Radio Broadcast (May 1928-Apr 1929)

C2 R8 R3 An A.G A LTERNATING-current operation of /-\ screen-grid tubes has been in the minds ^ of many experimenters, judging from the amount of correspondence received by Radio Broadcast, and by the number of visitors to the Laboratory who have broached this subject. The receiver described here is the first that has come into the Laboratory which shews how this may be accomplished. After all the speculation regarding the possibilities of a. c. operation of this new tube, the trick of how to do it seems to be no trick at all; all one needs is a source of a. c. voltage of the proper value — 3.3. volts. The receiver, originally designed for d. c. ■Grid Receiver operation, was described in the March Radio Broadcast. It covers, with plug-in coils, all frequencies between 100 and 10,000 kc, and consists simply of a stage of radio-frequency amplification using the screen grid-tube, a regenerative detector, and two stages of audiofrequency amplification, transformer-coupled. All tubes in the present adaptation of this receiver operate from a. c, the voltage for the screen-grid tube being obtained by connecting in series the 1.5 and the 2.5-volt windings of a standard filament transformer, and then dropping the resultant 4 volts to the proper value, 3.3, by means of a 4-ohm resistance. The output tube is a 1 12-A and the detector and first audio amplifier are heater type c-327 or UY-227 tubes. In the proper places in the circuit are bias resistances so that not even C batteries are necessary for the receiver's operation. In the Laboratory the use of a. c. on the screengrid tube's filament contributed no a. c. hum to the output from the loud speaker. When listening with a pair of phones across the output, the hum which is audible is no greater than that of any two-stage audio amplifier and detector operating entirely from a. c. The difference in circuit between the original d. c. receiver and the present one can be determined by reference to the accompanying diagrams, Figs. 1 and 2. Aside from the a. c. wiring, and the addition of C bias resistors in their proper places, another change is that the grid leak is placed across the grid condenser instead of from grid to plus filament. This is because the heater type of tube has no filament proper, and all grid and plate returns are connected to the fifth or cathode post of the tube. Reference to the diagram of the a. c. model, Fig. 2, shows the following resistances which are not in the d. c. set: Ri, 1500 ohms, to furnish C bias for the screen-grid tube; R2, 64 ohms, centertapped, across the filament of this tube, the center point connecting to ground through the bias resistance; Ri; R3, 4 ohms, to drop the output voltage of the transformer to 3.3 volts for the filament of the screen-grid tube; R4, 1500 ohms, in the grid return lead of the first audio tube to supply C bias to this tube; Ri, 2000 ohms, to furnish C bias to the last tube; and R6, another 64-ohm center-tapped resistance for the last tube, the center connecting through the bias resistance to ground. There are also two o. i-mfd. condensers across the center-tapped and bias resistances on the screen-grid tube to act as radiofrequency bypasses, and there is a 500,000-ohm potentiometer across the secondary of the first audio transformer to act as a volume control. A i-mfd. condenser across the C bias resistor of the final tube is optional. Its inclusion will provide better bass note reproduction. Naturally, uy sockets must be used in place of standard sockets, for the two heater type tubes now used in the detector and first audio stages. Otherwise the present receiver is exactly like the one described in March. It covers the same frequency ranges, UX-112-A (CX-312-A) = Ground FIG. I A schematic diagram of the four-tube receiver as originally designed for d.c. operation 37