article is provided as a reference to how these early vacuum tube
transmitters were designed and built. Modifications in the circuit
would be required to adapt this transmitter to modern standards.
It is also a good reference for theory and operation of some of
the older equipment that might be valuable for hobbyists who restore
old radios - and there are a lot of us out here!
February 1941 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
A Simple 5- and 10-Meter Transmitter
For Portable/Mobile and Home Station Use
|Here is a rig to satisfy anyone's yen for
a small transmitter for the 5- and 10-meter bands. Small
enough to make a good 56-Mc. mobile rig, it is large enough
to provide plenty of 28-Mc. contacts from home.
With the lid clamped down on foreign DX, the
high-power rig seems to be a waste of energy nowadays. Why not reduce
power to the point where distances allowed can be spanned with some
pride of accomplishment and at frequencies that are not jammed with
QRM? For those who wish to "down" their power and "up" their frequency,
this article describes a 5- and 10-meter 40-watt rig that can be
operated as a mobile 'unit on 5 meters and in a fixed location on
10 meters, in compliance with F.C.C. ruling.
In spite of its orthodox appearance, as shown in the photographs,
this little transmitter brought up some interesting points that
I believe to be of interest. The front panel contains the meter
which can be plugged into the crystal oscillator, r.f. amplifier
and the modulator circuits. The left-hand dial tunes the 6J5G oscillator,
the right-hand dial tunes the 807 amplifier, and the antenna is
connected to the right-hand feed-through insulators. The jacks under
the meter are, left to right, oscillator, amplifier, and modulator
cathodes. The two red lamps indicate crystal current on the left
and filament "on" on the right. The microphone jack and stand-by
switch are immediately below. The bottom row left to right are the
6-volt receptacle, the audio gain control and the 400-volt d.c.
receptacle. The entire unit is housed in a 7- by 9- by 15-inch metal
case with a handle added.
A 5 & 10 transmitter in a 7- by 9- by 15-inch cabinet.
good for a 15- to 20-watt carrier. The two main dials control
the oscillator and amplifier tuning, and below the dials
can he seen jacks for metering the various cathode circuits.
The two buttons directly below the dials are dial lamps
used to indicate crystal current and filament "on".
A rear view of the transmitter shows the r.f. portion on
the upper chassis and the modulator below. The construction
is conventional throughout.
There is nothing new or novel
about the circuit. The original layout used a 40-meter crystal and
a 6L6 quadrupling to 10 meters, with an 807 as a straight amplifier,
but the new ruling of the F.C.C. caused the redesign so that 5 meters
could be used for mobile work, leaving the 10-meter operation for
fixed use only. As most fellows know, even the old stand-by circuits
are often critical. With this in mind, care was taken in using fairly
good parts and in making short leads. For reference, QST of January,
1938, the 1940 Handbook, and the Bliley Bulletin E-6 were read and
reread, but still the unit had several unsuspected "bugs."
In the 6J5G oscillator circuit, the only deviation from recommended
practice was the grounding of the tank condenser. This offered no
apparent difficulties. Much trouble was had, however, in making
the oscillator function. This trouble was finally traced to a dirty
crystal. I hope that anyone trying this circuit has a good crystal
to start with, because much "trouble shooting" will be eliminated.
Carbon resistors are recommended for the cathode. Wirewound resistors
were tried, but found to be less satisfactory. In all cases, low-loss
condensers should be used, not only for greater efficiency, but
also because it may mean the difference between success and failure
of the oscillator circuit.
The final amplifier circuit can
be found in any radio book, hence no trouble should be expected
here. Again Lady Luck frowned on this circuit, because a defective
807 resulted in considerable "trouble shooting." But RCA gives new
"lamps" for old (with reservations).
For simplicity, no
bias batteries were used on the 807 final, sufficient bias being
developed by the grid leak. Screen-plate modulation was found entirely
satisfactory, thus allowing for a simple modulation transformer.
The output circuit can be any standard style to meet existing antennas.
With mobile use in mind, link coupling with a short twisted feeder
was used. Antennas of the half-wave or quarter-wave variety are
very easy to use; in fact, odd lengths were tried with surprising
results. The audio section is just as straightforward as the high-frequency
section. A good single button carbon "mike" gave good intelligibility
to the signal with plenty of drive. A 6N7 dual triode operated Class
B gives good volume with good economy. The total current from a
power pack of the vibrator or generator type doesn't exceed 150
ma. This keeps the mobile power-supply costs fairly low. Attention
should be called to the lack of batteries. Microphone current is
obtained from a resistor in the "B" minus lead, bypassed for audio
frequencies. Any voltage from 2 to 10 seems to operate the average
microphone well. The entire audio is mounted on the lower deck of
The oscillator plate current runs 20 to 25 ma.
when tuned to resonance. Unlike common grid-leak-biased tubes, resonance
is indicated by maximum plate current. The final amplifier plate
dips to 20 or 25 milliamperes. Since the meter is in the cathode
circuit, it reads combined grid, screen grid, and plate current.
The grid current of only a few milliamperes is disregarded in the
meter reading. With 8-10 milliamperes screen current I find that
the drive to the 807 final is sufficient. This results in fairly
good efficiency on 10 meters. With antenna or dummy load, it is
possible to load up the final to about 55 ma. This results in a
power input of approximately 22 watts and an output of about 12
jack was included in the modulator plate circuit more for convenience
than necessity, so that the meter can be used as a volume indicator
if desired. The no-signal current runs about 40 ma., while average
speech sends the current up to 60 ma. Steady sine wave input for
maximum output (100 per cent modulation) runs about 70 ma.
While this transmitter was originally designed for portable
and portable-mobile use on 5 and 10 meters, it seems not undesirable
to have one of these units around the shack for emergency or local
rag chews. With the commercial plug-in coils and several crystals,
band change can be quickly accomplished. In spite of the difficulties
encountered, this little outfit gave much satisfaction in its operation
Fig. 1 - Circuit of the 5- and 10-meter transmitter.
I wish to express my appreciation to W8QOG, Queen City Radio
Club, for the tests on the signal, Mr. W. Cheshire, W8UPC, and Mr.
W. A. Phillips and his associates in the laboratory for their assistance.
Posted January 23,