Radio Broadcast (May 1928-Apr 1929)

R4 LI R1 Fuse R1 Tl R2 R3 UX-201-A UX-171-A (CX-301-A) (CX-371-A) THE SIMPLICITY OF THE D C. AMPLIFIER IS EVIDENT FROM THIS PHOTOGRAPH A D.CL Power Amplifier and B Supply By Victor L. Osgood THE problem of getting really fine tone quality, power, and volume from an amplifier entirely electrically operated from a 1 1 5-volt direct-current supply is one that has frequently stumped the fan who does not live in a district where a.c. is available. Many still believe that it is not possible to do away with B batteries where the supply is d.c. in nature, and still get good quality and volume. The problem for a time was a baffling one, but the introduction of power tubes, especially those of the 1 7 1 -a type, has made possible the design of a power amplifier that has won the approval of all those who have heard it. The solution lies in combining a stage of transformer-coupled amplification with a second stage of push-pull, using two tubes in parallel on each side of the push-pull system. Ninety volts are placed on the plate of the first tube (a ux-201-A van nsv About the Amplifier — THE majority of self-contained electricallyoperated receivers now on the market are designed to function by plugging into an alternating-current source. While there are some manufactured receivers now available for use where the supply is d.c, these are few and far between. Constructional articles on d.c. equipment have been equally scarce, mainly due to the fact that the districts where d.c. is supplied are considerably in a minority. The combined power amplifier and power-supply unit described in this article is especially for the much-neglected constructor whose house supply is d.c. It will handle as much undistorted output as will a single iyi type tube with 180 volts on the plate and 40.5 volts on the grid — sufficient for ordinary home purposes. — The Editor or a cx-301-A) with a grid bias of 4.5 volts, and power tubes of the 171-A type are employed in the push-pull system with a plate potential of 105 to 110 volts and a grid bias of 22.5 volts. The input connection to the amplifier may be made direttly to the plate of the detector tube in the receiver itself. Four ux-171-A (cx-371-A) tubes are used in the push-pull system, as shown in the schematic diagram, Fig. 1, and they can supply an output of 700 milliwatts to the loud speaker. This will be found ample to give excellent tone quality and volume. A push-pull amplifier sometimes has a tendency to oscillate and for this reason a 25,000-ohm resistance, R3, is connected in the center tap lead of the input push-pull transformer to absorb any unbalance in the circuit which would otherwise tend to make the amplifier oscillate. The filaments of the 171-A type tubes in the power amplifier are lighted with power obtained from the 11 5-volt line after the voltage has been reduced by resistance Ri. The circuit is arranged so that plate potential for the radio-frequency amplifying tubes and the detector is available; thus the necessity for B batteries is altogether eliminated. Even though the supply is d.c. some filtering is necessary to eliminate the commutator ripple in the voltage. The filter circuit used here, however, is very simple, consisting of one choke coil, Li, and a 4-mfd. filter condenser, O. Two 1 -r.ifd . bypass condensers, C2 and C3, are also necessary across the two intermediate voltage taps. The d.c. voltage rating of these condensers need not be more than 160 volts. Either 67 or 90 volts are available for the plate circuits of the r.f. amplifiers, the choice of voltage depending upon the individual set and the owner's preference. Biasing voltages are supplied by dry batteries. Were we to take the required 22.5 volts C bias from the power line, we would, of course, have to deduct just that much from the total voltage