RADIO AND ITS RECEIVERS

Sun (Auckland), Volume I, Issue 243, 4 January 1928, Page 14

RADIO AND ITS RECEIVERS

Conducted for THE SUN by

C. M. TAYLOR, B.Sc., M.I.R.E.

AUCKLAND NOTES During lYA’s carnival week, when the average set is working about three times as long as is usual, without the same intervals for recuperation of the batteries, considerably more attention should be given to both the A and the B supplies, so that the accumulators will not be over-discharged or conditions blamed when it is the batteries that have lost their punch. To those away on holiday recharging may become a problem, but if the holidayj maker has access to a car, recharging may be done by connecting the radio battery in parallel with the car battery, care being taken to connect the two positive terminals together, and the two negative terminals likewise together. If there is any doubt about the car battery positive and negative it may be tested by a couple of pieces of wire dipped into a saucer of water into which one of the battery vents has been dipped. The negative terminal will gas much more freely than the positive terminal. The battery will certainly not be fully charged on short runs, but will be kept from becoming completely run down, and will have sufficient charge to give reasonable radio service. AUCKLAND PROGRAMMES The Auckland station had bad luck with its relay of London, especially as 2FC has had such remarkable success, but one cannot pick the weather or atmospheric conditions, and those responsible deserve congratulations for their efforts to make this radio Christmas a success. One little point which can easily happen, and which should be just as easily avoided is the repetition of the same record two and three times in the same day. One realises the very large number of records necessary to carry out the service given, but a few more of the old favourites would be more welcome than a rather monotonous repetition of fox-trot after foxtrot, or whatever these jazz records are supposed to represent. COURSING POPULAR Those in whose hands lies the directing of the stations’ programmes find a constant source of surprise in the whole-heartedness, which in a few rare instances amounts to an obsession, with which listeners regard their own particular broadcast feature. Not long ago a woman wrote to 2BL almost appealing that no more coursing meetings be broadcast, as she was so obsessed with them that she was neglecting her duties in the home. In this way wrestling and boxing broadcasts have their special devotees. One listener from Queensland told 2BL recently that he was going back to his home with a five-valve set he had procured specially to listen to the wrestling items. “As a serious listener,” he said, “I make this request —keep going with the wrestling. It will certainly add to the number of wireless listeners.” THE PARIS SHOW This year the Radio Show was held at the same time and in the same building as the Motor Show, the motors having the ground floor and the radio the gallery. (What an idea for next year’s Olympia, Radio and Motors.) There were 222 exhibitors, of whom set dealers predominated, and components were in the minority. High-quality apparatus is dearer in proportion to its quality and yet is cheap compared with British. A high-quality six-valve super-het. costing complete only £2O 163 Bd. The four-electrode valve is being used a great deal in the first stage of supersonic circuits and the popular set has this valve followed by three stages of intermediate frequency amplification, a detector and one stage of audio, giving good results on all European stations on a loop or indoor aerial. Portables are now appearing in France, but are in general rather crude and designed mainly for the motorist. Loud speakers showed the straightout horn to have practically disappeared and the mos; v , popular type being the small cabinet, with a decorated silk panel behind a fretw'ork grille. HYPNOTISED BY WIRELESS At different times when the Sydney stations have “staged” a hypnotic or spiritualistic seance, listeners have written in all seriousness telling of inexplicable and weird occurrences that happened during the broadcast—showing that in more directions than one many listeners have vivid imaginations. Now, however, comes an authentic instance. A Sydney man has received an account of a scene which took place in Boston recently, when two men were hypnotised by the broadcast voice of Mr. G. M. P. Fitzgibbon, a well-known hypnotist. He spoke from WBZ, Springfield, and evidently his voice was reproduced with such faithfulness that the two men immediately went into a trance. They had previously been under the hypnotic influence of Mr. Fitzgibbon, which probably accounted for their response to his voice over the air. "ATMOSPHERICS" There was a time when static was considered almost fatal to summertime reception, which was one reason for the legend that broadcasting was for the winter only. But now highpowered stations, in the region of the city at least, have practically overcome this disability, although in the country static can still prove troublesome. It must always be borne in mind that the larger an aerial is the more static will it collect simply because it collects a greater amount of energy of any description. Consequently an indoor aerial, if possible, a frame aerial, will always help towards the removal of static—but it must not be thought that these, or anything else, will eliminate the trouble completely. A “counterpoise”—a smaller aerial, strung a little distance above the ground, directly under the ordinary aerial, and used as an earth—will also minimise static, but this is perhaps a contrivance which not everyone finds possible. Reception of nearby stations is often possible without any aerial of any sort, and if static does by any chance prove troublesome under these circumstances, the removal of the aerial connection—and the earth too, if possible—will go a long way toward reducing the trouble, without cutting down the signal strength a great deal. EDUCATIONAL EXPERIMENT

The writer has refrained from any comment on this belated experiment until the official organ of the Radio Broadcasting Company made a statement. The “Radio Record” of December 23 heads its remarks as highly successful and excellent reception in most parts of the Dominion. On their own showing 22 schools out of 34 reported excellent results, but when one examines this report one finds that of this 22. all but five of these schools are within easy reach of station 2YA, being either in Wellington Province or just across Cook Strait. The fact that stations within the radius of 2YA report enthusiastically on the experiment should indicate to the Education Department that it has a new agent within its power, whereby its experts ccm further the cause of education to avery marked degree. But

| to label this experiment as it was conducted as highly successful is, to put it I very mildly, grossly misleading, and where there are any doubts in such a matter, more harm than good is done. To take the case of .various schools in Auckland it leads to all sorts of conflicting statements. To quote from what the writer has heard from various dealers and teachers in Auckland —Grafton, a -wash-out on account of induction interference; Mount Eden, one report wonderful reception, volume \ had to be turned down. Another report was that it was not a success. Point Chevylier, wonderful —a fivevalve set worked three speakers then had to be cut down by two-thirds. Ellerslie, moderate results, not enough to be used for really useful educational purposes. I leave it to my readers to sort this out. It is beyond me. The essentials of such a service are that it shall be of sufficient strength to be received above the noise level, and 2YA cannot do this in the clay time. It is ■ further interesting that the “Radio Record” should now find so much good in radio as an educational factor, when THE SUN has been advocating its use for the last six months. CHOOSING A POWER VALVE If one goes into a dealer’s shop and asks for a power valve, the first question to be asked is what is the voltage of the low tension supply used, as these valves are obtainable in all voltages from two volts to six volts. It is well to bear in mind that there are other considerations which must be taken into account if the purchaser is to get just what he needs. Speak/ing generally most valve manufacturers give very full technical details of their products and this is particularly so with British valves. The points of chief interest are, apart from filament voltage and filament current, the amplification factor, the A.C. resistance and the maximum permissable grid swing for a given plate voltage. The first gives the actual magnification produced by the valve and the last gives an idea of the power that the valve can handle without distortion. The permissible grid swing is rarely given by the maker, but it can be obtained from the curves which are nearly always supplied. The diagram shows how to obtain the maximum grid-swing for any given plate voltage, from the usual grid voltage anode current curves which are found inside the carton the valve is packed in. In the diagram the voltages taken are 60, 130 which is very near to the usual one used and 180. The straight portions of th© curves will be noticed to end at A, B and C respectively. Now the grid must never be allowed to become positive, or a grid current will flow and distortion results and so we are limited to the portions of the curves between the zero grid voltage ordinate (the anode current line) and the points A, B and C. By dropping lines from A, B and C to the grid volts line we get the values 4, 12 and 24 volts negative. These are the maximum grid voltage swings and the correct bias to use is approximately half these values, i.e., 2,5, 7 and 13 volts respectively. The point now arises as to the necessary grid swing under operating conditions. This depends on the type of loud speaker in use, and the amplification factor of the valve. To operate a loud speaker at comfortable volume in an average room, there should be a voltage swing in the anode circuit of about 100 volts. It will be less for a speaker of 1,000 ohms than for one of 2,000 ohms. Taking 100 volts as the desirable swing and if the amplification factor of the valve is 5 the corresponding grid swing will be 100 divided by 5 or 20. Thus it will be seen from the diagram that the valve under consideration will require a plate voltage of about 180 volts if it is to handle enough power with safety. Many valves are operated with lower plate voltages and smaller grid bias voltages without causing obvious distortion, but for true and undistorted signals a high anode voltage is essential. ELECTRIC GRAMOPHONE “PICK-UP” One of the most interesting developments in gramophone design in the last year has been the introduction of the so-called “electric pick-up” by which the music from the gramophone is reproduced on a loud speaker operated by a valve amplifier, instead of on the ordinary gramophone reproducer. The new system frequently gives a much improved quality of reproduction, and it also makes possible the production of a far greater volume of sound than can be obtained from the ordinary gramophone. This is a distinct advantage when the instrument is being used for dancing. Although the commercially made electric pick-up devices are fairly expensive, a simple experimental pick-up, capable of giving excellent results, can easily be made from an old wireless earpiece. The earpiece is detached from the headband, and the diaphragm beneath the cap is removed by screwing off the cap. The orifice in the cap is enlarged until the cap takes the form of a locking ring, which can be replaced on the earpiece to hold the diaphragm in position. The centre of the diaphragm is carefully burnished with sandpaper and a length of bus bar, or some similar rigid metal rod —an old bicycle spoke will do well — is soldered vertically on to the centre of the diaphragm, using as little solder as possible. Care must be taken not to dent the diaphragm. The vertical rod is then bent over in the same shape as the needle arm, which is attached to the diaphragm on the reproducer of an ordinary gramophone, and a needle-holder taken from an old gramophone reproducer is soldered to the other end of it. If no needleholder is available a light brass screw connecter, which can be bought for about 3d. will do instead. Level with the edge of the receiver cap a light supporting hinge should be attached to the bus bar in the manner in which the supporting hinge at the edge of the reproducer is attached to the needle arm on the ordinary gramophone reproducer. The connections for the apparatus are simple. On e of the two wires in I the cord from the headphone is conS nected to the grid of the first valve ; in a valve amplifier, and the second i wire is connected to the negative ter- ; minal of the filament lighting battery, j If desired, the pick-up can be attached to a wireless receiver by removing the detector valve from its socket and connecting one of the conductors in the cord to one terminal of the primarjof the first intervalve transformer, and the other wire from the cord to tho second terminal of the transformer primary. In use the telephone earpiece can be attached to the tone arm of the gramophone by rubber bands, with the needle bearing on the record* in the ordinary way. No needle, of course, is used in the gramophone reproducer.