(c) 1999,2008 Peter McCollum
The RT-4 Transmitter
This transmitter is basically a larger version of the RT-1-B,
and was also used in embassies. It operates from 4-25 MC in four bands, with
four crystal sockets (FT-243 style) plus a selection for an external VFO. The
meters include the "RDR" logo. The antenna output is for balanced
line, using a final-tank roller inductor with dual taps.
The separate power supply unit provides the final's plate
voltage, plus a variac-controlled output of 115V to operate the main
transmitter chassis (all other voltages are generated in the transmitter
chassis). The supply has two meters: one to indicate the input voltage, and one
for the variac-controlled output voltage. The power input can be strapped for
either 115 or 230 volt input.
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RT-4 Transmitter Specifications |
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Transmitter |
Power Supply |
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Weight |
about 160 lbs |
about 180 lbs |
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Dimensions |
19"W X 14"H X 21"D |
19"W X 14"H X 21"D |
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Tube complement |
6AG7 osc./VFO buffer |
866A (two) rect. |
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2E26 driver |
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4-125A (two) final |
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83 rectifier |
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0C3 regulator |
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0B2 (two) regulators |
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Frequency Bands |
4-6.5, 6.5-11, 11-17, 17-25 MC |
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Power Output |
about 500-600 watts |
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The front panel of the RT-4 transmitter. The
identification has been thoroughly scratched off. Author's collection, photo by
Bill Strangfeld.
Rear view of the RT-4. On the rear panel skirt, the
VFO input is visible on the left, and the high voltage connector is on the
right. Author's collection, photo by Bill Strangfeld.
Top view of the RT-4, with the cover removed. Of the
two large inductors visible, the top one is adjustable, the other fixed. Note
the taps on the coils. Under the two strips near the middle is a pair of
bandswitches. The two 4-125A's are visible at the bottom. Author's collection,
photo by Bill Stranfeld.
Bottom view of the RT-4, with compartment covers
removed. Author's collection, photo by Bill Strangfeld.
RT-4 notes from a user:
I remember installing one in
1952. Once the networks began changing to radio teletype we started using all
commercial gear for anything over 100 watts. After we ran out of a lot of the
military hand-me-downs such as the Hallicrafters BC-610 (military HT-4) we used
a lot of the TMC GPT-750 since it could be used key down at around 500 watts 12
to 14 hours at a time. It was a big old thing with three big pull out drawers.
It was complete with it's own frequency shift exciter. Only problem was it
required a good remotely tunable antenna, otherwise a high standing VSWR was
reflected back to ground through the filament transformer. In emergencies I used
to put the transformer on styrofoam blocks, lifting it above ground. Of course
I locked the bottom drawer to prevent some poor soul from getting electrocuted.
RT-4 and GPT-750 notes from another user, “kwcactus”:
[At one time] I was given an
RT-4. I put the RT-4 on the air and used it on the ham bands. Finally I tore it apart and rebuilt it
as a pair of 4-400A tubes. It was, though, a pretty good transmitter as a stock
RT-4, though it was quite heavy for what it did. Incidentally we also used the
GPT-750 in some places, and found it to be very reliable. We had it with the
FSK deck as well as with the SBE deck. We even had one with the AM modulator,
for special purposes. For most
installations we used a 48 foot cage dipole, fed with a Harris remote antenna
tuner, servo operated. We ran the RTTY version at 500 watts output in 24 hour a
day service. The GPT-750 was the mainstay in most embassies around the world.
It was also used in remote field sites (such as Quemoy) and in Vietnam. Some (a lot of) embassy stations had
two or three GPT-750s, one reserved for RTTY, one for standby/backup, and one
for CW operations. The matching receiver, the GPR90RX, never made the grade,
and the GPT-750s were teamed with R390As for the most part.
Following are images of the RT-4's power supply:
Front view. The left meter
indicates input AC voltage, the right meter indicates 'output' voltage, which
is set to 115 V using the large variac knob. The two switches and round lights
are for filament and plate power. Between the meters is a circuit breaker.
Author's collection.
Top View.
Bottom view.
Rear view.
Rear view, with case.