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APRIL, 1928 AN EXPERIMENTAL SCREEN-GRID RECEIVER 435 THE FRONT PANEL OF THE TWO-TUBE SET The two variable condenser dials are made by Genera Radio and they come with the condensers. The smaller knobs provide for vernier tuning. The panel is of aluminum and is 7 x 14 inches in size. generation is controlled by varying the effective inductance of the plate circuit. The effective value of L t , shunted by the resistance, is varied by changing Ri. In this type of plate circuit the effective inductance increases with increasing wavelength, which tends to minimize the range L 4 ,93-95 Turns No.30 D.C.C. FIG. I of adjustment of the regeneration control as the tuning is varied. So far as sensitivity and selec- tivity are concerned a plain variometer might be used in the detector plate circuit, or a fixed inductance with a small variable condenser coupling back to the detector grid. As shown, the circuit is wired for use with a 6-volt battery and 2OI-A. type detector tube. Satisfactory operation using the method of ob- taining regeneration described was not obtained with a 199 type tube as detector. The uv-227 (0-327) a. c. type tube may be used as a de- tector if a transformer of proper voltage is used to feed the heater. This tuning and detector unit should be satisfactory for use with alter- nating-current screened-grid tubes when, as, available. The parts used in the receiver shown in the accompanying photographs are as follows: LIST OF PARTS LI Lj—45 Turns No. 26 D. S. C. 2f" Diameter Li —General Radio No. 37 Choke of 60 Millihenries Li —Regeneration Coil (Construc- tional Data Below) Ci, Cj—General Radio Type 334 or 247 5Oo-Mmfd. Variable Con- densers Cs, Ci—i-Mfd. Bypass Condensers C 6 —o.ooj-Mfd. Fixed Condenser C»—o.ooo25-Mfd Fixed Condenser Cj—o.ooi-Mfd. Fixed Condenser Ri—Carter 5000 Ohms, Variable Rs—3-Carter Megohm Resistor Ri—4-Ohm Fixed Resistor Ri—General Radio Type 214 50- Ohm Variable Resistor R 6 —General Radio Type 301 30- Ohm Variable Resistor Two Sockets Eight Binding Posts One Copper Shield for Tube, 2" Inside Diameter and 5j" High One ux-222 (cx-322) One ux-2oi-A (cx-3Oi-A) One Aluminum Front Panel, 7" x 14" One Aluminum Sub-Panel, 13" x 9" Hardware, Etc. The construction of the regeneration coil, Lt, is shown in Fig. I. The coil form is a wooden spool with an inside winding diameter of J" and a groove J" wide. An outside diameter of IJ" pro- vides sufficient winding space. A coil wound with 93-95 turns of No. 30 d. c. c. will be found suffi- cient for the general run of 2OI-A tubes. Coils Li and Lz may be General Radio coils type 2?7C with 10 turns removed. If loud speaker operation is desired, any stan- dard d. c. audio amplifier or a. c. power audio am- plifier may be used with this unit. UX-222 (CX-322) 0.005 mfd. 0.00025 mfd. UX-201-A (CX-301-A) 135 A SCHEMATIC DIAGRAM OF THE TWO-TUBE SCREEN-GRID RECEIVER