[Amps] Filtering capacitor in a HV PS

[Will Matney]

Geo,
Here's pretty much what I know on the subject going from the 
manufacturers point of view. Taking both sides from the component 
manufacturer and the OEM's. First, keep in mind that a choke input 
really shouldn't be used on SSB service by what I was taught, only a C 
input. In my opinion, I would use a C with a capacitance just as high as 
I would for a C input. The reason being is the changing inductance of L 
due to the varying DC load will allow more AC product to be at the 
output of the choke. In a C input, this would be a lesser value as C1 
would deal with the AC first before the choke. Now the formula which 
Rich gave is correct in determining C for the capacitance value (70,000 
/ RL). The voltage rating however of X2 is something I don't quite agree 
with. This is due to some papers written in "Passive Component" magazine 
by the capacitor manufacturers. According to them, the dialectic is 
designed around a certain voltage for the capacitance needed. They said 
that by not using a capacitor close to its voltage value, the 
capacitance itself can change over time. Needless to say, this is very 
unwanted. I will have to try and dig this info up if I still have it as 
this is a printed magazine I get bi-monthly. A lot of this finds file 
13. It could possibly be in an archive somehow at their website, I'm not 
sure. Now, I'm strongly thinking that one would be better off using a C 
with a voltage rating of only 1-1/2 X voltage. As Rich points out, the 
actual failure voltage is considerably more than the rating itself 
anyhow. I can also state from experience that a cap bank of two (2) 450 
Vdc caps will withstand a +900 Vdc source on them for years too as I 
have seen it time and time again. I have actually seen + 1000 Vdc on 
only this same 900 Vdc setup without any pre-mature failure. I myself 
use 6 X 450 Vdc caps for 2500 Vdc with no premature failures. All 
electrolytics will fail over time as they age and the dialectic gives 
out. From what I've read and from general experience of 22+ years of 
making amps, I would have to concur with the capacitor manufacturers. 
Hopefully, I might locate this article, but if not, one might find 
something on the net about it.

Will Matney

Interesting  information Will, Can you tell me what C is typical for a choke input?

Geo W7LFD