Radio Broadcast (May 1928-Apr 1929)

MAY, 1928 A FIVE'TUBE SCREEN GRID RECEIVER 21 c c ■1,0* A A B 45 90 135 180 FIG. I The schematic diagram of the five-tube screen-grid receiver is incorporated as part of the 1st stage National Impedaformer specified in the list of parts. THE AUDIO-FREQUENCY AMPLIFIER THE impedance resistance coupled amplification employed is capable of excellent tone quality at a reasonable cost. The first stage Impedaformer consists of an r. f. choke coil, plate impedance, coupling condenser, and grid resistor. The second stage unit differs from the first only in that it does not include an r.f. choke, while the third stage unit comprises a plate resistor, coupling condenser, and grid impedance. There are several advantages in the use of this "reversed" Impedaformer as the last coupling unit. First, it improves the operation of the amplifier when used with B-power units by overcoming any tendency toward "motor-boating," which is encountered at times with straight impedancecoupled amplification, and second, it makes possible the use of a high-mu tube in the second a.f. stage. In the plate circuit of the power tube is incorporated a tone filter to protect the loud speaker. The grid bias for the first audio tube is obtained from the voltage drop across the filament equalizor while that for the power tube is obtained from the voltage drop across the filament equalizor, while that for the power tube is obtained from a dry C battery. It will be noticed that a 0.8-ohm filament equalizor is employed in the negative A lead to drop the A battery potential down to the 5 volts required for all the tubes save the 222. The 15ohm equalizor then drops the 5 volts down to 3.3 volts for the filament supply of the 222, as mentioned before. The filament current required for the dial light when added to that of the 222, makes a total of a. quarter of an ampere, so that the standard 0.8-ohm equalizor provides the proper conditions. The use of 5 volts instead of 6 across the dial light filament makes no appreciable difference in the brilliancy of the scale illumination, while it does materially increase the life of the small bulb. HOW THE RECEIVER LOOKS WITH FRONT PANEL REMOVED The prototype of the receiver described in the article The actual assembly and wiring of the receiver is quite simple and straightforward. The first step is to prepare the front and sub-panels (See Fig. 2). The tuning unit, sockets, audio components, and all other parts, are then mounted in place on the sub-panel, and the set is completely wired before mounting the front panel. There is only one long lead carrying any r.f. current, and that is the one from the plate of the 222 tube to the primary of the r.f. transformer. The heavy line in Fig. 1 indicates the frame of the tuning unit, and by making connection to this frame at several convenient points the wiring is considerably simplified. A short length of flexible rubber-covered hook-up wire with a fuse clip or other suitable connector at one end is used as the contact to the "cap" or control grid of the 222 tube. OPERATING NOTES AS ALREADY intimated, very excellent performance may readily be obtained with a short wire connected directly to the control grid of the ux-222 (the cap on top of the tube) as an antenna. I'n fact, such an antenna is to be recommended for use wherever extreme selectivity is desired. Where the set is not located very close to any broadcasting stations, however, a 20 or 25-ft. indoor antenna may be used if preferred. Such an antenna should be connected directly to the tap of the antenna coil. Where the conventional 50 to 60 ft. outdoor antenna is to be used, the series antenna condenser must be employed, and the lead taken to the tap provided in the coil, as in Fig. 1. Such an antenna will be found of considerable aid in increasing the range and volume of the receiver on distant stations in any location where local in -W — -2%- All Holes ■%! Drill Sub-panel Drilling Template H Drill ~ H Drill 1 CenterLine ofCondenser Units^ % Drill ' -7&" • #6 Drill Top of Sub-panel "y Front Panel Drilling Template FIG. 2 Front and sub-panel drilling instructions