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tuning facilities; minimum size;
comparable resettability; similar
range of permissible output loads;
freedom from r -f or control circuit
instability; and good dynamic
regulation of power supply. A
study of existing linear amplifiers
led to the conclusion that a new
concept of tank circuit was necessary
to meet these objectives with
minimum complexity.
Tank Circuit Design
A study of existing tubes and
tank circuit configurations resulted
in the conclusion that a
grounded grid single stage amplifier
with a pi type plate tank circuit
was desirable. To obtain the
desired tuning range of 4 -26.5 Mc
a single pi section would require
varying the series inductance as
well as the shunt capacitance elements.
Previous experience with
sliding contacts and tap switches
at this power level made it desirable
to seek some other means
for extending the tuning range of
the tank which would reduce the
maintenance a n d resettability
problems which such contacts and
switches, carrying circulating current,
have experienced in the past.
By taking advantage of the low
minimum capacitances of modern
vacuum variable capacitors; it
was possible to devise a pi type
tank circuit which utilizes fixed
inductances, has no switches or
sliding contacts which carry circulating
tank current, and in
Fig. 2: Prototype design of amplifier. R -f and
rectifier cabinets shown require floor space
6 ft. wide by 3 ft. deep
"9a
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á
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20
- 30
ó 40
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w 50
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60
70 -16
IIÏIIIIII1
LINEAR
INPUT
POWER
TONES
AMPLIFIER
TYPICAL TWO -TONE DISTORTION CHARACTERISTICS
.
?TH
-14 -12 -10 -
11000 1575
CYCLES
CYCLES
3RD ORDER
/ i-- __ 5TH ORDER
-
ORDER
RELATIVE OUTPUT TONE- do
i
Iy_rr
Fig. 3: Distortion data. Further improvements have
reduced third order distortion products even further
which the resettability is determined
primarily by precision lead
screws in the capacitors. This
tank circuit, along with some of
the associated circuits, is shown
in Fig. 1. At the lowest operating
frequency. C -8, C -9, and C -10 are
adjusted to minimum capacitance,
effectively removing them from the
circuit. The remaining circuit
consists of the tuning capacitor
C -7; coils L -4, L -5, L -6 and L -7;
and loading capacitance C -11. Relay
K -5 is closed, connecting the
output circuit to the loading capacitance.
As frequency is increased
C -10, C -9 and C -8 successively
become the loading capacitance
and vacuum switches K -4,
K -3 and K -2 transfer the output
circuit. That portion of the tank
circuit which is not used at any one
time is effectively removed by adjusting
its capacitance value to
maximum. The input circuit consists
of a relatively low Q parallel
tuned circuit to adjust input impedance
of the amplifier to provide
a suitable termination for the
driving transmitters.
Amplifier Tube
For improved distortion, it was
decided to operate the LD -T2
driver at 2.5 kw peak output or
less. To obtain desired output, a
power gain in the linear amplifier
of the order of 15 was, therefore,
necessary. This dictated the
choice of a relatively high mu
íc 3
n
7 __
i
I_ _
tube. Other desirable characteristics
for the tube included small
size, air cooling with minimum air
volume, relatively high plate dissipation,
and long life. Two Type
6423 coaxial terminal triodes were
selected for use in the amplifier.
General characteristics of these
tubes are given below:
Type 6423 Triode
Filament Voltage
Filament Current
Amplification Factor
Interelectrode Capacity
Grid -plate
Grid -filament
Plate filament
Max. de Plate Voltage
Max. dc Grid Voltage
Max. dc Plate Current
Max. Plate Input
Max. Plate Dissipation
Approximate weight
Max. diameter
Height
62 v.
81 a.
90
25 µµf
82 µµf
1.5 µµf
1.1,500 v.
-1400 v.
2.5 a.
so kw
12.5 kw
18 lbs.
4 -% in.
15-9/16 in.
Prototype Linear Amplifier
A prototype linear amplifier has
been constructed for full scale
confirmation of excellent results
obtained in testing a 3 kw breadboard
model of the pi tank circuit.
This prototype, shown in Fig. 2,
consists of the r -f and rectifier
cabinets with a total floor space
6 ft. wide and 3 ft. deep. Exhaustive
tests isolated and eliminated
parasitic oscillation and resulted
in circuit stability permitting
full plate voltage to be applied
at zero grid bias with no
tendency toward self -oscillation.
(Continued on page 154)
Tele -Tech & ELECTRONIC INDUSTRIES June 1956
109