February 1941 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights are hereby acknowledged.
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!
See all available
vintage QST articles
A Simple 5- and 10-Meter Transmitter
|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.
For Portable/Mobile and Home Station Use
By Wilbert L.
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.
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
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.
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
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 unit.
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 watts.
A 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 and appearance.
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, 2014