T-368 AM/CW/RTTY Transmitter
T-368 Line art
Produced in large numbers during the 50's and 60's, the T-368 transmitter was
used by the military as a multi-purpose unit. Affectionately known as the "T-3"
a great many were placed in stationary RTTY service, while others were used for
AM and CW operation. As a medium power transmitter the T-368 was overbuilt for
continuous 7/24 service. Originally designated as the T-368/URT, the T-368 has
since undergone several updates, culminating in the T-368F model. Major updates
were the addition of a vacuum T/R relay, and modified protection circuits. Most
of the older non-lettered models have undergone MWOs (Modification Work Orders)
for service updates. The MWO tags are typically affixed to the front of the
The unit weighs about 700 lbs., and is roughly cube shaped. Height and length
are close to 41". It's modular construction allows easy access for service. The
lower deck, and the most heavy, is the power supply section. The middle deck,
and next lightest, is the modulator, and the top deck is the RF section. All
decks are equipped with Cannon connectors and can easily be removed for service
and troubleshooting. It fact it is possible to "hot swap" defective decks while
the power is still energized with no adverse effects.
Conversion to Amateur Use
The T-368 requires very little in the way of conversion for use on the
amateur bands. With its 450 watts of carrier, and heavy duty construction, the
T-3 makes an outstanding amateur transmitter. A few simple safety and
convenience changes are all that is required. See the
Conversion button for more information.
The T-368 uses a total of 29 vacuum tubes. Aside from the "6000" tube
mentioned later, all are quite readily available from commercial vendors and
hamfests. A pair of 4-125A tubes are used for modulators, and a single 4-400
for the PA. Power output is 400-450 watts carrier in the AM mode. The unit is
quite frequency agile, capable of a tuning range from 1.5 Mc to 20 Mc with the built-in
exciter unit. The exciter tube, know as a "6000" is a bit unusual, and becoming
more difficult to find. I have replaced the rectifier tubes in my T-3 #1 unit
with solid state found that the unit runs considerably cooler. Although this
does raise the supply voltage somewhat, I believe that the design offers
significant engineering headroom as not to cause a problem. The tube lineup can
be viewed in detail by clicking on the
Tube Use button.
Two blowers are utilized to keep the units interior temperature within a safe
operating range. One blower located on the RF deck force cools the 4-400 final
tube. The second blower, and the larger of the two, is located on the rear cover
of the unit. This blower forces air into the power supply deck. Despite having
two blowers, the T-3 relies to a large extent upon convection cooling. Cool air
is drawn through a filter located on the rear of the back cover, forced into the
unit by the larger blower, and exhausted via a vent in the top right rear of the
Power requirements are simple, 110-120VAC at 20 amps. A stiff service is
required to allow the T-368 to run at it's full potential. Some of the earlier
units, the non-lettered models, were supplied with a power supply that could be
used on 230VAC, but the later units lack this feature.
In it's stock form, the T-368 suffers from classical military limited
frequency response in the audio. When used with the ubiquitous D-104 mic the
audio is acceptable. However, with a few simple and almost painless mods the
unit can easily produce audio very close to broadcast quality.
One of the more popular audio mods, and one which I use in my T-3 #1, is the
N9FOY speech amplifier mod. It is based loosely upon the AF
amplifier used in older RCA Broadcast Transmitters. Click on the
N9FOY Button on the left side of the screen for more
information. Steve has also designed a simple circuit that allows individual
bias adjustment of the modulator tubes.
A simpler audio mod, and one which I use on T-3 #2, is described under the
KG2IC Mod button on the left side of the screen. This mod
does not offer the audio quality of the N9FOY, but does make a noticeable
difference over the stock unit. As a plus it retains the stock speech amp, and
is easily reversible.
Tim WA1HLR has designed an extensive series of mods for the
T-368 transmitters. These mods encompass both the audio and the RF sections of
the unit, and the results are quite impressive. The RF deck mods permit a
considerable improvement in the transmit audio. Click on the
WA1HLR Mod button on the left side of the screen for details and
Although the unit is exceptionally rugged, over time some weak points have
appeared. They are quite minor, especially in the light the vintage of the T-3,
now approaching 40+ years in age! One of the more common problems has to do with
the safety interlocks on each deck. Over time they pit and do not allow a good
connection. The fix is simple, just bypass the interlock units by jumpering out
the interlock wires on each deck. My second T-3 suffered from this problem, and
since the interlocks were jumped out it has worked just fine. A possible fix for
this problem is listed under the
Interlocks button. A word of
warning here. If the deck
interlocks are bypassed, the high voltage (2,400 volts) will not be disabled
when the decks are removed!
The exciter tube, a type 6000, is also becoming a bit rare, although alternatives
Fair Radio was stocking new replacements for this tube. It
would be an excellent idea to obtain a few spares.
As these units were often used in continuous service, it is not unusual to
find the blower bearings are worn and noisy. Both of my units suffered this
fate. The blower motors are serviceable, and replacement bearing can be readily
found at a good bearing supply warehouse. The cost of replacement bearings was
quite reasonable at $1.83 each.
The plate current meter is susceptible to failure as it carries HV on it. The
later meters were mounted with an insulated standoff ring. The exact replacement
meters are becoming quite scare, although another meter maybe substituted in
it's place. A various number of mods are available to correct this problem. The
majority deal with removing the HV from the meter circuit. The
WA1HLR Mod in particular works quite well, and offers improved
transmit audio as well.
The BC-939-B antenna tuner of BC-610 fame was also used with
the T-368. The top of the T-368 cabinet contains mounting studs to secure the
BC-939-B. Tuning range of this unit is from 2MC to 20MC.
The MA-29A/U carbon mic was designated to be used with the
The ME-165/G combination dummy load/SWR/Power meter is an
especially useful accessory to have. The ME-165/G is a 600 watt dummy load and
Power meter which is invaluable for tuning up the T-3.
As it was designed for military service, the T-368 is exceptionally simple to
operate. It is also equipped with numerous safeguards both for the protection of
the transmitter, as well as the operator. The T-3 employs a overload protection
relay which engages when excessive plate current is detected. A thermal
protection switch is also placed over the 4-400 final tube to protect it in the
event of blower failure. Any frequency between 1.5KC and 20KC can be readily
selected with the built-in exciter unit. The two large vacuum variable
capacitors used in the loading and tuning circuit allow rapid and painless
frequency changes. The design of the Pi-L tuning network allows the T-3 to match
a wide range of antenna loads.
The T-3 is really overbuilt for amateur service, and it is a pleasure to
operate on the air. When used in intermittent amateur service, the unit will
easily outlast its owner. Service parts are readily available from a variety of
vendors, see the
Resources button for more info. Failures are infrequent,
and when they do occur, it is usually a simple matter to diagnose and correct.
There is very little, short a commercial broadcast transmitter, that can
offer the "brick on the key", 7/24 service of a T-368. Given some simple mods,
and a little time, the T-368 is capable of outstanding transmit audio. When
matched with the R-390A receiver, the T-368 indeed offers a bulletproof and
formidable vintage AM station.
See you on the air!