100uF/500V vs 2x220uF/350V

[GainFreak]

Hi,

Obviously most of the time 2x220uF/350V caps in series are preferred instead of a single 100uF/500V one. Provided the anode voltage would be 450-460V at idle what are the pros and cons in both cases?

[Alex R]

Well I'm prepared to stand corrected but personally I wouldn't expect there to be any important difference between the two. Series caps are used to handle a higher voltage, typically over 450v or 500v. An amp that idles at 460v might go over 500v at startup till the power valves start passing current.

[GainFreak]

An amp that idles at 460v might go over 500v at startup till the power valves start passing current

It will certainly will especially when in Standby mode, but according to specs such caps /designed mainly for switching supplies/ can handle 550V surge.

[Earl]

From Ailen's site -

"Q: I've heard that it is best to run electrolytic capacitors as close as possible to their rated voltage. Is this true?
A: No. Electrolytic capacitor life is not infinite. It is controlled by a number of factors, but mainly the temperature at which it operates, which is set by two things: ambient temperature and internal heating. Ambient temperature is the biggest factor, so if you are running a capacitor in a hot tube amp, be sure to use one with extended temperature rating if possible. The second factor, internal heating, depends on the ripple current in the capacitor and it's internal resistance.

Capacitor life is directly proportional to the ratio of rated voltage/operating voltage. If you run a 500V cap that has a specified lifetime of 10,000 hours at 250V, it will instead last 20,000 hours. If you run it at 500V as specified, however, it will only last 10,000 hours, which is what it is rated for. So, as you see, there is no benefit to running a capacitor right at it's rated voltage, it is better to run them conservatively with respect to voltage and temperature for maximum lifespan.

Film capacitors and ceramic capacitors are even more dramatic - their lifespan is proportional to the rated/operating voltage raised to the 7th power and 3rd power, respectively, so a film cap rated at 10,000 hours at 500V run at 250v will last 1,280,000 hours, and a ceramic cap under the same conditions will last 80,000 hours, while an electrolytic will last 20,000 hours as I mentioned previously.

Go here for more info and a handy web calculator for capacitor lifespan: http://www.illinoiscapacitor.com/tec...alculators.asp "

[bluefoxicy]

Capacitors in series act like one cap. Since the connection between them is of zero resistance, the plate distance between the two plates (- on one and + on the other) is zero, thus it's one plate. Thus you're just talking about a bigger capacitor with less capacity (use the resistors in parallel formula for caps in series).

I don't know the voltage formula. The bigger voltage tolerance, the better.

[Synaptic Amps]

I recall being warned by either Kevin O'Connor or one of the folks from Jensen Capacitors in Denmark that due to the poor tolerance of capacitors and the resulting likelihood that charge will not be stored equally on two 'equivalent' capacitors in series that it's always best whenever possible to use a single capacitor of the desired value. Of course, in high voltage power supplies it's sometimes necessary to use two caps in series in which case it's essential to use shunting resistors across each cap to prevent the buildup of excessive voltage on either one.

[Enzo]

When series wiring filter caps, I think it should be assumed we would install equalizing resistors. Not assume everyone does it, it should be assumed as good practice though.

[mooreamps]

Hi,

Obviously most of the time 2x220uF/350V caps in series are preferred instead of a single 100uF/500V one. Provided the anode voltage would be 450-460V at idle what are the pros and cons in both cases?

I would believe it to be better to use the 2x220/350 set-up, with equilizing resistors, primarily for the benefit of the increased margin in it's ability to handle the high voltage. We also understand the voltage spec is based on the rating at sea level, and the rating decreases as a function of altitude. Now, we are probably not too concerned about installing a tube amp inside a launch vehicle and sending it into space.... But, there can be a factor of up to 10% reduction in rated voltage at 5000 feet, which is about the altitude of Denver, Colorado. So, if this amp is to be used in "higher" places in the country, we certainly would want the extra margin for rated voltage.

-g