Radio Broadcast (May 1928-Apr 1929)

258 RADIO BROADCAST SEPTEMBER, 1928 voltage the rectifier tube will be operated well under its maximum rated value. The power transformer, because of its several low-voltage windings, also supplies a.c. filament voltage for the rectifier tube, the power amplifier tubes, for a number of 326 types, and also sufficient current for two 327 tubes — thus complete receiving set electrification is available. C-bias potentials for the power amplifier tubes are obtained in quite the ordinary manner by means of the several bypassed wire wound resistors going to the mid-point tap of the transformer filament winding feeding the power tubes. A fixed resistor in series with one of variable type efficiently prevents one from accidentally cutting out the entire resistance and thereby operating the cx-3 10 tubes without a C potential, which would result in damage to both tubes. THE FILTER NETWORK THE filter network is probably the most important unit of a power supply system. In the apparatus under discussion the writer employed choke coils of high inductance and of a type which could be operated at high current densities without fear of loss of inductance, or possible core saturation. Across the filter output is connected an efficient wire wound potential dividing resistor, Ri. Its total resistance amounts to 41,000 ohms and is tapped to provide the more commonly required B voltages. Each tap point is bypassed by means of condensers, as shown. Some readers may want to use this power amplifier in conjunction with a dynamic type loud speaker. Loud speakers of this type can be ob tained, designed to operate with a field excitation of about 60 milliamperes. This current can be obtained by connecting the field winding of the loud speaker in series with the filter system at the point marked X in Fig. 2. GENERAL ASSEMBLY AND WIRING THE preceding remarks may be considered in the light of the writer's idea of a set of specifications for building a high-grade power supply and amplifier system. And from it the reader may readily determine if the layout meets his requirements, As to making up the general assembly no working dimensions are necessary, as Fig. 1 shows the best part arrangements, considered from an electrical viewpoint, and just where each piece of apparatus is to be placed on the 17" x 21" inch drafting board used for mounting purposes. The wiring plan adopted is somewhat novel and seldom seen. It will be noted that a series of braided lead wires are employed throughout, and in final form it is apparent that a neat and workmanlike job results. The scheme is also one giving maximum insulation, since each lead is kept in a definite position with freedom from short circuits, broken connections, etc. High-grade rubber-covered stranded copper wire should be used for this purpose. The wire employed in the author's model was called " Rise Wire," made by the Belden Company; of course, any other equivalent type should prove suitable. Perhaps the reader shall wonder what part is played by the several two-post terminal blocks. The block shown in Fig. 1 to the right of the resistor R3 is merely a simple arrangement for feeding either 550 or 750 volts to the rectifier plate. A similar block will also be found facing the input push-pull audio transformer, Ti. A jumper normally closes the latter terminals, since these happen to be in the grid return circuit of the power tubes. If a galvanometer is placed in this circuit we at once have a visual indication of the flow of grid current and, of course, such a condition simply means a distorted power output. Connecting a high-resistance voltmeter across points Y and Z of the power tube C-bias resistors, as shown in Fig. 2, permits a correct voltage adjustment by means of the variable resistor. At a plate potential of 425 volts for the 310 tubes the normal resulting bias voltage should be 35. The only nut-clamped connection of the set wiring will be found to be the lead wire going to the first choke coil. Connecting a milliammeter between these points permits taking a reading of the total direct current drain of the system. It is most important to determine that in no case should the plate voltage of the power tubes ever exceed 425 volts; filament voltages, both a.c. and d.c, should also be measured; in this way high quality reproduction and long tube life will be realized. If the above instructions are carefully followed out, and the arrangement used as indicated, the amplifier will show an overall voltage amplification of about 3450 or a gain of 71 tu; while in terms of power amplification it is capable of producing up to 3 watts of undistorted energy. The maximum input signal voltage from the detector to produce maximum power output will not have to exceed 1 1 5 millivolts r. m. s., so there is no possibility that the detector will be overloaded. lists of parts THE parts listed below are those used in the particular unit described in this article, but since none of the parts are of special design there is no reason why the builder should not substitute for the parts indicated below, other units with equivalent characteristics. Frost, for example, makes wire-wound resistors that may be used for item Ri, and Centralab makes wire-wound resistors that can be utilized in place of resistor R3. A — 1 AmerTran power transformer, type PF281 B — 2 AmerTran choke coils, 20 henries, type 709 Ci — 1 Dubilier power condenser, 2 mfd., type 666 C2 — 2 Dubilier power condensers, 4 mfd., type 667 C3 — 6 Dubilier bypass condensers, 1 mfd., type 9°7\. C4 — 2 Dubilier moulded Micadon condensers, 0.01 mfd. Gi — 4 Dubilier Metaleak grid and -plate resistors G2 — 4 Daven grid leak mountings H — 2 Amperites, 0.25 ampere, No. 1 A Ri — 1 Ward Leonard AmerTran resistor, type 507-6 R2 — 1 Ward Leonard resistor, 25,000 ohms, type 507-65 R3 — 1 Ward Leonard Adjustat, 1000 ohms, type 507-7 R4 — 1 Ward Leonard resistor, 500 ohms, type 507^17 Ti — 1 Samson input transformer, type Y T-2 — 1 Samson output transformer, type 0-3 5 Benjamin 4-prong sockets 1 Mounting board, 17" x 21" 7 Composition terminal strips Hook-up wires, screws, nuts, etc. 1 cx-340 tube (Ceco type G high-mu tube for a.c. operation) 1 cx-i 12 tube(cx-327 tube for a.c. operation) 2 cx-3 10 tubes 1 cx-381 tube Note: A 6000-ohm and a 1000-ohm Cbias resistor are also needed, as shown in Fig. 2, if a.c. tubes are used in the a.f. amplifier