Filter cap value question (in a Dynaco SCA-35 hi-f amp)i

[philbo]

What is the voltages of caps in parallel rule? Two 200uf 350v caps parallel is now 400uf at 175v, right?? In series 100uf and 700v???

The voltage rating of the parallel combination will be equal to the smallest voltage rating of all the paralleled caps. In this case, if they're both 350V, then the rating would be 350V.

In series (NOT recommended, and usually provides no benefit) the voltage divides up in inverse proportion to the values of the caps. So, if you put a 10 uF and a 100 uF in series and apply 550VDC, the 10 uF cap will drop 500V and the 100 uF cap will drop 50V. That is assuming the individual voltage ratings of the caps will allow this without frying them.
The total Capacitance of that series pair can be found by using the product over sum: (100 uF x 10 uF)/(100 uF + 10 uF) = 9.09 uF

[Wrongdog]

Okay - thanks!!

[Chuck H]

In series (NOT recommended, and usually provides no benefit)

I beg to differ. In an ideal world we would just use a single, appropriately rated cap. But WRT tube amps it can be difficult and or expensive to find and use a single cap. Suppose you have a B+ of 500V with an off load voltage (that will be presented to the caps) of 575V. Ok, I need to use 600V caps... Oh darn. No one makes them!?!

Maybe you could find some. But they would cost too much, may be the wrong size or orientation and you would be limited by whatever qualities (or lack of) they possessed. A totem (series) arrangement solves for this.

Even many 500V rated caps have slipping reputations for quality. The best caps stop at 450V. So should all tube amps be made with less than 400Vp (considering off load voltage that the main filters should see and the inrush spike)??? That doesn't seem practical. In fact, a great many mass produced amps with a B+ over 400 that use a single main filter cap have had on and off trouble with premature cap failure. Component manufacturers just aren't giving much respect to electro's rated over 450V. A series arrangement solves for this too.

Just sayin'

[Enzo]

Series caps are extremely common. One generally uses two of the same value anyway. But even if they are odd value, you put parallel equalizing resistors in the circuit to even out the voltage. A couple of 350v caps in series each with a 220k in parallel very neatly shares the vcoltage equally and acts like a 700v cap.

[philbo]

Series caps are extremely common. One generally uses two of the same value anyway. But even if they are odd value, you put parallel equalizing resistors in the circuit to even out the voltage. A couple of 350v caps in series each with a 220k in parallel very neatly shares the vcoltage equally and acts like a 700v cap.

You are right, it can be done. And, again, you are right (generally speaking) about the equalizing resistors. No arguments there.
But in my experience (working professionally in electronics design since 1974) there is only rarely a need to do so. I have yet to see a schematic, aside from a 40KW radio transmitter power amp stage, that called for 2 caps in series.

BTW (for Chuck): Here are the 60000+ capacitors that are 600V and higher from just one vendor. If you trim the selection to just film capacitors >600V, you still get around 35000 to pick from.

I'm really not trying to be obnoxious; I'm just posting my take on the subject. So, feel free to take it with a grain of salt, or whatever. To be fair, if you are fixing something and HAVE to get it done today, and HAVE to use what you have in stock, then I could see an argument for hacking a couple series caps together. But as far as not being available.... well, the answer's in the link above.

[Enzo]

FIlm caps? We are talking about electrolytics used as power supply filters. There are darn few 600v filter caps out there. I am looking at 20uf or 40uf or 100uf. 1000pf doesn't help. And in guitar amps we really cannot consider the $150 1100v jobs, they wouldn't fit into most amps anyway. The contention wasn't that there are zero 600v caps, just that there are not reasonably useable ones for guitar amp power supply filters. There are amps like Music Man with 800v B+. Very hard to filter with single caps.

I have every respect for a long career in design. But here are some common schematics to demonstrate series caps. Not cobble jobs, these are mainstream commercial amps.

Fender Twin reverb 1963
http://www.webphix.com/schematic%20h...b763_schem.pdf

Fender "The Twin" 1990
http://www.webphix.com/schematic%20h...s/the_twin.pdf

Fender Hot Rod DeVille 1995
http://www.webphix.com/schematic%20h...od_deville.pdf

Soldano
http://www.webphix.com/schematic%20h...100_hotrod.pdf

Yamaha
http://www.webphix.com/schematic%20h...a_t50_t100.pdf

SOvtek
http://www.webphix.com/schematic%20h...ek_mig100h.pdf

Crate
http://www.webphix.com/schematic%20h...voodoo_60w.pdf

Peavey
http://www.webphix.com/schematic%20h...vy_vtm_120.pdf

[Enzo]

FIlm caps? We are talking about electrolytics used as power supply filters. There are darn few 600v filter caps out there. I am looking at 20uf or 40uf or 100uf. 1000pf doesn't help. And in guitar amps we really cannot consider the $150 1100v jobs, they wouldn't fit into most amps anyway. The contention wasn't that there are zero 600v caps, just that there are not reasonably useable ones for guitar amp power supply filters. There are amps like Music Man with 800v B+. Very hard to filter with single caps.

I have every respect for a long career in design. But here are some common schematics to demonstrate series caps. Not cobble jobs, these are mainstream commercial amps.

Fender Twin reverb 1963
http://www.webphix.com/schematic%20h...b763_schem.pdf

Fender "The Twin" 1990
http://www.webphix.com/schematic%20h...s/the_twin.pdf

Fender Hot Rod DeVille 1995
http://www.webphix.com/schematic%20h...od_deville.pdf

Soldano
http://www.webphix.com/schematic%20h...100_hotrod.pdf

Yamaha
http://www.webphix.com/schematic%20h...a_t50_t100.pdf

SOvtek
http://www.webphix.com/schematic%20h...ek_mig100h.pdf

Crate
http://www.webphix.com/schematic%20h...voodoo_60w.pdf

Peavey
http://www.webphix.com/schematic%20h...vy_vtm_120.pdf