Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

The Grid 747
structed as a portable set, having the batteries contained
in the cabinet as a part of the receiver. A
loop consisting of about 12 turns of No. 18 d.c.c.
wire wound on a 2 ft.
square frame, shunted by a
.0005 mfd. condenser will effectively cover the
broadcast range of wavelengths. The use of an
antenna and coupler is inadvisable as the chances
for radiation are too great. The radio-frequency
transformers to be used in this circuit are of the
untuned type having an average wavelength range
of 200 to 550 meters. If it is desired, the tuned type
of transformer with a variable condenser shunting
each secondary may be employed, but the tuning becomes
increasingly difficult for each stage used.
FIG. I
The potentiometer allows the circuit to be adjusted
to its most efficient point of operation and also controls
to a large degree the tendency of the receiver
into proper operating condition. Here, the radiofrequency
amplifier with reflexed audio amplifier is to oscillate. As a further oscillation control it may
eliminated from the main circuit so that the result be necessary to connect the return lead of the grid
is a straight three-circuit regenerative receiver. circuits of the second and third tubes to the potentiometer
arm.
The primary of the audio-frequency transformer is
shorted and a pair of phones inserted in series in the
A THREE-TUBE R. F.-A. F. REFLEX RECEIVER
plate lead of the detector tube. The first tube is
removed from the socket and the antenna and
ground are connected to the plate terminal of the
first socket and the 890 post, respectively. By MR. T. L. G. asks for a circuit consisting of
two stages of radio-frequency amplification,
crystal detector, one stage of reflexed
bringing the tickler close to the secondary and rotating
the variable condenser, a regenerative squeal stage of radio and a stage of straight audio. This
audio-frequency amplification through the second
should be heard in the phones and if the detector circuit appears in Fig. 3.
One rheostat of ten ohms
responds correctly, carrier waves of transmitting is sufficient for controlling all three tubes. The
stations will be tuned-in. If this is not the case radio-frequency transformers used are standard
then the coils should be inspected for reversals of neutroformers, the secondaries of which are shunted
connections, or reversals of windings. The grid by .00037 mfd. condensers. The audio reflex transformer
should be of a low ratio as should the stage
leak and condenser may be defective or the tube is
not making proper contact with the socket blades. of straight audio. A C-battery
is inserted in the
By means of a progressive trouble-elimination reflex stage and the last audio stage for stabilization
system it is possible finally tomake the necessary corrections
so that the first tube in the radio-frequency shown allow the use of two or three tubes. In this
purposes. Two jacks situated in the circuit as
amplifier circuit may again be thrown in and the circuit it is absolutely essential that a good crystal
operation observed. Other trouble-shooting suggestions
were contained in the January, 1925, GRID. throughout.
be used for satisfactory results. uv-2Oi-A's are used
A SIX-TUBE LOOP SET
A ONE-CONTROL, ONE-TUBE REFLEX RECEIVER
those who want a multi-tube radio frequency
amplifier-detector-audio-frequency
flex receiver having one control. The
4 shows a revamped one-tube crystal re-
FOR
coupler
amplifier circuit with preferably one tuning FIGURE Ti may be the standard variocoupler obtainable
on the market or it may be of the type employed
control, the circuit shown in Fig. 2 is suggested. A
receiver employing such a circuit may be easily con- in the RADIO BROADCAST one-tube Knockout receiver
AFT
AFT
FIG. 2