Neat idea, once you figure out the size of the iron. I didn't know that
an overloaded ferroresonant transformer would drop voltage that much (to
a few % of nominal line voltage). I suppose there might be a bit of
trial-and-error design work to get things matched up for a particular tube.
I am not suggesting that having a ramped up AC supply isn't doable, just
not as straight forward as using remote programming of a big commercial
DC power supply. If it weren't for the AC hum/vibration concern, these
VHF cavity-circuited tubes usually have one of the conductors of the
filament resonator (being cathode driven) hard grounded to the
structure. The tube has a cathode and a heater connection, cathode being
the hard grounded one. As such, one cannot use a CT transformer to
reduce AM noise (hum modulation).
In 1998 I designed an AC filament power supply for another Thales
tetrode, TH555A, that needed 15 VAC at several hundreds of Amperes.
Since it was a shortwave tube, it could use conventional center-tapped
filament transformer to prevent hum modulation. The -555A had similar
filament ramp up requirements, requiring about 8 minutes minimum ramp
time. I talked to the late Carl Seivers at SNC in Oshkosh, who designed
plenty of big and small iron for broadcast transmitters, EF Johnson and
even some for Benton Harbor. He said he'd be happy to take on a special
project, tiring of wall-wart transformer production engineering. He made
a very sweet saturable reactor, that had a DC coil and a dual wound AC
coil that was in series with the 480 VAC single phase input. It wasn't
small, mind you! I provided a 2 Amp DC ramped power supply (Lambda),
that controlled the reactor at any rate I desired. This essentially made
a precisely-controlled zero to 460 VAC source, that drove my filament
transformer, with no moving parts. Two of those things have been running
ever since, for both tubes of the push-pull pulsed amplifier.
> Filaments can just as easily be ramped up on ac, this way with no
> fuss. I have seen ferro-resonant regulator feeding the filament
> transformer, the size was carefully chosen so the low resistance of the
> cold filament "bogged down" the regulator to a few % out, as the heater
> warmed, the resistance rose as did the output of the regulator until
> opium temperature, current and solid regulation were achieved.
> I first saw this on 8.5 Volt 25 Amp Varian klystrons.
> -- Ron KA4INM
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|