Message text written by "Steve Eldridge"
>John Nelson G(w)4FRX published a design for a Thyristor controlled HV
supply as part of a series on the 4CX250/350 in the " Short Wave Magazine",
June '81 to Jan '82. This enabled a wide range of control of final HT
voltage, along with regulation due to feedback. I'm not too sure how many
were built, or whether RFI problems were significant. The article claimed
that these could be tamed. Hope this helps and good luck with the amp.<
The things I did in my giddy youth <g>. I know of about 20 that were built,
with some still in use. Two are still going strong at this QTH, with one
having supplied the HV for a pair of 4CX600s on 144MHz for about 14 years
now. I also know of several folks who just built the thyristor stack and
used a variable DC control voltage to make a species of solid-state Variac.
Given the benefit of 20/20 hindsight, the feedback regulation wasn't really
worth having but the protection circuitry works well and the ability to
soft-start the HV and set its value is useful on occasions. RFI isn't a
problem at all if the thyristor stack is built in a screened box and the
mains filter included as described. I'm sure the job could be tackled a
whole lot easier these days with modern components.
BTW and FWIW, I've never had any problems running Variacs in front of
transformers feeding capacitor-input filters. But in the circumstances
described, I'd opt for the choke-input filter every time -- even if it
meant reinforcing the floorboards.
73 and HNY to all
John GW4FRX
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