>
>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.
For 100% duty-cycle RTTY and FM use, a FWD is not wonderful. However,
for SSB (which is c.35% duty cycle), a FWD makes much DC potential, which
is precisely what one needs for linear amplification. . On my Web site
there is a circuit which allows the operator to switch between FWD and
FWB (full wave bridge) operation. One SPST switch is used.
> 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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