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

To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] Filtering capacitor in a HV PS: How low can you get
From: Will Matney <craxd1@ezwv.com>
Date: Tue, 17 Aug 2004 06:37:10 -0400
List-post: <mailto:amps@contesting.com>
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
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