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[AMPS] Additional Info Voltage Doubler/Filter Capacitance

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Subject: [AMPS] Additional Info Voltage Doubler/Filter Capacitance
From: (Tom Rauch)
Date: Fri, 21 Sep 2001 09:20:15 -0400
>    Thanks for the info so far. Additional info is as follows: The
>    transformer was designed to be used with a doubling rectifier. The
>    secondary is 900 volts +or- 5%. The secondary resistance is 16
>    Ohms. (primary will be wired for 240V).

Ouch Kurt!! That is very high resistance, because the secondary 
ESR (equivalent series resistance) will be substantially higher than 
the secondary dc resistance. The actual resistance seen by the 
rectifiers and capacitors will include 16 ohms plus core losses and 
primary/power mains resistances!

You want to use a conventional (full-wave) doubler, and not a 
cascade (half-wave) doubler. Cascade multipliers have less 
regulation and more ripple, and are mostly used where one side of 
the source is grounded or you need high multiplications.

Doublers are actually just as good as bridge rectifiers IF the 
transformer is the same size and fully optimized. It does not sound 
like your transformer is sized for four 572B's at all.  

Assuming you have 2500 ohms of DC load resistance from the 
tubes (1600 watts tube input power), and 30 ohms of ESR (which 
is reasonable for a 16 ohm DC resistance secondary when used on 
a very stiff power line) and 2 ohms of secondary ESR caused by 
the capacitors and power line resistance, around 80-100 
microfarads will be optimum.

The voltage will be 2500 volts no load and almost certainly well 
under 2100 volts full load. Any amount of capacitance above 80-100 
uF (for each half of the series string) will NOT improve things.

The reason the regulation is so poor is the transformer resistance 
is excessively high. Looking at a doubler transformer I have laying 
around for the same approximate application, the secondary dc 
resistance is under 8 ohms.

Your transformer will be usable, but the regulation will probably 
only be around 20% at 800ma (1600 watts PA input power) 
assuming they sized the primary wires with a slightly better design 
margin than secondary wires. 

Your transformer will dissipate around 300-400 watts as heat when 
delivering 1600 watts output. Keep that in mind, because more 
than likely it is not designed to handle the temperature rise with 
more than casual SSB or CW operation.

73, Tom W8JI 

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