Dumb question on Coupling cap ratings...

[john65b]

Forgive me on such a lame question, but why would a coupling cap (or any cap for that matter) in a tube pre or tube amp have to be rated any higher than its power supply caps?

I understand that in event of a problem, the cap could see full voltage, but why are there 630v caps on amps with 450v PS caps?

Sorry - but it has been driving me nuts. The cost is a bit more for a .47uf 600v cap vs a .47uf 250v cap...

If my B+ was 200v, why wouldn't 250v coupling cap work out just fine?

[kevinkr]

Originally posted by john65b
Forgive me on such a lame question, but why would a coupling cap (or any cap for that matter) in a tube pre or tube amp have to be rated any higher than its power supply caps?

I understand that in event of a problem, the cap could see full voltage, but why are there 630v caps on amps with 450v PS caps?

Sorry - but it has been driving me nuts. The cost is a bit more for a .47uf 600v cap vs a .47uf 250v cap...

If my B+ was 200v, why wouldn't 250v coupling cap work out just fine?

Yes, but in many cases the chosen manufacturer may not offer the dielectric in a suitable lower voltage, and in general with films it does seem that the higher voltage film caps have slightly better electrical characteristics and may (allegedly) sound noticeably better. (I think I have noticed this effect with some caps at least.)

[Jeb-D.]

I understand that in event of a problem, the cap could see full voltage, but why are there 630v caps on amps with 450v PS caps?

As Kevin explained. A lot of time it is just availability. For film caps the most common ratings are usually 200/250V, 400V and then 600/630V. Finding something between 400V and 600V can be slim pickings. Especially if you only like particular brands.

You defiantly want a coupling cap voltage rating to be at least equal to the B+ under worst circumstance. Think of start-up condition, a tube going bad, bad connection with the socket pin, accidentally powered up with no tube plugged in, or initial power up of an amp you mis-wired and haven't caught yet.. Your B+ will be higher than steady state (Just while the tube warms up if there is no fault) because it is not loaded down. Also during any of these conditions a coupling cap will see the full B+. Since the tube is not conducting yet, it is an open circuit. Which means the plate voltage will be the value of B+. So that is will be the voltage across the cap. Since one end is tied to the plate , and the other side of the cap is usually connected to grid resistor, which is at DC ground potential.

Measure your B+ during initial start up, or un-loaded. If it does not significantly exceed 250V. There shouldn't be much of a problem using 250V coupling caps.

[HollowState]

Originally posted by john65b
but why are there 630v caps on amps with 450v PS caps?

Two big reasons for this. First, it's good and proper design criteria to have a voltage margin of 50%. Any electrical design engineer worth their salt will tell you this. Second, modern film and foil capacitors come is fixed discrete voltage steps. Typically 250V, 400V, 630V (used to be 600V) and 1000V. So the logical choice is 630V.

[Jeb-D.]

Measure your B+ during initial start up, or un-loaded. If it does not significantly exceed 250V. There shouldn't be much of a problem using 250V coupling caps.

This will usually give you significant voltage margin for you steady state condition as well.

[john65b]

Two big reasons for this. First, it's good and proper design criteria to have a voltage margin of 50%.

OK, but why does this rule not apply to the PS cap? Cost can't be the reason, as the coupling caps are often more expensive than the PS cap...this is what confuses me (doesn't take much nowadays)

[dsavitsk]

Originally posted by john65b


OK, but why does this rule not apply to the PS cap? Cost can't be the reason, as the coupling caps are often more expensive than the PS cap...this is what confuses me (doesn't take much nowadays)

Because if a PS cap blows, it is generally a smelly mess and not much more. If a coupling cap blows, it can mean taking out other components, other parts of your amp, or you.

[HollowState]

Originally posted by john65b
OK, but why does this rule not apply to the PS cap? Cost can't be the reason, as the coupling caps are often more expensive than the PS cap nowadays

Part of the answer is in the way electrolytic capacitors work. They have an electrolyte and an electrode that serve as plates. The dielectric is a film of gas that's formed by electrolysis. In order to reach full capacitance they must be "formed" by operating near their full voltage rating. Also, operating an electrolytic capacitor well below their rated voltage can increase it's power factor, through incomplete forming, which limits it's usefulness.

The audiophool market has a large amount of custom "boutique" film and foil capacitors for sale at rip-off prices. However I see very little "custom" electrolytics being offered.

It may also be interesting to know that electrolytic capacitors age oppositely from other types. They deteriorate when not in use on the shelf, but very little under power. Film and foils do just the reverse.

Victor

[woody]

You might look at the classic article on caps ((picking audio capacitors)) walterjung.org/classic/articles.html

Well it seems that if manufacturer X has a 50v poly cap
and a 600v poly cap the 50v cap will distort the signal
just a little more than the 600v cap. But I read that article
25 years ago so I could be wrong.....

[Robert McLean]

You might look at the classic article on caps ((picking audio capacitors)) walterjung.org/classic/articles.html

A correction to the link :
http://waltjung.org/Classic_Articles.html