Question on Voltage Ratings on Caps

[pompeiisneaks]

This has possibly been asked before, but I have such bad luck searching this forum.  Apologies for the newb question, but if I have a voltage rating on the B+ at 470 or more, and the highest 100uF Cap Doug sells is 450, is that safe?  I know caps tend to be over engineered to handle more than they print, but you should never abuse that rating.  If I need a 100uF cap that rates at 500, do I need to search elsewhere or will the 450's work?

~Phil

[pompeiisneaks]

Or do I just do two 50uF at 350 to get 100uF at 700 in series? Is that safer?

~Phil

[eleventeen]

It's significantly safer, and generally not much more expensive. With series "stacked" power supply filter caps, it's considered good form to place (usually) 220K resistors across each of your caps. See: Fender Twin Reverb. Those resistors which are there to "balance" charging levels on the caps also act as bleeder resistors for the supply. S'good.

[sluckey]

if I have a voltage rating on the B+ at 470 or more, and the highest 100uF Cap Doug sells is 450, is that safe?

Not in my opinion. I would never use a cap in a circuit that exceeds it's voltage rating.

Or do I just do two 50uF at 350 to get 100uF at 700 in series? Is that safer?

While the voltage rating for caps in series does increase, the capacitance does not. Two 50µF caps in series yields 25µF total.

Doug sells 47µF @ 500v. Put two of those in PARALLEL. Or use his 50/50 @ 500V F&T dual can with the caps in parallel.

[HotBluePlates]

Also, you should select caps for the highest voltage which could be present in the circuit under any circumstance.

Say you have a PT with a 350-0-350v high voltage winding. In an amp with a tube rectifier, you might only have a B+ of 430-450vdc, depending on the specific rectifier, filter cap values and circuit current draw.

But what happens at switch-on, or when you pull all but the rectifier tube from the amp? 350vac * 1.414 = ~495v. This is the peak value of the incoming rectified a.c., and also the d.c. voltage you'll have when there is no load on the power supply. So the filter caps used in the circuit should be rated for 500vdc.

[EL34]

This page has cap hookup info
http://el34world.com/charts/filtercaps.htm

[vibrolax]

Or do I just do two 50uF at 350 to get 100uF at 700 in series? Is that safer?

~Phil

It is safer, you don't get 100 uF when the caps are in series.  The voltages add, but the capacitance is  1/(1/50 uF + 1/50uF) = 25 uF

Calculate series caps like parallel resistors

[pompeiisneaks]

Excellent,  thanks guys. makes sense.

~Phil

[jojokeo]

But what happens at switch-on, or when you pull all but the rectifier tube from the amp? 350vac * 1.414 = ~495v. This is the peak value of the incoming rectified a.c., and also the d.c. voltage you'll have when there is no load on the power supply.

When I service or see any amps that use a tube rectifier I like put a 220k/1w resistor across the standby switch as insurance to act as an inrush current limiter that allows the filter caps to slowly charge (or to a keep charge) while in standby mode. Then when the standby switch is flipped ON it (is shorted by the switch, effectively out of the circuit) keeps the rectifier from possibly shorting & blowing by not taking that full load draw across it every time it is used since the filter caps already then have charging capacity to prevent this, and this also helps preserve the rectifier's lifespan.