Radio broadcast .. (1922-30)

316 RADIO BROADCAST JANUARY, 1926 that matter, any other type of ux tube. With the uv bases, the tube pins are inserted in holes in the socket, with the bayonet pin over a corresponding mark on the socket. With other makes of ux sockets, uv-2oiA tubes may not be used — that is, ux or uv tubes may not be interchanged at will, except in a few cases. Therefore, the builder, if he uses ux sockets, should be sure that his tubes are ux bases. IMPEDANCE AMPLIFICATION DURING the war, "choke" or impedance amplification was quite popular, and was used in various airplane, ground and ship low powered telephone transmitter stations where quality of reproduction was important. The first practical chokes available are the Thordarson autoformers. which are inductances of 350 henrys, tapped so that there is a voltage step-up of i:ij. These chokes satisfy the impedance requirements of a distortionless amplifier very nicely, and will give greater amplification per stage than a resistancecoupled amplifier but not quite as much as a transformer amplifier. They require but 90 volts for their operation, in this respect being similar to transformers. A three-stage choke amplifier added to a receiver described by Mr. Silver in the November issue is shown in the circuit diagram, Fig. 3. While .5 mfd. coupling condensers are shown, those of .01 mfd. will probably be more practical, due to the tendency of the amplifier to block with higher capacities, for reasons previously given. One interesting feature of this amplifier is the use of the modulator to control its volume. Instead of the conventional shunt resistance which is not entirely satisfactory for audio volume control, the modulator consists of a 500,000ohm resistance across the output circuit of the second stage. The grid of the tube into which this circuit normally feeds is then tapped in at any desired point on the resistance, thus giving a smooth even volume control from zero to the maximum available. The modulator may be used on an amplifier, of any type, and is to be recommended as a volume control instead of the conventional jack arrangement. It also serves, in this instance, as a grid leak. POWER TUBES "THE use of ux-i 12 tubes in both stages *• of a transformer amplifier will improve quality considerably, since, in the case of the Thordarson 35:1 transformers, at 50 cycles, the tube impedance is about onethird that of the transformer primary. However, it is doubtful if any increase in volume will result from the use of ux-H2 tubes instead of uv-2OiA's. Other than improved quality, increased handling power will be obtained. This means that with a very strong signal, the ux-i 12 will function only without distortion, whereas a UV-2OIA would probably overload and distort. This is true, provided the proper C battery voltage is used. ux-i 12 tubes should really be used with 135 volts plate potential, and about 9 volts negative grid bias. They will function moderately well at 90 volts plate potential with the conventional bias, however. For resistance amplification, Daven and Cleartron make high-Mu tubes which are ideal for interstage use, and both make a power tube which is excellent for use in the last audio stage. The use of these tubes cannot be too highly recommended, as they will do much to bring resistance amplification into its own, for the improved volume resulting from the use of two high-Mu and one output tube is surprising when compared with the output of three standard tubes. If Daven tubes are used, no rheostat will be required for them. Thus, the rheostat or ballast resistor if used would be connected in the filament circuits of the first three tubes, while the last three, being Daven tubes designed for 6 volts, would connect directly across the battery. ux-i 99 tubes may be used throughout the set if it is intended to operate it on dry cells. If this is done, the ux-iao should be used for the output stage, or entirely in the audio amplifier if transformer coupling is used. OPERATION IT APPEARS that in preliminary operation, builders have experienced some difficulty in getting distance until they were familiar with this receiver's operating peculiarities. It might therefore be well to go over the mode of operation again. Assuming locals to be received properly, the antenna coupling coil should gradually be turned out until the desired degree of selectivity on the first, or left, dial is obtained. This setting will react on the volume control, and may be found by tuning-in a high frequency (short wave) station, setting the volume control so that the arrow points straight to the right, and gradually loosening the antenna coupling until the set breaks into oscillation. This is a proper operating position for the antenna rotor, where it may be left permanently set. If the volume control is then turned counter-clockwise, the set will stop oscillating and it may be tuned as one would a neutrodyne — that is, all three dials will be set about alike for a given station. If maximum sensitivity is desired, the volume knob should be turned clockwise until, with all three dials set in resonance, i. e., in a position where a station would come in, the set breaks into oscillation. Then, if one dial is moved, it will click out of oscillation, but if both dials are moved after the first a corresponding amount, oscillation will start again. In this fashion the three dials may be moved up the entire scale in hunting for a station in steps of one or two degrees at a time. Using this method, a station will come in as a squeal. To tune-in properly, the first and third dials should be set right on the squeal — that is, so moving either one in either direction will cause the pitch to increase. Then, with the middle dial set dead on the squeal, the volume knob should be retarded until the squeal stops and the signal is audible. No squeals will be heard if tubes or batteries are not good, or if too long an antenna is used. In some cases it may be necessary to add two or three turns to windings 5-6 of the radio frequency transformers. They were described in the November, 1925, RADIO BROADCAST. In case too short an antenna is used, it may have to be connected to point 4-5 of the antenna coil socket, with the ground to the minus filament line. If one stage of r. f. amplification is to be cut out, the antenna connects to No. 5 of the middle socket, with the ground to the filament, and the antenna coil removed. To use a loop, the antenna coil is removed and the loop connected to points 3 and 6 of the coil socket. SINGLE OR DUAL CONTROL C INGLE control will not be found en^ tirely satisfactory in this receiver, but dual control will work quite nicely. This is because of the extreme selectivity of the outfit, which renders each circuit so sharp that minor variations which cannot be overcome cause trouble. The second and third controls may be connected so that they will work together if only one knob is turned by tying a piece of heavy braided fishline around the pulleys on the two condenser shafts. The ends of the line should meet in a light coil spring which can be gotten from a hardware store, and which serves to take up any play. It also allows either dial to be turned independently by loosening up whenever pressure is applied to either dial separately. With this arrangement, only the first and second dials need be manipulated for tuning, and operating the set becomes a real pleasure.