4 tubes and sockets, 12 capacitors, 7 resistors,
4 inductors, 3 transformers, a crystal, a meter movement, a switch , a bulb, 3 jacks
(for a tuning meter), a project box, a handle and and little hookup wire and solder.
That's all it used to take to construct a home brew dual band (5- and 10-meters)
amateur radio transmitter as featured in the February 1941 edition of QST.
You can probably find all the parts at a Hamfest to make one today, but you will
need to modify the 5-meter band circuit to current 6-meter band operation since
there is no 5-meter band anymore (lost to VHF television).
February 1941 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
5- and 10-Meter Transmitter
For Portable/Mobile and Home Station Use
By Wilbert L. Thompson*
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 be 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.
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.
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 insula-tors. 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.
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.
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 re-read,
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 straight-forward 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.
Fig. 1 - Circuit of the 5- and 10-meter transmitter.
- 50 μμfd. variable.
C2 - 0.005 μfd. mica.
C3,C7 - 0.1 μfd. 600.volt.
C4 - 100 μμfd. mica.
C5 -10 μfd. 50-volt electrolytic.
C6 -100-μfd. 25-volt electrolytic.
R1 - 20 ohms, 10-watt.
R2 - 200 ohms, 2-watt.
R3 - 50,000 ohms, 1-watt.
R4 - 25,000 ohms, 10-watt.
R5 - 15,000 ohms, 10-watt.
R6 - 1000 ohms, 1-watt.
- Microphone-to-grid transformer.
T2 - Single-plate to p. p. grids.
T3 - P.P. plates to r.f. load (6000 ohms)
B - 2-volt 60-ma. bulb (or larger - up to 200 ma.).
X - 10-meter crystal (Bliley).
M - 0-100 milliammeter.
Sw - S.p.s.t. toggle switch.
RFC - 2.1-mh. chokes 125 ma.
J - Closed circuit jack.
L1 - 6 t. No. 12 wire 3/4" diameter spaced diameter of wire.
L2 - Commercial 10-meter plug-in coil. Same for 5 meters.
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.
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.
Got Your Code Certificate Yet?
Have you got your code attainment award certificate from A.R.R.L.? This League
award is available to every United States amateur li-censed. The program aims to
recognize your code ability. WIA W prac-tice transmissions take place on 1761, 3825,
7280, 14,253 and 28,510 kcs. daily except Friday starting at 9:15 p.m. C.S.T. These
will help you add to your ability to read code the knack of copying code. It is
time now to prepare for the next official qualifying run from WIAW which will take
place Friday, February 21st at 9:30 p.m, C.S.T. Aim to get your certificate or endorsement
sticker for higher speed on that date.
October 13, 2021
(updated from original post on 9/20/2012)