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Simple Bulb Transmitters
323
FIG. 3
The circuit used with the equipment shown in Fig. 2. This system is fundamentally the Colpitts oscillator with Heising modulation
tubes are burned below their normal brilliancy.
The special transformer for rectification is best purchased, but the intrepid experimenter may build his own/ following the general procedure outlined in the preceding issue of RADIO BROADCAST for the construction of a filament lighting transformer. The core should be built up two inches high in the same manner, using 10" x 2" soft iron strips. The exact dimension of the core may be varied as occasion necessitates, but the cross-section must in all cases approximate four square inches. The secondaries are best wound on three legs of the transformer. Care must be taken to insulate the four windings for 1100 volts, and empire cloth or tape should be used generously.
The primary is wound with 300 turns of No. 15 single cotton covered wire. The two filament lighting secondaries are each wound to 24 turns with No. 9 single cotton covered wire; and the high-voltage secondary with 3000 turns of No. 27 double cotton covered wire. The secondaries are wound in two sections or pies (both windings in the same direction) ; the finish of the first pie connecting to the first turn on the second, and the joint brought out for the centre tap. Every other layer of the highvoltage secondary should be insulated with a single covering of empire cloth or tape.
The various units used in the construction of a bulb rectifying set are shown in Fig. 2. The
circuit is indicated in Fig. 3, and is fundamentally the Colpitts oscillator with Heising modulation, the operation and values of which were described in detail in the January number. Tuning and the adjustments of the modulating circuit are effected in the same manner as was suggested for the D. C. apparatus. The substitution of rectified A. C. for the motorgenerator, necessitates no change in the constructional details, excepting the chokes Xi and X2, and the condenser Ci and C2. These chokes and condensers constitute the filter system, and as the ripple a'ccompanying rectification is more difficult to eliminate than the comparatively gentle hum of a D. C. generator, they must be of larger sizes. The reactance coils (chokes) are each wound with three pounds of No. 27 single cotton covered wire, on lo-inch cores with cross sections of approximately three square inches. Ci and C2 should have a combined capacity of at least ten microfarads. C3 and C4 are capacities similar to those shunting the filament lighting transformer in the motor-generator installation, and may be paper-foil condensers such as are used across spark-coil vibrators.
ELECTROLYTIC RECTIFIERS
ECTIFICATION is accomplished in a chemical rectifier by the electrolytic action of different solutions on aluminum which, when that metal is used as an electrode,