Wireless Broadcast

Star (Christchurch), Issue 17863, 3 June 1926, Page 13

Wireless Broadcast

CONTRIBUTED BY

“ AERIAL.”

THE TUNED ANODE RECEIVER. In days gone by, when broadcasting was very young, and even before that time, the most popular circuit which used radio frequency amplification was the well-known tuned anode circuit. It was known to every radio experimenter and the great majority looked upon it as the king of R.F. amplifying circuits, placing it second only to a well-designed superheterodyne. Its fame was world wide, and for a long time it was accepted as a standard in English radio circles. The coming of the transformer coupled neutrodyne amplifier placed it at a disadvantage, but not for always, as the neutrodyne principle of capacity balancing can be applied to the tuned anode receiver, and this once again makes it a circuit to be recommended. At this point a description of the circuit is in order. The secondary coil of the tuner is connected to the grid and filament of a radio frequency amplifying valve and tuned by means of a variable condenser in the usual manner. The plate of this valve is connected to one end of the plate coil, and also to one side of the coupling condenser, which has its other terminal connected to the grid of the next valve. The plate coil is simply an ordinary coil having a suitable number of turns to cover a given tuning range. It may be of the basket weave type, a honeycomb coil or a single layer coil. The last named is perhaps the best in the long run. As already mentioned, one end of the plate coil is connected to the plate of the first valve, and its other end goes to the positive terminal of the B battery. A variable condenser is connected to the extremities of this coil also, in order to tune it to resonance with the grid circuit. So far, this is the plain tuned anode circuit, and it has advantages and disadvantages. Chief among its advantages are its low cost, and secondly it is very simple and easy to tune. Let the grid circuit and input tuner be tuned to an incoming signal, be it either spark, or a radiophone signal. It will be found that as the plate coil is tuned closer and closer to ihe grid circuit the signals become louder and louder, just as they do when the tickler coil on an ordinary set is used. As the tuning of the plate coil approaches resonance, its impedance to that particular frequency becomes greater, and there is an increasing potential difference or voltage across the coil, until, when the resonance point is reached, the voltage is at a maximum. It is this voltage which is applied to the grid of the next valve through the coupling grid-plate con-: denser. The disadvantage of the cir-; cuit is now manifested, as when resonance and at the same time maximum signal strength is reached, conditions in the circuit are just right for oscillations to be set up, with the risk that the receiver will act like a small transmitter and radiate a continuous wave from the aerial, which results in a whistling note m a nearby receiver, which may happen to be tuned to the same- wave. This difficulty can be easily overcome by a slight modification of the circuit, and yet it will still retain all its amplifying properties. To cover the broadcast waves, the plate coil usually has from forty-five to sixty turns, depending on the capacity of the variable condenser which is used in conjunction with it. For the purposes of this article, let the plate coil have forty-five turns of wire wound in a single layer, on a tube of insulating material three inches in diameter. A single stage of R.F. amplification is being employed, and the second tube is therefore the detector. To modify the receiver, a neutralising condenser and some more wire is required. The forty-five turns are at present occupying the full winding space of the former, so this winding may be removed. Two methods of rewinding the plate coil suggest themselves, and this time the former has to accommodate more turns of wire, so either a finer wire must be used to re-wind in a single layer, or else the same gauge wire may be adhered to, and bank winding be resorted to. in order to get enough turns on. Since the coil is in the plate circuit, which has a very high resistance, the use of a finer wire, with its slight additional resistance, will be no disadvantage, while, on the other hand, the use of a bank wound coil increases the selfcapacity, and also increases the stray field of the coil, owing to the slightly larger diameter. The better way then, evidently, is to use finer wire and keep the coil small. Then, instead of forty-five turns, wind on sixty-five turns, and tap the coil at the twentieth turn from the start. Call the starting point A, the tap B, and the finishing end C. To place the new coil in circuit, connect the finishing end, C, to the plate, and the tap, B, to the positive of the B battery. The variable condenser is connected between the points B and C. It will

now be seen that only a part of the plate coil is being tuned, and that the tuned portion consists of forty-five turns. This will make the tuning approximately the same as before the circuit was altered. A slight difference will be noted, however, this being due to the altered inductance and selfcapacity of the coil, and also in some measure to the untuned portion between A and B. The end of the coil which has been designated.A is connected to one terminal of the neutralising condenser, the other side of which is connected to the grid of the first valve. We are now ready to neutralise the receiver, or more properly to neutralise or balance out the selfcapacity of the first tube. On lighting the filaments, having connected the batteries to the set, it may be found that the receiver still oscillates when the two coils are in tune with each other. The aerial should now be disconnected, and the set tuned to produce the strongest oscillations. The neutralising condenser is now adjusted l until oscillations cease, and then the | condensers must be varied to see if any sign of oscillation can be produced. If it refuses to oscillate all is well, but it may be found that the set will not neutralise readily. In this case the lay-out should be looked to. The plate coil should be at right angles to the other tuner coils, and the wiring must be done neatly. A receiver which is wired higgledly-piggled-ly will never neutralise properly. Assuming that all oscillations have been prevented, it will be found that music comes in clearly and undistorted from fairly powerful stations, but tuning the weak and distant station is not at all eas\ r . To receive a distant station it is highly desirable to use regeneration, and also to be able to make the set capable of picking up carrier waves. This may be easily accomplished by using a tickler coil in the plate circuit of the detector tube, and coupling it to the plate coil which has been under discussion. Alternatively, a small variometer may be used in the detector plate circuit, or the tuned anode trick may be again employed, i A forty-five turn coil shunted by a ; variable condenser in the detector circuit, but not neutralised, can be used. Any of these three methods will produce oscillations, so that carrier waves I may be located, and at the same time ino inconvenience will be caused to > others. The tuned anode circuit, done ... up a little in the manner indicated in this article, is worth the trouble, and S compares very favourably indeed with other similar types of receivers.

JOTTINGS. The relaying of the. description of last Saturday’s football match from Lancaster Park was a decided success, as letters from country districts testify. Mr W. Allardyce, who described the game, move by move, performed . his part very well indeed—so well that j it was not at all difficult for the lisI tener-in to follow the fortunes of either side. We certainly hope for more of i this kind of broadcasting. On Saturday evening the programme from SAC will consist of dance music, ' and will be relayed from the Winter Garden. The makers of most dull emitter valves recommend that when they are used as audio amplifiers, the rheostat be placed in the negative lead, and the grid return be taken to the negative lead on the battery side of i the rheo. This places a certain bias ! on the grid of the tube, to make it function properly. A local enthusiast points out that when the rheostat is altered the grid bias also alters, owing i to the variation of voltage drop across | the rheostat. To avoid this, he re- : commends placing the rheostats in the positive lead, and using a C battery to provide the grid bias. In this case any alteration of the rheostat setting makes no difference to the grid potential, but it must be borne in mind that with the rheostat in the positive lead the C battery is necessary, even if only 45 volts is being used on the plates. Omission of the C battery will, in nine cases out of ten, result in a loss of amplification and also in distortion. The same authority also advocates the scrapping of the grid condenser and leak, which we have been accustomed to use with our detector tubes. In its place a C battery is again used to bias the grid, so that the tube works on the proper part of the curve for detection, in this case, the lower bend. The upper bend could be used with equal results as far as sensitivity is concerned, but a waste of plate current ■would result. The reason for substituting a C battery for the condenser and leak is that many grid leaks are noisy, and all condensers are not above suspicion. But then, the same applies to C batteries.