Single Signal Receiver

T HERE is no doubt that with amateur
activity at its present height something
considerably better than the t.r.f. receiver
is not only desirable but essential. But recognition
of that self-evident fact does not give material
aid to the fellow with a t.r.f. pocketbook. Some-
thing more concrete than sympathy is needed-
to wit, suggestions for what to do about it. We
propose to offer some herein, in the form of a
description of a supcrhet which will do a real
single-signal job, but which costs little if any
more than the ave.rage t.r.f. set--maybc less.
Figures prove the case, and we rest ours on the
fact that the receiver pictured here can be built
for $21, plus approximately $4.50 for the six
tubes used in it. It is a ham-band outfit, using
plug-in coils adjusted so that each band is spread
ovi•r pract.ically the whole dial range. What you
/l;et out uf it, dcpendR to Rome- <'Xt0ut, on how
The circuit, shown in Fig. 1, will not be hard
to follow by anyone having an understanding of
the operation of a superhet receiver. The mixer, a
6L7, is coupled to the antenna. 'fo reduce image
response and provide additional gain, this stage
is made regenerative. The oscillator is 6J5 triode,
one of the most satisfactory types for this pur-
pose. There is a single i.f. stage, using a 6K7
and iron-core transformers. The second detector
is a 6C5 operated as a plate rectifier to handle
large signals and to provide good headphone
output; this type of operation, incidentally, does
not load the i.f. transformer and hence better
overall selectivity results than when a diode
detector is used. The audio output tube is a 6F6.
A 6C5 heat oscillator completes the tube com-
plement. Metal tubes were used throughout be-
cause they are self-s.hielding and thus eliminate
the need for extra tube shields. Although not
indicated on the circuit diagram, the
i.f. amplifier is made regenerative by
a very simple method to give the
single-signal effect.
THIS SIX,TUBE S.S. RECEIVER CAN BE BUILT FOR ABOUT
TWENTY-FIVE DOLLARS, WITH TUBES
Taking the circuit features indi-
vidually, the mixer stage uses t,he
familiar tickler circuit to obtain
regeneration. This method was used
in preference to the popular cathode-
tap arrangement for three reasons:
fi'irst, it is much easier to change the
number of turns on a separate coil in
making preliminary adjustments
than it is to move a tap; second, the
possibility of hum, always likely to
be present in regenerative circuits
using 6-volt tubes, is lessened by
having the cathode at ground poten-
tial for r.f.; third, with the cathode
grounded it is less likely that any
oscillator voltage will appear in the
mixer grid circuit. The appearance
of oscillator voltage on the No. 1 grid
of the mixer is not only undesirable
from the standpoint of tube perform-
ance, but also, since in this case the
1nixer works directly from the an-
.A. regeneratioi•e i.f. stage give$ single-signal selectivity: a regeneratfoe
mixer is used to provide good signal-to-image ratio. Power supply re-
quirements are 2.1 amperes at 6.3 volts, and 60 milliamperes a.c. at
200-250 volts, for loud-speaker operation.
intelligently you use it--but then, that is also
true of high-priced receivers. It is quite easy to
build and put into operation, with only one origi-
nal adjustment which requires more than ordi-
nary care.
• Asst. Technical Editor, QST.
14
tenna, is likely to bring in unwanted
signals. This is particularly so on the lower-
frequency bands, where high-frequency commer-
cial signals can be picked up on oscillator
harmonics.
To avoid eonstructional eomplications, the
mixer tuning is not ganged with the oscillator.