Submarine Hull Is "Mike" - AmericanRadioHistory.Com

526 RADIO -CRAFT :March, 1932
Imp roving An AUTO RADIO
In this article, the author outlines some of the problems
that were encountered in the design of an automobile
receiver and describes the methods used to overcome
them. Just how he proceeded, makes interesting reading.
T
IIE trials and tribulations of the
makers of automobile radio sets are
many; all too often the greatest
profits made from these sets have
accrued to the purveyor of hair dyes to
whom the harassed and grayed engineer has
finally been driven.
Recently it was the writer's pleasure ( ?)
to become acquainted with some of these
problems. The results of his labors are
embodied in the description of the set illustrated
in Fig. A which is the basis of
this article.
Before taking up the constructional details
of the set itself, a brief review of the
major difficulties to be ehcountered in this
field will perhaps be of interest. These are,
in the orderAn which they rank, as follows:
(1) The inadequacy of the signal pickup
system (i.e., the antenna and ground substitute)
for furnishing a large signal input;
(2) The difficulty of obtaining a high gain
:n a very small set;
(3) The close voltage limits within which
the type '36 tubes must be held to secure
a maximum of efficiency (this is distinctly
at variance with the elauiuls made for the
tube which, supposedly, was designed to be
non -critical as to these factors);
(4) The difficulty of providing in an automobile
set, where the leads are long, a satisfactory
volume- control that shall operate
in the R.F. stages without introducing a loss
of amplification, audio howling, R.F. oscillation
or tube blocking.
In solving these problems, it would also
be well if we could achieve an equality of
gain throughout the broadcast hand so that
our little automobile set would be in no
way inferior to its big brother, the home
radio set.
Analyzing the Problems
Analysis of these factors at once rules
out the first item from consideration; we
r- r --- --a R2, R3 iL5
By OSCAR BLOCK
can do nothing to secure a good pickup in
a moving car without adopting unsightly and
impractical expedients. Since our little set
Fig. A
Photograph of the efficient automobile receiver
described by Mr. Block.
shall indeed have to be long on performance,
in it we shall incorporate high -gain high -primary-
inductance R.F. coils, carefully isolated
R.F. circuits, and the maximum of regeneration
possible.
We shall do away with taps on our "B"
batteries and shall use, instead, a voltage -
dividing system; since by this method we
can hold our voltages within closer limits
and, once adjusted, our set shall be more
nearly independent of battery fluctuations
than would otherwise be the case.
As to the volume- control circuit, we have
adopted a really simple expedient that at
once removes our control from any signal
circuit and allows us to operate our tubes
at their most critical point without in any
way increasing the fear of oscillation or
tube blocking.
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Fig. 3
The values of the resistors are such that practically no
changes in plate and screen -grid voltages resIt sullen the volume is changed.
Complete diagram of the auto receiver.
Novel Volume -Control
In Fig. 1, this circuit is outlined. Here
111, R2, R3, and R4 form a voltage- divider
circuit across the "II" supply. R1, 1t3,
and 114 are fixed resistors; 11.2 is variable.
Any variation in 1t2 will change the current
flow through the resistor network and,
of course, change the voltage drop through
each resistance. Thus, if the resistance of
112 is decreased to reduce the amplification,
the increased current flow through the circuit
results in a simultaneous decrease of
the plate and screen -grid voltages, as well
as an increase in the negative grid bias;
in combination, these provide infallible volume-
control. This means of control gets
away from time increase in plate and screen -
grid voltages which accompanies the conventional
control -grid bias -variation methods
where the plate and screen -grid circuits are
fed through resistances.
Further, this method defeats the detection
which often results at low volume in an
R.F. stage when time screen -grid bias is re-
Fig. 1
.4 novel method of volume control; resistor R2
is variable.
duced. (Such detection is the result of low
current through the cathode resistance causing
tau -low control -grid bias.) Another
advantage of this method of control is that
howling, which sometimes comes as the result
of the screen -grid's voltage becoming
close to that of the plate, is made impossible.
Such a condition is apt to occur when, as
the result of decreased screen -grid current
or increased plate current, the altered voltage
drop through the filter resistances tends
to equalize the plate and screen -grid
voltages.
The smooth operation of this control is
no small factor in allowing the close voltage
settings which the type '36 tube requires for
peak operation and which no other simple
method can insure.
Uniform R.F. Gain
We have mentioned the desirability of
providing uniform over -all gain throughout
the broadcast hand. Mechanical means
through movable primary coils could he
used but for the ideals of compactness and
(Continued on page 556)