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. All copyrights hereby acknowledged.
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 throughout.
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.
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 (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.
0.1 μfd. 600.volt.
C4 - 100 μμfd.
C5 -10 μfd. 50-volt electrolytic.
C6 -100-μfd. 25-volt electrolytic.
R1 - 20 ohms, 10-watt.
R2 - 200
R3 - 50,000 ohms, 1-watt.
R4 - 25,000 ohms, 10-watt.
- 15,000 ohms, 10-watt.
R6 - 1000 ohms, 1-watt.
|T1 - Microphone-to-grid transformer.
T2 - Single-plate to p. p. grids.
- 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
M - 0-100 milliammeter.
Sw - S.p.s.t.
RFC - 2.1-mh. chokes 125 ma.
Closed circuit jack.
L1 - 6 t. No. 12 wire
3/4" diameter spaced diameter of wire.
- 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
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
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.