[Amps] Filtering capacitor in a HV PS: How low can you get

[Will Matney]

Angel,
About 20 to 25 uF is about a minimum used by all amp manufacturers on 
the B+ supply. Most circuits are at least sized X2 when considering the 
voltage. However, I've seen a bunch by manufacturers such as Alpha, QRO, 
etc use Qty. (8) 220 uF @ 450V in series giving a 27.5 uF @ 3600 Vdc 
rating. This is on a 2500 Vdc supply. I have also seen some use just 
slightly over the B+ voltage in lower voltage amps around 900-1500 Vdc 
range, maybe 100 to 450 Vdc over. From this, I can say your 25 uF @ 4.5 
Kv should be just fine in my opinion. You really don't want more than 
20-30 uF as the amount of joules of energy stored could destroy the 
supply and or tube upon any malfunction. Kind of think about it as 
letting loose bottled lightning. The bleeder resistors should be about 
100K ohm 2 watt across each to prevent heating of the electrolytic caps. 
Heat causes caps to fail early. For one single capacitor, use about 820K 
at 15-25 watts or so. That would consume around 7.62 watts of power. 
Each two watt resistor in the string for 8 capacitors will give a 16 
watt rating still consuming 7.62 watts. This again is X2 the power. The 
main thing is to check the resistors voltage rating to see if it will 
handle the voltage being applied across it. There is formulas for 
determining the minimum and optimum capacitance values and I hope to 
show them on the transformer website under filters. Either there or in 
an e-book I'm doing. Actually, the rule is to kill out any audible 120 
Hz hum or 100 Hz in Europe. In my low voltage supplies, I use a 1000 uF 
@ 35 Vdc for the 12 Vdc supply, a 100 uF @ 250 Vdc for the control grid 
and a 100 uF @ 450 Vdc for the screen supply. Keep in mind that the 
screen supply is not X2 and I have had good luck without any drastic 
failures. Anyhow, hope this helps.

Will Matney