Radio Broadcast (May 1928-Apr 1929)
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172
RADIO BROADCAST
JANUARY, 1929
•O B+ 90
^ — O B + 45
-O B + Det.
FIG. I. RECEIVER CIRCUIT
The above is the complete schematic diagram of the r.f. and detector stages of the author's loop-operated Browning-Drake receiver. The set is operated in connection with the external a.f. amplifier and power
unit described in Fig. 2.
increase the length of its useful life, and we want our rather-expensive screen-grid tube to live to a ripe old age.
If the source of plate voltage is equipped with variable resistors, it will be found well worth while to adjust carefully the voltage for the screen grid and the plate of the screen-grid tube. The efficiency will be maximum, however, when both potentials are in the neighborhood of the figures mentioned on the diagram, so tint those using dry batteries are also assured of good results.
Mr. Browning's suggestions regarding neutralizing could not be followed, as the coil used is an old one, not having the special winding for neutralizing. It will be noted on the pictures that the position of this coil has been reversed, not only to bring the tickler knob into the center of the panel for appearance sake, but chiefly to shorten the plate lead. This position of the coil is, of course, made possible by the removal of the antenna coupler from the receiver.
Other deviations from the list of parts suggested in Mr. Browning's article, such as C3 and C4, do not necessarily indicate improvements, but that these parts happened to be on hand — and happened to work.
If a ten-inch base panel is used, sufficient room will be found on it to add the audio stages, but we have found it more desirable to place both audio stages in the power pack, even though the power transformer does not supply filament voltage for the first audio tube. The seven wires of
the cable are then used as follows: A-plus, ABminus, Detector plate, B-plus 45-volt, B-plus 90-volt, F-minus for first audio tube, and F-pIus for first audio tube. The F-minus connection on the plug must, of course, be carried to a point on
C3, C6Two Sangamo fixed condensers, 0.001-mfd
C4 One Sangamo fixed condenser, 0.0002 5-mfd
C5 One Dubilier by-pass condenser, o. 5-mfd
Ri One potentiometer, 15-ohm (The writer used a Buffalo rheostat after converting it into a potentiometer);
R-> One Carter midget rheostat, 30-ohm;
R3 One Carter midget rheostat, 20-ohm;
R4 One Durham grid leak, 2-megohm;
One front panel, 7 x 1 8-inch;
One base panel, 7 x 17-inch (Use a 10 x 17-inch base panel if the amplifier is to be mounted on the base panel);
One Tyrman shielded socket, screen-grid type;
One Benjamin Cle-ra-tone socket, ux-type;
One Yaxley cable and connector plug, 7-wire;
Two Carter tip jacks (for loop-antenna connections) ;
One loop antenna, 0.0005-mfd. or 0.0003 5-mfd. type;
Two National vernier dials, type-c; One Carter "Imp" battery switch.
The following accessories are required to place the receiver in operation: One screen-grid tube, 222-type; One detector tube, 20iA-type; One storage battery, 6volt; One B-supply unit; One a.f. amplifier, with tubes; One loud speaker.
TOP VIEW OF RECEIVER
the proper side of the receiver switch, so that this switch will also control the first a.f. tube.
LIST OF PARTS
THE apparatus used by the writer in the construction of his loop-operated BrowningDrake receiver are given below. In some cases the various items were not selected especially for this receiver, but were employed because they happened to be on hand. Of course, electrically equivalent parts may be substituted if desired.
The complete list follows:
POWER UNIT
OF+1
\ From ( Receive/
FIG.
CIRCUIT OF POWER UNIT
A two-stage transformer-coupled amplifier is incorporated in the power unit designed by the writer for use with his loop-operated Browning-Drake.
Ci One National Equit u n e condenser, 0.0005-mfd. (Substitute a 0.0003 5-mfd. condenser if this size is required by the loop antenna selected) ;
C2 One National Equit u n e condenser, 0.0002 5-mfd.;
THE circuit diagram of the power unit used by the writer in connection with his loopoperated Browning-Drake receiver is given in Fig. 2. It will be noticed that the power circuit also includes a two-stage a.f. amplifier. The parts employed in the construction of the unit follow:
O C2 Two Acme "Parvolt" filter condensers,
2-mfd., 400-volt; Cs One Acme " Parvolt " filter condenser, 8-mfd.,
400-volt;
Ct, C5 Two Acme " Parvolt" by-pass condensers, i-mfd.;
Ri One Electrad fixed resistor, 10,000-ohm, type b1 00 ;
Ri One Electrad "Truvolt" variable resistor,
5000-ohm, type T-50; R3 One Electrad fixed resistor, 2000-ohm, type
B-20;
R4One Amperite (Radiall Co.), type i-a; Ti One AmerTran audio transformer, first-stage; T2 One AmerTran audio transformer, secondstage;
T3 One Thordarson output transformer, type R-76;
One Thordarson power compact, type R-171;
Three tube sockets, ux-type;
One vacuum tube, 20iA-type;
One power tube, i7iA-type;
One gaseous rectifier tube, BH-type.