Coll 35/31 'Bahrain: application for W/T amateur transmitting licence for employee of Bahrain Petroleum Company' [‎65v] (130/250)

48
A GUIDE TO AMATEUR RADIO
Meters
Meters represent an expensive part of a trans
mitter and it is desirable to keep down the number
if possible. One milliammeter is absolutely essential
and two very desirable, and these can be arranged
to measure all feed and grid currents. A 50 or
100 mA fullscale is most useful with a 10 mA for
grid currents. The meter can be arranged with a
plug and telephone jacks can be fitted in all circuits,
as in Fig. 13. Self-closing jacks must be used and
the preferred method of connection parallels the
closing contacts (see Fig. 13) by cross connection ;
in this way the circuit is not broken when inserting
or withdrawing the plug, thus there is no arcing at
the contacts. There is no reason why a low reading
meter should not be used with the appropriate shunt
across each jack, the multiplying factors being
marked over the jacks.
Examples
Two designs have been selected to illustrate the
principles of transmitter design. The first, known as
“ The D.X. Three,” is built to the circuit of Fig. 13.
It comprises a tritet oscillator which drives a
push-pull P.A. A receiving-type output pentode
in the tritet taking about 25 mA at 350 volts will
drive a pair of RFP 15 valves to about 50 watts
input, on any of four amateur bands.
Band change is made by changing the coils, and
normally the second harmonic of the tritet is used,
though for 3 • 5 Me is shorted to make it a straight
pentode C.O. The links are set up on the plug-in
coils.
The drive unit can be seen to the left of the
illustration, well separated from the P.A. Note the
link coupling. The P.A. uses two ganged condensers
on both grid and anode sides for condenser tap
balancing, and it will be seen that these are arranged
for short wiring to the valves. Since no external
screening is applied to the valves small neutralising
condensers are needed, as already described. The
whole transmitter is built up on a metal deck, with
R.F. wiring above and service wiring concealed
below.
The P.A. stage takes a current of about 100 mA
at 500 volts on load, the screens take about another
15 mA and the grid current is about 15 mA with
up to 70 volts negative bias. This bias, and up to
40 volts positive for the suppressor grids, can be
obtained from a single 120 volt dry battery, with
chassis connected at about 80 volts on the battery.
The suppressor current is only a few mA. (See
reference 14 at the end of this chapter.)
The second example is somewhat more elaborate
and its radio frequency circuit is given in Fig. 13,
whilst the illustration on page 46 shows its modulator
unit and aerial coupler and the rack type of assembly.
This transmitter will run at 100 watts on C.W. or
25 watts on telephony. The modulator unit and
monitor are on the bottom deck, the drive unit on
the next and the P.A. stage at the top. Above is
built out a universal type of aerial coupler.
The drive unit is of a type known as the " Jones
Exciter ” and uses two twin triode valves. V 3 is a
combined crystal oscillator and doubler, whilst V 4
is a second doubler of the push-push type. This unit
alone will give an output of 10 watts on any of
three bands and makes an interesting little trans
mitter on its own. By switching of the link circuit
the ouput can be taken from any of its three stages,
the valve V 4 being out of action in some cases. The
link coils are L 10 , L u and L 12 , and these are taken to
the switch S 2 , whence they may go to an aerial
coupler or via L 5 to drive the P.A. The unit can
be used thus as a single C.O. or a C.O. with one or
two doublers. With the aid of crystals in the 1-7,
3 -5 and 7 Me bands drive is available on any band
from 1 • 7 to 28 Me. Plug-in coils are used to give
the various combinations. Note that in the push-
push doubler a plain step-up transformer is used
in the grid circuit.
The P.A. stage is again push-pull with split
stator condensers in grid and anode, which can be
seen in the picture. The method of shielding the
input from the output circuit can also be seen, the
valves passing through holes in the metal screen.
Telephony modulation is done by the application
of audio frequency voltages to the suppressor grids
and for this purpose the suppressor must be biased
negative to reduce the standing carrier output
power as explained in Chapter 5. For telegraphy it
may be given positive voltage for maximum output,
and this change is made by the switch S 1 .
Jacks are shown at all points where anode or
grid currents are measured, but if sufficient meters
are available they may of course be wired in
permanently.
For detailed description see reference 12 at the
end of this chapter.
Tables
The following tables give the drive required for
certain valves frequently used in amateur trans
mitters and also suggested coil sizes.
Table 1.
Grid Drive Power required for certain types of valves
used in Amateur Transmitters.
Type.
Max. Plate
Dissipation
Grid Drive
Required
watts.
watts.
LS5 >
LS5B f
15
1 to 3
RFP15
15
1
RFP30
30
1 5
RFP60
60
2
RCA210
15
1 to 3
RCA802
15
•25 to -8
RK20
40
•9 to 2
RK25
12
•8
RK28
100
1 -8 to 2-7
Coil Tables
An accurate specification cannot be given for
each specific case, for the reason that valve capaci
ties, etc., vary considerably, but the following
information may be used as a guide for initial
experiments, after which better values may be
found by trial.
Table 2.
Triode and Pentode Crystal Oscillators
Anode Circuits.
Frequency.
V ariable
Condenser.
Turns.
Wire and
Former.
3-5 Me
100 ppF
35
14 s.w.g. enamel
7 Me
100 (j.p.F
11
or 16 s.w.g.
14 Me
50 ppF
8
d.c.c. on 1£ inch
former.