Howdy, Don!
At 02:26 AM 11/11/2007, you wrote:
Looking for a halfway decent condition smoothing choke, 30 to 50 Henrys,
hopefully insulated for 4 KV or more, and capable of handling maybe
300 MADC or more.
>>> Is this for a power supply filter choke, or a modulation reactor? <<<
This is for mod shunt reactor use. I'm already in good shape re. supply chokes.
To elaborate on this a little bit on it...
Repeat... NOT a swinging choke!!! I can't imagine what a swinger
would act like
as a shunt reactor!!! :o(
The closer to 300 MADC rating I can get on the choke, the better; as
we all know
a 50 Henry, 500 MA or more shunt reactor from a broadcast transmitter
is a thing
of beauty, but it's BIG... way too big for this project. My goal is
to stuff this rig into
a tabletop rack roughly the same size as the original Globe King 500;
I'm already
cheating a bit by using a computer server rack that's 8" deeper than
the Globe King.
I own, and have run for several years now, a Globe King 500 (no
suffix), on 160 AM.
Further, I've been acquainted with the rig for a LOT of years (I've
been licensed since
the early 1960s), and have heard all the tales about them...
including an oft repeated
tale about the rig CATCHING FIRE while on the air! Unfortunately, I
too have had that
experience; I now keep a CO2 bottle in the shack, just in case the
reengineering job
I did on the transmitter after the fire wasn't adequate.
The biggest problem with the rig's design is that the bean counters
got to it after the
engineers designed it; they were thinking bottom line on profits,
cutting production
costs anywhere they could, and the place they hit hardest was the
power supplies.
The plate supplies for PA and modulator are a bad joke; the filter
chokes are the point
where the fires started. They're grossly overloaded, and more
suitable for a receiver
than a 500 watt transmitter. They overheat and the insulation breaks
down; in the case
of my rig fire, the PA plate filter choke broke down to the case,
causing it to smolder
and burn, interestingly without taking out the line fuse. I was in
the middle of a rag chew
when I saw smoke coming out of the back of the rig; I had time to
tell the other guy what
was happening and do a legal sign-off, before cutting the power off
and ripping the PA
plate supply out of the bottom of the rack (the rack screws weren't
in place), and carrying
the smoking hulk out of the shack, through the garage, and into my driveway!!!
I wound up taking the choke apart and rewinding it (have you tried to
find "fish paper" for
transformer winding insulation these days? <<GRIN>>), and wrapping
the finished choke
winding with teflon tape to provide additional insulation. Likewise,
I wrapped all of the
other chokes and transformers while I was at it, and as a final
measure lined the end bells
of the chokes and transformers with sheet polyethylene cut from
plastic milk jugs.
Another power supply problem... low plate voltage on the PA. A
4-250A will work with only
1800 VDC on the plate, but the plate efficiency sucks. That tube
doesn't begin to come
into it's own with less than 2000 VDC on the plate. At 500 watts
input on 80 meters, my
rig (measured with a Bird 43 into a Bird dummy load) will produce an
AM carrier output of
270 watts; not real good plate efficiency for a class C final. IMHO
if the plate transformer
had been adequate (read that as more expensive) to provide 2000 VDC
under load, the rig
could get maybe 15 - 20 percent more efficiency... and maybe even
make the legal limit
output for AM (375 watts of carrier).
As it is... the PA stage runs hot; at this lower than optimum plate
voltage, the plate is
normally running a dull red with unmodulated carrier. Voice raises
the plate to cherry red.
These figures are legitimate; meter shunts have been put into
tolerance, and the PA bias
voltage has been optimized.
Other design penny pinching caused an oddity in the PA stage, one
that it seems only
Globe King 500 owners and operators know about.
Did you know that on 160 meters, the Globe King will NOT match a 50
ohm load? To
save a few of bucks, they cut down the size of the PA loading
variable, and likewise cut
the size of the PA pi net coil, designing it to match a minimum
impedance of 300 ohms.
That's one major change I'm employing to correct a problem in my
version of the rig.
A VERY bad mistake in the cost cutting effort was eliminating
protective circuits for the
PA tube... if the original rig's design used overload relays.
Tuning a Globe King 500 is touchy business. Unless plate loading is
kept heavy, and grid
drive is kept to a minimum, the screen grid current is sure as hell
going to soar to the point
where the maximum screen dissipation rating is going to be exceeded.
After blowing up one
tube while testing for maximum PA efficiency, I learned my lesson; I
keep the multimeter
on screen current and watch it like a hawk during tuneup.
I've never been able to understand the huge prices that the Globe
King 500 pulls on the used
rig market; IMHO it's a marginal transmitter at best, but the design
concept is a good one. I
want to use that concept to make a more robust rig that'll give GOOD
quality AM while coasting
along at 500 watts of carrier OUTPUT without straining anything.
73's,
Mr. T., W9LBB
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