Radio Broadcast (May 1928-Apr 1929)

204 RADIO BROADCAST ADVERTISER Safeguard Your A. C. Installation SATISFACTORY and economical operation of A. C. receivers is contingent upon maintaining close regulation of operating voltages, by means of suitable A. C. measuring instruments. This is necessary because of the wide fluctuation in the potential of secondary lines furnishing current to house lighting circuits. Set manufacturers, dealers and electric light and power companies everywhere are cooperating to the end that voltage regulation, both on supply lines and in connection with voltage control equipment of the receivers themselves, may be effected for the better operating service of all set owners. For this reason, as well as for other testing requirements outlined in the following, all purchasers of A. C. receivers are urged to provide themselves with an instrument such as is shown in the illustration — known as the Weston Model 528 A. C. Voltmeter, range 150/8/4 volts. When you find that there is an excessive in-put voltage, it follows that there is too high a voltage on the filament which shortens the operating life of the rectifying tubes. The Model 528 Voltmeter therefore checks the line supply voltage at all times and indicates when adjustments should be made to manually operated line voltage regulators between the power supply and the power transformer. This voltmeter also indicates when the line voltage is over rated, thus enabling the operator to make an adjustment in the set for the higher line voltage so that normal life can be obtained from his tubes. The Model 528 is also made as Ammeters which are especially useful in checking the total load of the A. C. Set — in conformity with set manufacturers' instructions. The determination of A. C. filament flow in A. C. tube filament circuits is easily obtained by means of this instrument. Write for your copy of Circular J fully describing the Weston Radio Line. Weston Electrical Instrument Corporation 604 Frelinghuysen Ave., Newark, N. J. WESTON RADIO INSTRUMENTS No. 251 Radio Broadcast Laboratory Information Sheet January, 1929 Moving-Coil Loud Speakers DESIGN OF THE COUPLING TRANSFORMERS YKTHEN an engineer designs a moving-coil loud * ' speaker, he alro has to design the input transformer which is used to couple the loud speaker and receiver. The impedance ratio of this transformer will depend upon the impedance of the moving-coil system and upon the plate resistance of the power tube in the receiver. Since the engineer doesn't know what type of power tube the buyer of the loud speaker is going to use, upon what facts does he base his decision regarding the impedance ratio of the transformer which is finally incorporated in the loud speaker? The fact has been mentioned many times in these data sheets that the maximum undistorted output is obtained from a tube when the load into which it works is equal to twice the plate resistance of the tube. A curve was also given on Laboratory Sheet No. 237 showing how the power output changed with variations in load impedance and this curve indicated quite clearly that a large percentage of the maximum amount of undistorted power was still available in the load, even though the load resistance was 5 or 6 times greater than the plate resistance of the tube. Suppose the engineer designed the coupling transformer so that looking into the primary the impedance is 4000 ohms. The plate resistance of a 17lA-type tube is 2000 ohms and if this tube were used the maximum undistorted power output would be obtained (since 4000 ohms is twice the plate resistance of a 171a). If. however, this loud speaker were to be used with a 112a or 210-type tube, both of which have a plate resistance of about 5000 ohms, then only 40 per cent, of the maximum available power would appear across the loud-speaker circuit. Also, when a high plate resistance tube is used with a low-impedance load the tube characteristic is curved (see Laboratory Sheet No. 124) and this produces distortion. Evidently then, if such a design were decided upon, the power loss would be somewhat greater than half when using a 112a or 210-type tube and also distortion would be produced due to curvature of the tube's characteristic. If the transformer were designed so that from the primary the impedance was 10,000 ohms then the maximum amount of undistorted power would be obtained from a 112a or 210-type tube, since they are both 5000-ohm tubes. On the other hand, if a 171atype tube with a 2000-ohm plate resistance were used with this transformer we still would obtain 70 per cent, of the maximum power, and, since the plate load, 10,000 ohms, is much greater than the tube resistance, 2000 ohms, distortion would not be introduced due to curvature of the characteristic. This design of the transformer is obviously the correct one. lo. 252 Radio Broadcast Laboratory Information Sheet Audio Amplifiers January, 1929 IMPORTANCE OF BY-PASS CONDENSERS TN SKETCH A on this sheet we illustrate the cir-t cuit of a single-stage audio amplifier. Resistor R, being connected in series with the cathode of the tube, functions to supply C bias to the grid of the tube. Should the resistance, R, be bypassed with a condenser? If this circuit were casually analyzed one would be inclined to answer this question negatively, since this resistance is in series with the primary of the audio transformer, T, and the impedance of this circuit is very high. Consequently the a.c. currents around through the plate circuit and through the resistance ought to be very small. If, however, we draw out the equivalent circuit, as we have done in sketch B, a different condition is seen to exist. This equivalent circuit represents what the tube and transformer look like at high audio frequencies, La being the leakage inductance in the transformer and C the distributed capacity reflected into the primary. R is the grid resistor and Rp the plate resistance of the tube. At high frequencies this is a series resonant circuit and the currents are, therefore, quite large. For this reason a comparatively large voltage may be developed across the resistor R which supplies the C-bias voltage, Ec, to the grid of the tube. This voltage, Ec, should obviously be only a d.c. voltage, but, since the circuit is a series resonant one, considerable a.c. voltage will be developed across the resistance and be impressed back on the grid of the tube. This voltage impressed back on the grid will be out of phase with the original voltage and it will, therefore, reduce the amplification at high frequencies. These facts were checked on an amplifier in the Laboratory a short while ago and proved to be true. The low-frequency response of the amplifier was unaffected by the condenser across the C-bias resistor. At high frequencies, however, there was a very considerable loss in gain unless a by-pass condenser of 1 or 2 mfd. was placed across the resistance. It is therefore recommended that home constructors always make certain that all the C-bias resistors are properly bypassed. i — ^ — rJW) No, 253 Radio Broadcast Laboratory Information Sheet January, 1929 Shielding SUGGESTIONS REGARDING ITS USE SHIELDING is used in radio receivers for two purposes. First, it prevents direct pick-up, by the coils in a receiver, of signals from powerful local stations, for, when such pick-up exists, the receiver is likely to be non-selective. Second, the use of shielding prevents electrostatic and electromagnetic coupling between the various parts of the circuit, particularly the various inductance coils. Electrostatic coupling is readily prevented, thin sheets of shielding material between the apparatus to be shielded generally being sufficient. Electromagnetic coupling is more difficult to prevent. The prevention of such coupling necessitates the use of very complete shielding, the joints must be tight and a material with a low electrical , resistance must be used. The shielding in a receiver should be used for only one purpose — shielding. It should not be used to conduct currents, for example, between a coil and a condenser. If this is done the usefulness of the shielding frequently will be destroyed due to the fact that these currents flowing through the shielding material constitute circuits which can readily produce coupling to adjacent conductors. All the shielding in a receiver should be grounded and connected also to negative-B, negative-A, and plus-C wires. Except for the fact that the shield may be used for the A-minus conductor, the wiring of the set should be done as though the shielding were not present. In other words, the fact that some condenser, for example, one of the tuning condensers, is connected to the shield should not cause us to connect one end of a tuning coil to the shield and thereby complete the circuit through the shielding material. Instead a lead should be run from the tuning coil to the tuning condenser so that the currents in this circuit will pass through this lead and not through the shielding. The coils in a receiver should preferably be located about central within the shielding compartment, since in this position the increase in resistance of the coil due to the shielding will be a minimum. If these simple rules are followed in constructing a shielded receiver, many difficulties will be prevented.