>
>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
If one selects a transformer with low internal R, a FWD rectification is ideal
for SSB. However, FWB rectification is better for RTTY and FM. On my Web
site, there is a circuit which does both. The FWB produces double the current
at half the volts. One SPST switch is needed.
> a. When a tube is spec'd "Maximum anode voltage 3kv", I assume that
> means
>3kV under ANY conditions, right?
not for SSB
>So building a doubler that floats no-load
>up to 3.8kV or so is a no-no.
To get a rough idea of what the peak volts ability of the tube is, look at the
tube's AM rating. For example, an 8171 is rated at 7500v max. for N-Ø-N
(A-Ø) service. For AM service, the rating is 5000v, so with 100% modulation,
the peak potential ability is 10kV. For SSB service, an 8171 works without
quitewell with a 9kV no-load anode supply. (don't try this at home without
your parent's permission, kids)
> 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.
To obtain good regulation in a FWD, one needs to use a transformer with low pri
& sec winding resistances.
> c. Say I have a high-quality transformer (maybe only 50 ohms secondary
>resistance)
A bit much for typical FWD service. For my last anode supply I used a
transformer with a 10.5 ohm secondary. The Heath SB-220 uses a 12.2 ohm
secondary trans.
>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?
No. The accepted practice is to reduce R to a minimum. For SSB service,
maximal anode potential is usually better.
>Seems
>silly to blow 130W with a 2.5kVa supply...? But how else can you "dial in"
>the load voltage of a doubler?
I order the hv transformer to be built with 3 secondary taps. The low tap is
for FM/RTTY. The high tap is for SSB.
>Or perhaps the clue is to intentionally
>design as near to 2.8x Vac under load?
True. 2.5 - 2.6 x Esec is about the practical limit.
- later, Scott.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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