Radio Broadcast (May 1928-Apr 1929)

28 RADIO BROADCAST MAY, 1928 nected between the screen grid and the ground of the receiver. The grid post on an ordinary four-prong ux type socket is the connection for the screen grid while the cap on the top of the tube connects to the working or control grid. In connecting up the circuit the two grid leads should be kept in mind. When the connections from the screen grid tube are made as shown, it will be noted that the capacity between the plate and filament is placed directly across the second tuning circuit. For this reason it is sometimes necessary to add an equal capacity of about 15 mmfd. (shown dotted across the antenna tuning system) so that the trimmer condenser will keep the two tuning circuits in step all over the wave band. Sometimes it is not necessary to add this small condenser, for a similar result may be obtained by connecting the 0.0001-mfd. condenser in series with the antenna to the stator plates of the first tuning condenser. This puts the capacity of the antenna across the antenna tuning system, and as a result the compensating condenser has sufficient variation to take care of the difference in tuning of the two condensers which are operated on the same shaft. It will be noted that the primary of the Browning-Drake r.f. transformer specified is not used with the 222 tube, but this may be left in position as it does not affect operation of the receiver. NEUTRALIZATION WHEN the receiver is completely constructed and connected up, neutralization may be accomplished in the following manner: The compartment which contains the detector circuit should have the shields completely on and fastened down tightly (instructions for the assembly of the shields accompanies each set if those specified are used). The rear shield on the first compartment may be left off until after the neutralization process takes place. The best way to neutralize is to set the dial at about 20 on the scale and then turn the tickler either in one direction or the other, until a distinct click is heard in the loud speaker or phones. This means that the detector circuit is oscillating. Adjust the tickler coil until this circuit is not oscillating. A test to determine whether or not the circuit is oscillating is to place the finger on the stator plates of the second tuning circuit, when a distinct click will be heard if the circuit is oscillating. Now turn the tickler back until oscillations just cease. Turning the trimmer condenser will then throw this circuit into oscillation if the neutralizing condenser is not properly set. The neutralizing condenser should be adjusted until the above test is satisfactory and the trimmer condenser has no effect on oscillations produced in the second circuit. It will be found that the Browning-Drake neutralizing condenser will be almost at a minimum value for correct neutralization. In the sets constructed with the screen-grid tube, tremendous amplification has been obtained, so that it fs hardly necessary to use the tickler. As mentioned previously, a very short antenna should be employed. Its overall length need not exceed 20 feet, and it should be vertical, or nearly so. In this article a little different viewpoint has been given on the 222 type tube — one which the writer believes to be scientifically correct, and it is sincerely hoped that these data may be helpful to experimenters as well as to those who desire to build an up-to-date and sensitive receiver. (R,+ R;)(R,Lu-jLj-(R,Rz<.-i I (L"--rJ FIG. 2 Shield Grid B+45 FIG. 3 A picture diagram of the screen-grid Browning-Drake receiver The following parts were used in the twotube receiver described. The choice of an audio amplifier is left at the discretion of the constructor: LIST OF PARTS Official Browning-Drake Single-Drum Control Kit, Comprising Drum Dial, Antenna and Detector Tuning Condensers, All Coils, and Pilot Light Socket Official Browning-Drake Type T-2 Foundation Unit, Consisting of Westinghouse Micarta Drilled and Engraved Front Panel, Base Panel, Complete with Mounting Hardware. Also Miscellaneous Machine Screws, Nuts, and Wire. Browning-Drake Radio-Frequency Choke B. D. 135-Mmfd. Trimmer Condenser B. D. 30-Mmfd. Neutralizing Condenser Yaxley Filament Switch, No. 10 B-D Two Aerovox 0.5-Mfd. Condensers Tobe 8-Meg. Grid Leak Fixed Condensecs (0.001, 0.0001, and 0.00007 Mfd.) Six Eby Binding Posts (Ant., Gnd., B + Amp., B + Det., B— , B + 45) Set of Browning-Drake Shields Yaxley 30-Ohm Rheostat, No. 130 K-BD Type 1 12 Amperite (0.5-Amp.) Two Benjamin ux-Type Spring Sockets Browning-Drake 15-Ohm Biasing Resistor Clips for A Battery Leads, Rubber-Covered Wire, Etc.