[R-390] Capacitor replacement C609

Charles P. Steinmetz charles_steinmetz at lavabit.com
Sat Mar 2 19:09:04 EST 2013


>The value of the listing is actually 6.8 uf, 6R8 uf, with the "R" 
>representing a
>decimal point. I have a number of them I got at a surplus dealer in El Cajon
>(San Diego area). Also, I worked for the large component distributor,
>Hamilton/Avnet in the 1970's and sold Kemet tantalum capacitors for 
>many years.
>The seller clearly doesn't know their part numbering system. I've 
>tried both 6.8
>uf 35v and 10 uf 35v solid tantalums as replacements and didn't see a
>discernible difference. Either should work.

Indeed.  C609 is just a cathode bypass capacitor on a 5814 (= 12AU7) 
audio stage.  There is absolutely nothing critical about it, although 
if you're going to use a different value it should be larger than 8 
uF, not smaller.  Mouser has three pages of 10 uF/35 V solid (dry) 
tantalums in stock, from about $1 to $40 in single quantity.

But there is no need for C609 to be tantalum, wet or dry -- any plain 
aluminum electrolytic will do (and is the capacitor of choice among 
audiophile designers in a cathode bypass application).

In this application, reliability at temperature is much more 
important than getting the theoretically most ideal capacitor.  A 
capacitor rated for at least 105 degrees C is advisable, as is a 
higher voltage rating.  ESR doesn't matter, ESL doesn't matter, 
dissipation factor doesn't matter, dielectric absorption doesn't 
matter, voltage coefficient of capacitance doesn't matter, and 
high-frequency resonance doesn't matter (in the sense that any 
newly-manufactured capacitor you buy will be way better than 
necessary in all of these areas, assuming it is operating nominally). 
Note that aluminum electrolytics are readily available with 105C 
temperature ratings, but solid tantalums are generally available only 
with an 85C rating.

I use a Vishay TE1305-E3 (Mouser 75-TE1305-E3) 20 uF/50 V 
ultra-reliable high-temperature aluminum electrolytic when I replace 
C609.  If I were to use a tantalum, it would be the Kemet 
T322E106K050AT (Mouser 80-T322E106K050AT) 10 uF/50 V high-reliability 
solid tantalum (but note the 85C rating).

Best regards,

Charles










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