The Ely miner. [volume] (Ely, Minn.) 1895-1986, 1923-08-31
BY CHARLES DAWSON

by CHNRlES DAVlSoN
Now that standard power tubes are
available for amateur use it is prob
able that considerable experimental
work will be done in building tube
transmitters. The writer has had a
number of years experience in the de-
sign and operation of continuous-wave
equipment and hopes to provide some
practical information for the amateur
who is about to build his first CIV.
set.
We shall first consider a circuit that
Is to be used solely for straight CIV.
and LC.W. interrupted continuous
wave) transmission.
Working on the basis that two of the
five-watt power tubes are to be used
as Oscillators, the following units are
required:
A 40-watt transformer giving a Sec
ondary voltage of 700 to 800 with a
tap brought out at the midpoint of the
secondary winding.
A filament transformer with an out
put of five amperes at 100 volts.
Two radio chokes for the plate cir
cult.
Two bypass condensers.
A chopper or grid circuit interrupter.
An oscillation transformer.
An antenna series condenser
A grid leak and grid condenser.
These units are connected as shown
in Fig l.
The plate and filament transformers
may be purchased as one unit. Each
radio choke consists of 80 turns of
No. "a DEC. wire wound on core
of 21a inch outside diameter. The by
pass condensers should each have a
capacity Of .001 MID. and be insulated
for 500 volts.
A single-cOil oscillation transformer
will be found to give good results in
this circuit as we need not worry
about decrement with a CIV. set. This
coil consists of 80 turns of No. 6 bare
copper wire, the turns being spaced
" Inch apart with an outside diameter
of inches.
The antenna series condenser may
be variable from 00O3 to 000S mid. If
the antenna fundamental is low
enough this condenser is not required.
For the grid condenser we may use a
capacity ranging between .002 and OO4
with insulation to withstand 300 volts.
The grid leak should be about $500 ar
5000 ohms resistance, with carrying
capacity of 80 milliamperes.
Adjustments
After the various units have been
connected as shown in Fig. /, the
transmitter is ready to be adjusted for
maximum output at the desired wave
length. The general characteristics
Of this type of circuit are as follows
The variable contact A determines
the wave-length at which the circuit
will oscillate. The voltage built up
between A and B supplies the neces-
sary grid excitatiOn. For operation
with high resistance antenna more
grid voltage is required, therefore a
greater number of turns must be in
Guided between A and B. In order to
secure good operation the plate i'm
pedance must equal the antenna impe-
dance. The condition is obtained by
adjusting the turns included between
B and c.
1t will be observed that bringing B
nearer A increases the plate induc-
lance, but decreases the grid Undue
lance. AT all times the three variables
should occupy the same relative posi-
lions as shown. In other words B
must connect to some turn of the coil
between A and c. This makes the
plate and grid radio frequency volt
ages $50 degrees out of phase, which
is necessary for oscillation in any
vacuum tube circuit.
The amount of current obtained in
the antenna will, of course, depend
largely upon the radiation resistance.
This set will radiate from / to 1.75 am
peres with the average amateur an
tenna The power input into the tubes
can be roughly determined by the color
of the plates. These should not run
hotter than dull red If long tube life
Is desired
Fig. / shows the key and chopper in
series with the grid leak resistance.
This is the best manner In which to
control q tube set TO" telegraphy as