T EVIS i - AmericanRadioHistory.Com

Fiq.6. Method by which a mast- mounted boost
er is connected to its power supply and TV
receiver using one antenna transmission line.
condenser C,. The greater the feedback
through C,, the lower the gain
of the amplifier and the better the
noise factor. The "Q" of the grid circuit
is reduced by this arrangement,
and the frequency response extended
slightly. Fig. 5B is the schematic of
a commercial version of the same circuit
using a bifilar coil in place of
the two coil sections of Fig. 5A. The
only tricky adjustment is that of the
variable neutralizing condenser C,.
Without equipment for adjustment,
this condenser can be set using a receiver.
Normal procedure is to tune
the plate and grid slugs to the proper
channel with C, set at minimum capacity.
The booster will oscillate as
the second slug is brought to resonance.
C, is adjusted (higher capacity)
until the oscillation stops. This is the
point of maximum gain, but not that
of best noise factor. Adding more
capacity to C, will result in lowered
gain and internal noise.
In making bifilar coils only one
slug is used and both sections of the
coil are wound on a common form.
The center or r.f. ground point of both
coils is located at one end (the other
taps are one turn down), and the
windings "criss- crossed" at opposite
sides of the coil. This method has
been employed in the interests of
economy rather than engineering perfection.
Since at any instant opposite
sides of the coils are 180 degrees out -
of- phase, a small voltage cancellation
occurs at the crossover points of the
individual windings. The arrangement
assumes that tube and distributed
capacity is equal across the four separate
coils involved.
Production tubes and coils cannot
be held to these exacting tolerances,
and small circuit unbalances occur
which vary the amount of effective
feedback controlling the circuit gain
and bandwidth. Tunable boosters
definitely have their place in multichannel
areas, but do not represent
the ultimate in engineering design.
Probably the best approach to signal
preamplification is to design the
booster for a specific frequency and
to incorporate proper circuit parameters
for low noise and high selectivity.
Single -channel boosters have recently
become popular in fringe areas due
to their improved performance vs the
tunable types. A booster operating
at a single frequency has the advantage
of controlled feedback, which directly
determines the amount of gain,
and the all- important noise factor.
Since the plate and grid coils can be
designed for optimum "Q," selectivity
can be predetermined. When the
booster is attached to the TV receiver,
the plate and grid slugs may be adjusted
to balance out the reactive components
of the transmission line, resulting
in better selectivity. The cir-
Fig. 7. (A) Typical cascade type booster amplifier circuit using one of the new
dual -triode tubes developed for this application. (B) High -band coupling circuit.
V2
661(7
VI.
OL2
0
CI
2
C3
LT
R,- 100,000 ohm, r/2 w. res.
Re ---500 ohm, t/2 w. res.
Co C,-5 µµld. cond.
C,, Co C. -300 µµfl. ceramic tond.
L, -Ch. 2.6 3 t. #20 hook -up wire.
Ch. 7.13 2 t. #20 hook -up wire.
72
2 -4 10 t. #20 en.
Ch. 3.6 8 t. #20 en.
Ch. 7 -13 3 r. #20 en.
17 r. #20 en.
L L,-Ch. 2 -6
ce
1/2
6B1(7
VI.
Op
J -
L.O/
a
R2
etA11111N
.
(A)
LB
NOV
PLATE OF
VIA
Ch. 7 -13 8 t. #20 en.
12 I. #20 en.
Ch. 5.6 9 I. #20 en.
Ch. 7 -13 4 I. #20 en.
3 t. #20 en.
Ch. 7.13 2 t. #20 en.
L.-Ch. 2.4
L,,-Ch. 2.6
CATHODE
Cl- 20YV1a.
L1- 4 TURNSa203/16"qA.
(B)
L.-16 t. #28 c.c., 3/16" form.
V, -68K7 tube
Note: L,-L. wound on sí" s 2" form. Slugs
are 1 /s' + V." threaded ¡or form.
cuit of Fig. 5B is supplied with values
for single -channel design.
In the quest for extended television
reception, mast -mounted equipment
has been used extensively in fringe
areas. Circuitry is the same as that
of the receiver booster with the exception
of the power supply. In the
case of mast -mounted equipment, a
maximum of twenty -four volts is permitted
on transmission lines under the
national electrical code. This means
that the power supply at the receiver
must be capable of overcoming the
resistive loss of the transmission line
and still deliver twenty -four volts to
the mast -mounted booster. Variable
supplies are necessary to insure proper
operation of such external equipment.
Fig. lA is a popular mast -
mounted booster, and Fig. 1B a variable
power supply. Fig 6 is a commercial
method for employing a common
transmission line for both the
power and r.f. signal without loss to
either.
Mast -mounted equipment has two
distinct advantages over single -channel
boosters installed at the receiver.
Noise picked up by the transmission
line is overridden by preamplification.
The r.f. losses of the transmission line
are minimized by the higher signal -
to -noise ratio at the booster output.
Common 300 -ohm polyethylene twin -
lead has an average loss of 1.2 db per
100 feet on the low channels, and 3.2
db per 100 feet on the high.
Incorporation of cascode r.f. amplifiers
by set manufacturers has created
quite an interest in the industry for
improved receiver front ends. These
sets show much better pickup in areas
of weak signal level. This may be
directly accounted for by the quieter
operation of the cascode r.f. stage.
However, if a comparison is made
between the cascode circuit and a
properly designed push -pull triode
amplifier, the difference would be
barely noticeable. Push -pull amplifiers
were never extensively used by
set manufacturers mainly because of
mechanical circuit complexity. The
cascode circuit, on the other hand,
adapts itself nicely to pentode circuitry.
Fig. 7A is the schematic of a commercial
version of a cascode amplifier.
The controlling element in this
circuit is the feedback condenser C,.
Positive feedback is accomplished
from the plate -to-grid circuit. The
grid circuit "Q" and the first stage
gain is controlled directly by this
feedback path. As with the circuit in
Fig. 5, noise and gain go in opposite
directions, and the value of C, is a corn -
promise between the two. The inter -
stage coupling transformer is impor-
tant in the design of low- channel
cascode boosters, as the parallel coils
L, and L, (in shunt) represent a
broadband tuned load. Throughout
Channels 7 to 13 the input impedance
of the grounded -grid stage is of such
a low value that the coupling arrangement
of Fig. 7B is recommended.
RADIO & TELEVISION NEWS