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[AMPS] ? Voltage Doublers as Anode Supplies

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Subject: [AMPS] ? Voltage Doublers as Anode Supplies
From: measures@vc.net (Rich Measures)
Date: Tue, 1 Sep 98 05:40:46 -0800
>
>Heath and I'm sure many others made a bit of coin using full-wave voltage
>doublers for their anode supplies.
>I've been trying to "design" one myself.  I've been using the output
>voltage curves in Langford-Smith (p. 1174, 4th ed.) (published in slightly
>different format in any ARRL handbook; recent ones are chapter 6) to find
>Vout given C, Rload, and Rsec.  Rsec is generally given as the sum of the
>forward resistance of the rectifiers and the transformer itself, and in the
>curves the parameter for finding Vout/Vac is Rsec/Rload.  And of course
>Iload=Vout/Rload.
>(I use Langford-Smith because many more values of Rsec/Rload are avaiable,
>especially for the doubler case).
>Findings with some questions:
>Vout/Vac is *very* sensitive to Rsec/Rload in a doubler configuration,
>compared to a standard full-wave bridge.  I suppose this is illustrated by
>the oft-published statement "doublers have crappy static regulation".  It
>almost seems unusable...under load you might be 2x Vac and with no load
>2.8x Vac...yuk
>       a. When a tube is spec'd "Maximum anode voltage 3kv", I assume that 
> means
>3kV under ANY conditions, right?  So building a doubler that floats no-load
>up to 3.8kV or so is a no-no.
>       b. If memory serves the Heath SB-200 doubler supply was 2750V no-load 
> and
>2400V under load.  How did they get that good of regulation?  Practically
>as good as a full-wave bridge.
>       c. Say I have a high-quality transformer (maybe only 50 ohms secondary
>resistance) but I want 2x Vac,rms, not 2.8x...the curves would imply that
>the path to get there is adding some resistance in the secondary line (my
>case says 130-140 ohms)...is this a real and accepted practice?  Seems
>silly to blow 130W with a 2.5kVa supply...?  But how else can you "dial in"
>the load voltage of a doubler?  Or perhaps the clue is to intentionally
>design as near to 2.8x Vac under load?
>       d. As an exercise, I wanted to see the variation of load voltage with 
> load
>resistance.  So I started at a design point and found the resultant
>voltages for load resistances doubling and then halving from the design
>point, to make a curve of Vout vs. Rload.  It's interesting in that the
>quantity Vout^2/Rload (or, kVa) moves around a LOT...increasing
>significantly with low load resistances (high currents).  Obviously one
>cannot draw infinite current at zero load resistance; what physical effect
>am I missing?  Do these design graphs assume some effect to be zero, which
>becomes dominant at low load resistances?  I don't have the original
>Schrade (sp?) article in 1930-something Trans. IRE.
>
>I'm inclined to think that Heath did it by using a transformer that many
>would consider sub-standard; that is, having a fairly high secondary
>resistance.  Again, though, all that excess heat to dissipate seems like a
>poor design choice.  Does anyone have any particular insight along this line?
>
>Would love to hear anyone's experiences, etc.  And,
>THANKS IN ADVANCE!!!
>------------
>Scott Townley          
>nx7u@primenet.com
>------------
>Collector of:
>       Stoddard Aircraft EMI/RFI receivers and accessories
>       Big Parts for that Big Linear Amp 
>       70's era RF test equipment HP/GR/Tek
>       Radio-related technical reference material 1940+
>       ...anything else that will keep me off the streets at night
>
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>



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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