Electronics-World-1959-05

ligible. Consequently, the voltage
waveform at the output of the modulator
and on the screen of the 807
consists of positive half -cycles of audio.
This is an important requirement. See
"Transmitter Adjustments."
Tests have shown that the power
output of the 807 buffer on modulation
peaks is four times that of the stage
under conditions of no modulation or
"idling." Also, with proper adjustment
of R, as well as buffer and final amplifier
grid drive (which is quite easily
obtained), the resultant carrier power
is proportional to the voltage change
on the screen of the 807. (Note: This
does not mean that the plate current of
the 807 is also proportional to screen
voltage change.)
Resistor R, provides excellent load for
the output circuit of the modulator. Its
value is fixed at 25,000 ohms and requires
no initial or periodic adjustment.
Also, it will be noted that the resistor
handles a maximum current of only 10
to 13 ma., so that the power rating
need be only 4 watts, and a minimum of
source voltage is required.
Design of R.F. Stages
Conventional input and output circuitry
is employed in buffer and final
amplifier stages, as shown in Fig. 3. Important
design requirements are: (a)
sufficient drive for each stage to provide
recommended operating grid current
values for the type tubes used;
(b) a fixed source of grid bias and
screen supply voltage for the buffer
and final amplifier; (c) complete isolation
of stages to prevent both regenerative
and degenerative feedback; and
(d) provision for r.f. drive adjustment
to both stages. Of course, the plate
supply sources for both stages should
have reasonably good regulation to
prevent peak clipping. Also, the antenna
system to be used with the
transmitter should be capable of loading
the transmitter to at least 75% full
power output. Inadequate antenna
loading results in a lower depth of
modulation than is otherwise obtainable.
The r.f. drive adjustment for the
buffer, and to some extent for the final
amplifier, is provided by the adjustable
grid resistor R. Other methods can be
employed for the purpose, of course,
but if more than ample drive power is
available from the preceding stage, the
adjustable resistor method is particularly
effective; moreover, it has the advantage
of maintaining high efficiency
in the buffer and thus high peak -power
output from the stage. Optimum grid
current values for the final amplifier
can be obtained conveniently by readjustment
of the tuned grid circuit.
In fact, use of the tuned circuit in this
manner is the preferred method of adjusting
the grid current for the final
stage.
One more point should be mentioned
in regard to r.f. design and that is the
advisability of using a slightly higher
"Q" in the plate circuit of the buffer
and in the grid circuit of the final. This
provides better load regulation for the
N
150
LEGEND
R.F DRIVE VARIATIONS
UPWARD AT AUDIO FRED.
I 1
Fig. 4. How r.f. drive to power amplifier pre.
vents p.a. cut -off, and constantly changes
grid operating point at a.f. frequency.
's
r izo
Slo
Ñ T
S
U .
,el 11
5 45 50 55 60
ANTENNA CURRENT
65
CLASS A82
«.GLAS$ ADI
I
FOR
Fig. 5. Actual antenna current values for
several values of buffer screen voltage.
large variations in r.f. current in these
circuits.
Final Amplifier Efficiency
Fig. 6 is a diagram showing the pertinent
elements and values of a 1 kw.
input p.e.p. (peak- envelope -power)
transmitter which is currently being
used by the author on the 80 -, 40- and
20 -meter phone bands. It was originally
a 500 -watt rig, designed and built by
the author in 1950, and utilized a pair
of 803's in the final. QSO reports on the
transmitter were excellent. In 1951 the
803 tubes were replaced by the new
and more efficient 4 -125A type tubes,
Fig. 6. Shown at the
right is the schematic
diagram of the modulator,
buffer, and the
final power amplifier
stage of the one -kilowatt
(peak- envelopepower
input) transmitter
that is employed
by the author. Refer
to the text for a
complete description.
AUDIO FROM
$PEECN AMP
445 V)
807
R F DRIVER
which were operated from the original
clamp tube exciter at the 500 -watt
p.e.p. input. Recently, however, the
author obtained a new plate supply
which provides the power increase.
Since modulation is applied to the
driver stage, it might be assumed that
the final amplifier is necessarily operated
as a class B linear. This, however.
is not the case. Those who are
familiar with the 4 -125A tubes will
note that the grid bias value used is
that specified by the manufacturer for
class C c.w. operation. Also, the transmitter
is initially tuned up for class C
operation in the same manner as for
any c.w. transmitter. This is possible
because the system described is a form
of "efficiency modulation," that is, the
grid operating point of the final amplifier
is constantly changing as modulation
is applied to the preceding driver
and remains at a fixed point only when
no modulation is applied. Another very
important reason is that modulation is
applied to the driver, only in the form
of positive cycles of audio, as explained
under modulator operation. Thus, the
driver screen (and plate) voltages
never go negative with respect to the
fixed d.c. potential ( "B + ") on the stage.
This of course results in a grid operating
point for the final amplifier which,
although constantly changing, is never
so far down on the plate current curve
as to cause cut -off. Observation of the
r.f. modulated waveform on an oscilloscope
shows the negative modulation
peaks to be only a small percentage of
the amplitude of the positive peaks.
See Figs. 4 and 5.
While it is practical to adjust this
system so that the carrier will be near
zero with no modulation, it is not considered
desirable. It can be seen that
the average carrier power will be lower.
Experience with this system of modulation
has indicated that it is preferable
to maintain an unmodulated carrier
(Continued on page 144)
4- 125A'S
FA
375 V (TOTAL CJRRENT 15 MA)
ANT
May. 1959 61