these posts.  Can someone 
>clarify for me: 1.  The self-healing film capacitors only heal with 
>HV transits?

No, as I posted yesterday morning they heal just about any time, from 
imperfections and pinholes in the extremely thin and overstressed dielectric.

>3.  Do self-healing film capacitors have a shorter MTBF than other tyoes?

Depends on what you mean by "failure" (the "F" in MTBF).  The 
manufacturers of metallized film caps do not call a self-healing 
event a "failure." But as far as tube circuits go, they are failures 
that recur as often as 100 times a day.  Note that one of the reasons 
the electronics industry doesn't care about self-healing is that 
today's high-voltage cap applications are not tube circuits.  There 
are plenty of circuits where self-healing is not a big problem, but 
tube radios is not one of them.  However, the tube device market is 
too small for any major manufacturer to care about.  There are a few 
small manufacturers of film-and-foil caps that cater to the 
audiophile market, but their prices are insane and their production 
is so low that it's impossible to know if they are reliable.  I would 
not trust them.

>4.  I just re-capped my SP600 using ceramic discs in the RF deck, 
>but metal film bypass capacitors in the rest of the radio.  Do I 
>need to swap out the metal film caps with ceramic discs (or metal foil, or???)?

Hard call, when you have already put in the time to do it 
once.  Personally, I'd go back and replace all of the bypass caps 
with ceramic disks, both for reliability and because SP-600s are 
notorious for IF and IF harmonic leakage problems and you want all 
the help you can get from your decoupling.  That is what I ended up 
doing with my SP-600, which the previous owner had re-capped with all 
Orange Drops.  (The very last revision of the SP-600 addressed the IF 
leakage problem -- but those radios also came from the factory with 
ceramic bypass caps, so you wouldn't be replacing them now.  Very, 
very few of these "last generation" units were made.)

Best regards,

Charles