[Milsurplus] GRT21/22, AM6154/55

[Charlie L.]

 "...we got some VHF transmitters that used a 10 watt AM exciter to drive a
one tube linear to make 40 watts."

If the GRT's for V/UHF aviation freqs and their matching amps is what you
are talking about, the exciter ran 10 watts out and the amp 50. They were
hooked together and the exciter output would rise as the tube aged, ALC
controlled to 50 watts of resting carrier, peaking to about 200 watts PEP
AM.  If the amp failed, the exciter would continue to work, bypassing the
amp.  At first they were set at about 70 or so MA in stby with 1200 on the
plates cooking away at 80 watts or so, but a mod came out to reduce stby
current to zero.   When keyed, they went to 170 ma unmodulated. The amps
make great U/VHF amps for CW and SSB and you can get upto 350 watts out
with a good tube.  You have to remove the bandpass filter in the output,
and the special mica insulator between the plate and the grounded cavity
style tank circuit is hard to find and hard to substitute, and is a common
fail point where it would pinhole arc through with the sound of a .22
Magnum. . The final is nothing more than a 4CX250 with a larger set of
cooling fins on the top, making it a 4CX350 and rated at 350 watts.  I have
had  one 2 meters for over 30 years running 300 out to a 13 element yagi,
but lo, no one to talk to on SSB anymore in my area.  They were designed in
the time where the idea was to throw away bad modules and put in new.  But
like all good government plans, the gear stayed in service so long that you
had to repair the modules, or send them back to the depot as the new ones
were astronomical in price if you could get them.  There is a 100 watt
screen resistor in their you do not want to burn up, it is the poster child
for unobtanium.  Fair Radio had the transmitters and the amps for years,
and spare PA modules, but that option is gone forever now.  In the late
70's, I swapped out every tube transmitter and receiver at 3 different
sites on the same airport to these ITT GRR and GRT rigs.  There were over
100 transmitters, and receivers (mains and standby plus single freqs).
With the 100+ tube type receiver site at that time, one man was assigned as
his only job to take care of it.  After the GRR's went in, the resulting
drip in maintenance allow that fellow to take on more stuff, and he picked
up the 2 transmitter sites for his workload.  It also dropped the cooling
demand for the buildings but we had to buy space heaters for the winter
where there was no heat required before.  In the receiver site, there were
about 1000 tubes burning all the time.

de W4MEC
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