Radio Broadcast (Nov 1926-Apr 1927)

Record Details:

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468 RADIO BROADCAST MARCH, 1927 Fj} — £ device furnishes the B and C potentials. However, where B batteries are employed, the grid filter resistors may be dispensed with. Note, however, that only the grid resistors are omitted, not the grid bypass condensers. The several photographs accompanying this article serve to acquaint the builder with the mode of construction employed and very little effort is necessary to construct a similar amplifier. The materials required to duplicate the construction of the two-tube amplifier unit as shown here is as follows: A + -c -c +B +8 +B Det. 1st. Amp. 2nd Amp. FIG. 2 1 Wood Base-Board 8" x 14" xf, Ti, T2 — 2 Amertran De Luxe Audio Transformers 2 Airgap Sockets f -25 20.00 1.20 contend with. In batteries, the resistance coupling effects are produced usually when the batteries become old. Considering this coupling action, it is obviously desirable to provide a return path for the audio currents, and also to prevent their dividing between the provided path and the battery path. In the first place, condensers will provide the necessary return path direct to the negative-A lead. In the second place, audio chokes and grid filter resistors, when placed in series with the plate and grid circuits respectively, will impede the flow of these currents to the batteries and will necessarily make the currents travel through the bypass condensers provided for that purpose. This is made more clear by referring to the circuit diagram Fig. 2, which is similar to Fig. 1 with the exception that the condensers, chokes, jacks, and filters have been included. The Samson audio chokes have been employed in the amplifier described. What the audio choke does for the plate circuit, the grid resistor does for the grid circuit. In both cases it will be noted that first an element impedes the flow of the audio currents through the leads which connect either to the batteries or the a. c. supply device, and, secondly, a bypath is provided for these currents so as to make a short return to the minus A of the tube. Both grid filters and plate impedances, with their associated bypass condensers, are to be highly recommended; in fact, may be an absolute necessity when an a. c. operated power-supply Radio Broadcast Photograph THE COMPLETE AMPLIFIER UNIT In this illustration, the box has been removed. Note that the sockets and transformers, etc., are arranged along the rear edge, while the associated apparatus is disposed on the forward part of the base-board. The double resistor mount, located in the left front part of the base, holds the resistors which make up part of the grid filter circuit, so necessary when an a.c. operated powersupply unit is employed to furnish the B and C voltages. 1 2 3 4 5 6 7 Cable Connector FIG. 3 Li — 1 Samson Output Impedance, Type O 5.00 C5 — I Tobe 4-Mfd. Output Condenser . 3.50 Ci, C., C3, Ci— 4 Tobe i-Mfd. Bypass Condensers 3.60 L2, L3 — 2 Samson Audio Chokes, No. 3 6.00 Ri, R2 — 2 Brachstats, j-Amp. and 5 Amp 2.00 1 Lynch Double Resistor Mount . . .50 R3, R4 — 2 Lynch Resistors, j -Meg. . . 1 .00 1 Yaxley Cable Connector and Plug . 3.50 Ji — 1 Yaxley Single-Circuit Filament Control Jack, No. 703C ... .65 J2 — 1 Yaxley Double-Circuit Jack, No. 703 »5 1 Binding Post .10 Bakelite, Screws, Wire, etc .50 Wood for Cabinet 1 .00 Aluminum Paint .50 Total $49-95 To assemble the amplifier, refer to Fig. 3, which shows the disposition of the various parts on the base-board, and to the photograph on this page. The bakelite strip is drilled in accordance with the details shown in Fig. 4, and is mounted on the front edge of the base in the position shown. After the parts have been securely mounted, the wiring is attended to. It will be observed