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
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.
See all available
vintage QST articles.
A Simple 5- and 10-Meter Transmitter
For Portable/Mobile and Home Station Use
By Wilbert L. Thompson*
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 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
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.
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."
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 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
|C1 - 50 μμfd. variable.
- 0.005 μfd. mica.
C3,C7 - 0.1 μfd.
C4 - 100 μμfd. mica.
-10 μfd. 50-volt electrolytic.
R1 - 20 ohms, 10-watt.
R2 - 200 ohms, 2-watt.
R3 - 50,000
R4 - 25,000 ohms, 10-watt.
R5 - 15,000 ohms, 10-watt.
R6 - 1000
|T1 - Microphone-to-grid transformer.
- 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).
Sw - S.p.s.t. toggle switch.
- 2.1-mh. chokes 125 ma.
J - Closed circuit jack.
- 6 t. No. 12 wire 3/4" diameter spaced diameter of wire.
L2 - Commercial 10-meter plug-in coil. Same for
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
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
Have you got your code attainment award certificate from A.R.R.L.?
This League award is available to every United States amateur licensed.
The program aims to recognize your code ability. WIA W practice 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.