Radio Broadcast (May 1928-Apr 1929)

218 RADIO BROADCAST AUGUST, 1928 Simplicity is the key note of this additional stage of r. f. amplification which can he added to your present receiver. It adds amplification and selectivity — and costs hut little place to connect the output wire is a matter to be determined by test only. There is no danger attached to such an experiment; nothing can happen if the wrong connection is made. The receiver is then tuned as usual or to the dial setting where signals are to be expected, and the amplifier dial turned until the signals are heard. Under some conditions a slight readjustment of the receiver dial or dials may be necessary. In general, however, the external amplifier will have no effect on the receiver tuning with the exception of the first tuning condenser of the receiver's r. f. amplifier. Since the antenna is removed from the first tube of the receiver's r. f. amplifier, the tuning of this circuit may be a degree or two different from usual. The volume control remains on the receiver itself. Power for the amplifier may be obtained from the receiver power supply unit. Several long leads may be cabled together and attached to plus and minus A, plus 45 and plus 90 to 135. These voltages are not critical. If the receiver's A-voltage supply is used, minus B will be already attached to the A supply; if an additional A battery is employed for the amplifier, minus B must be attached to it. Whether it is connected to minus or plus A makes little difference. If dry cells are used, six of them arranged in a series-parallel connection (Fig. 4) will be economical to operate. A rheostat should be used under these conditions to cut down the voltage from 4.5 to 3.3, and the C bias for the grid obtained as shown in Fig. 3. If an external battery is used, the amplifier should be grounded. If the set's battery is used, it is probably already grounded, automatically grounding the amplifier. COIL DATA THE coil used in this amplifier is the Remler plug-in type 550 coil, designed to cover the broadcast band with a 0.00035 mfd. condenser. 222-Type + 90 to 135 FIG. 2 The circuit diagram of the amplifier if a six-volt storage battery is used. The fixed resistor Ri drops the battery voltage to 3.3-3.4 required by the tube and supplies a constant grid bias of about 1.3 volts If the constructor desires to build his own coil, about 90 turns of No. 24 wire on a 2-inch form with about ten turns for the antenna-ground coil will do nicely. With a .0005 mfd. condenser about 75 turns will be correct. The kind of insulation on the wire is not important. A stage of r. f. amplification similar to the unit described here was described in the June Radio Broadcast (A Screen-Grid Booster) by Glenn Browning, and may be purchased complete from the Browning-Drake Corporation or the parts may be assembled at home. A similar unit is manufactured by the Sterling Manufacturing Company. Up to the present time, this unit has not been tested in the Laboratory. The parts used in the unit described in this article are as listed below. Other similar parts may be used, of course. LIST OF PARTS Ci I Remler condenser, 0.00035 mfd., type 638 G> 1 Sangamo fixed condenser, 0.001 mfd. Li, L2 1 Remler inductance, type 550 L3 1 Remler r. f. choke, No. 35 Ri 1 Frost De Luxe resistor, 20 ohms 1 Remler dial, type 637 1 Carter Imp on-and-off switch 2 Remler sockets 1 Baseboard, 7x8 inches, 5 ply 1 Panel,. 7x8 inches, f-inch walnut 25 feet Celatsite Hook-up wire To place the amplifier in operation the following apparatus will be necessary: 1 Screen-grid tube, ux-222 type 1 A Battery supplying not less than 4 volts 1 45-volt screen-grid voltage source 1 90-135-volt plate voltage source 4 to 45 Volts FIG. 3 // dry cells are used a variable resistor should be used to lower the battery voltage to that required by the tube. This diagram shows how to connect it in the circuit FIG. 4 Six dry cells in this series-parallel connection will supply 4.3 volts and when used to light the filament of a screen grid tube taking 1 32 milliamperes should last a long time