Short Wave Sportfishing wrote in
:

Henry 2KD amp highly modified

Mr Henry made some really nice amps. I had a Collins 30S-1 for a while
until some ham in Georgia wanted it so bad he met my I-don't-really-want-
to-sell-it price I figured I'd never get. Don'tcha love hams with money?

I built a homebrew back in the 70's when an unlimited supply of 4-1000A
tetrodes became available from a broadcast biz run by a friend. Lots of
stations were converting to ceramic tubes in those days.

The amp was in a WW2 Navy M series rack that was about 7' high and 24"
wide. Power supply was a 7200V, 5KVA pole pig running off the output of
a 30A, 240VAC Variac marked "Plate Kilovolts" to match the 270 degree GE
monster meters up on the power amp module. A pair of 6080 power tubes
provided a nicely regulated screen power supply because, unlike most hams
that hook them up in grounded grid for simplicity, I was bound to use
them as they were intended, tetrode AB1, common cathode. A regulated
fixed grid bias supply had a front panel control marked "IDLE CURRENT".
The grid input circuit was a 200 watt carbon pile of resistors at 52
ohms, cooled by the fan pressurizing the chassis of the power amp module,
which blew up through the two 4-1000A proper airflow sockets into their
nice chimneys....all very commercial quality. A screen-shielded window
in the front of the amp module let you watch the pair of 4-1000As in
action.

Filament current for two tubes meant unwinding the secondaries off a big
240VAC primary transformer and winding my own to provide that much stable
current. A security keyswitch on the front panel and two 240VAC air
conditioner contactors provided primary power control. Coaxial relays on
the amp provided antenna changeover control. There was even an antenna
selection rotary relay with 6 outputs selectable from the front panel of
the amp. My favorite antenna was a 75 meter full wave loop strung
horizontally between 4 perfectly-place oak trees in my neighbor's pasture
he kept horses in next door, one of the trees right over my hamshack.

The power amp had a WW2 Navy rotary inductor with front panel turns
counter. The coil was square conductor about 1/8" square cross section
an about 8" in diameter. I never had it on 160M, but it would tune down
to the top of the AM broadcast band, easily. Plate tuning cap of this
shunt-fed pi output matching was a 30KV, 30A, 1500pf vacuum variable.
Output cap was an open air beast in 2 sections that came out of the
antenna tuning unit of a 5KW AM station that went dark, as did my
rectifier stack and filter chokes. Fed up with blowing the coupling
capacitor from the plates to the output pi, a rather large .01uf, 30A,
15KV broadcast ceramic cap was refitted. The others I had melted, not
flashed over...(c;

Of course, all this was overkill running it at 1KW on the ham bands, even
100% duty cycle on 20M RTTY for hours on end. I'm not a real math wiz
when it comes to calculating plate power, especially in the heat of
workin' some rare DX on a Pacific island that only is out of the water at
low tide, hmm.....950ma, 6200VDC....lessee....yeah, that's pretty close
to a kilowatt in, right?....(c; Hearing the 5KVA power transformer that
ran THREE houses buzzing away when it was on the air, I called the stupid
rural electric coop to warn them before it exploded. They poo-pooed my
request for more amps, but became believers when I blew the bottom out of
the transformer, setting an oil fire that burned up the whole pole to the
ground! A new pole and 15KVA beast seemed to cure the problem. I told
them my neighbor had a new hot water heater...(c;

You could get about 3500 W out of it with 10 watts of drive into the grid
dummy load resistor. It was so high gain I added a "volume control" pot
marked "GRID DRIVE" so you could turn the input power up to a more easily
manageable level for a 100W transceiver, then adjust the drive to what
you needed...minimum power, of course....and that prevented those
impressive flashovers unwanted modulation peaks created when you were
trying NOT to overdrive it running 10W from a 100W radio.

The crowning moment was when I came into the local ham club meeting with
a 2' section of completely melted RG-8 coax soon after building it. You
could see the smile on my face miles away...(c; One of our members was
chief engineer of another AM station that had an end-fed halfwave on 1220
Khz. He used RG-17A/U to feed it at that high voltage and gave me a few
hundred feet of 17A claiming I couldn't melt it. He was right...

About 10 years later, I moved to smaller quarters in Florida and sold
that amp to a North Carolina CBer who had been drooling over it for years
for $3500. I gave him 8 sets of 4-1000As in the deal, knowing he was
going to blow them trying to be king of the hill on CB...(c;

That was my last big amp....The next amp I had was the best one ever
made, a National NCL-2000 2KW desk amp with a pair of 8122 ceramic
tetrodes from RCA hooked up, also, as proper tetrode amps, common
cathode. Hams never learned how to properly tune them for SCREEN
CURRENT, not plate current, and blew lots of 8122s in the process.
Properly tuned, a desktop NCL2000 was has loud as a Collins or Henry
floor amp and would also operate on RTTY for hours without exploding at a
kilowatt. I've kicked myself ever since for selling it to W4ZMZ, a
Baptist preacher who lost the amp in a fire. What a waste....(snif)

Carrier on my signal? No problem.......(c; I used to tell them I was
rotating my 75M tower array around as I cranked up the big variac with my
foot. "How's that? Can you hear me over his carrier, now?", I'd ask.
My buddy a hundred miles away said I was the only one on the air that
could pin the S-meter on his Collins 75S3-B...