Effect of serial filter capacitance?

[azatplayer]

I understand that elytics in serial double the voltage capacity and halve the capacitance value.
But, I also get that they are a reservoir, and give you the stored power on tap as the amp needs it.
So.. if in the case of say..a superlead with a pair of 100uf caps in serial, you have 50uf total, but 4x50uf sections to get you there.
Question is, having that amount of volume to fill, even tho at 50uf in this example, thats gotta be hard if using a tube rec?
Also thats still gotta give you heaps of filtering doesnt it? Even if only a quarter of its rating?
Ive just built a plexi circuit with very low filtering and a tube rec. Im curious about using lower filtering in the preamp stages, but using bigger cans in serial to get the rating/frequency while having a storehouse of power on tap.

[Rich M]

In your example, when you put two 100 uF in series, for a given B+, each of those caps will store only 1/2 of the energy of a 50 uF cap at the same B+. This is because each cap only has B+/2 across it. The energy stored can be expressed as E = 1/2*C*V^2 and even though C is twice the value, B+/2 squared is B+^2/4 so the energy is halved.

[azatplayer]

So, then the only reason we do serial elytic caps, is for the extra voltage handling? Thats the only reason? Ok. cool. Learning more
Cheers Don

[Firestorm]

There's economics involved, too (well, not in the "wretched excess" 100 watt Marshalls with the six cans). Look at the oddball filter circuit in the Fender Deluxe Reverb reissue. GZ34 rectifier, 400V B+ and the first filter is a totem pole made up of a 220uF/100V cap on top of a 47uF/500V cap. The "bleeder" resistor across the first cap is 100K 1/2 W and across the second cap, 470K 1W. So not only do they combine (cheaper) different value caps to get the voltage rating and cap value they want, they "steer" the voltage to the caps with different value resistors. AND they keep the net value under the GZ34's max capacitance rating. This one works out to about 40uF. Haven't tried to figure out how the net voltage rating is affected by the assymetrical resistors, tho.

[ChrisM]

There's economics involved, too (well, not in the "wretched excess" 100 watt Marshalls with the six cans). Look at the oddball filter circuit in the Fender Deluxe Reverb reissue. GZ34 rectifier, 400V B+ and the first filter is a totem pole made up of a 220uF/100V cap on top of a 47uF/500V cap. The "bleeder" resistor across the first cap is 100K 1/2 W and across the second cap, 470K 1W. So not only do they combine (cheaper) different value caps to get the voltage rating and cap value they want, they "steer" the voltage to the caps with different value resistors. AND they keep the net value under the GZ34's max capacitance rating. This one works out to about 40uF. Haven't tried to figure out how the net voltage rating is affected by the assymetrical resistors, tho.

That's a neat idea!
From my math I get 70V across the 100V cap and 330V across the 500V cap. Not a bad design idea at all.

[Wayne]

It almost sounds like an idea that was hatched the week that inventory was running low on certain parts

When stacking caps, the ESR's would add in series, wouldn't they? I would think that would be a bad thing.

W

[Firestorm]

Yeah, I think the ESRs add, but unless you're designing really high-frequency stuff (computers, microwave transmitters), you're more concerned with the ESR of each individual cap because a high ESR can overheat/fast age the cap. But at the relatively low capacitances you see in MI amplifier power supplies, I doubt that ESR matters much except as a measure of cap quality.