[R-390] Capacitor replacement C609

[Charles P. Steinmetz]

Drew wrote:

>Note that the voltage rating of C-609 need not be this high.  In a 
>cathode-anode short condition in the associated tube, C-609 would 
>only see a bit less than 4V. Normal operating voltage would be much 
>less.  A 6V part would be more than adequate.

When you are considering only the voltage rating, that is 
true.  However, when you consider reliability, you find that there is 
very good reason to use a capacitor with a significantly higher 
voltage rating.

At elevated temperatures such as C609 experiences, electrolytics fail 
at much higher rates than they do at room temperature.  One must 
always choose a cap that is rated for the temperature it will 
experience; however, capacitor life is still radically reduced at 
high temperature even if the capacitor's temperature rating is not 
exceeded.  Using a capacitor with a significantly higher voltage 
rating than the actual voltage on the cap helps to mitigates this.

Best regards,

Charles