Filter caps in series.

[C Moore]

Working on a Ampeg B-25. The first two caps after the rectifier are of an unusual value (30/600). So I ordered some 100/350 caps and put them in series (50/700). Is that all I need to do, should I have a resistor somewhere?
Thank You

[M Fowler]

I've seen where one uses 220K 2w parallel to each cap.

[Bob-I]

Yes, you'll need resistors in parallel with the series caps to balance the voltage across the caps. I like 470k but I've seen folks use as low as 220k. Larger will keep the bottoms tight, lower will make it spongier.

[paulster]

+1 for the resistors.

I'd go with 470K since this design didn't originally have series caps and resistors so you want to retain the feel of the amp.

Have you seen that fliptops.net have a 25/800 for this particular application?

I haven't checked, but Ask Jan First (Google it) in Europe carries some 'exotic' values and can be good for parts for restorations.

[VacuumVoodoo]

There's no guesswork in determining value of balancing resistors in a capacitor stack. It's easy to calculate:

http://www.bhc.co.uk/pdf/AppNotes.pdf

All you need to find out to use the formulas in this app note correctly are min/max values of leakage current for the particular capacitors you intend to use.

[C Moore]

Hey Mark, that was kind of what I was thinking. Thanks.

Paulster - I had no idea they were selling caps of that value. Just out of curiosity, I will take a look. Thank You.

Voodoo - Thanks for the link. I passed it along to a few friends.
I appreciate it.

[tubeswell]

Yeah they are there to balance the load between the caps (as one of the others said) and there is an optimum size for those resistors. You don't want too-low a load to ground, because you need to maintain B+, but you don't want too-high a load either, because you don't want an infinite load. Lots of amps use 220k in parallel with each cap. There is also the resistor power rating to factor in. Those commonly used 220k resistors are 2W ones.

There is a handy calculator here (under 'Capacitor Discharge Calculator')

http://www.welwyn-tt.co.uk/CalcTools.asp

Here's another handy tip - you can also split the 'lower' resistor into a voltage divider with a parallel cap on the lower leg only, to provide an elevated heater supply.