RADIO AND ITS RECEIVERS

Sun (Auckland), Volume I, Issue 125, 17 August 1927, Page 14

RADIO AND ITS RECEIVERS

Conducted for THE SUN by

C. M. Taylor, B.Sc., A.M.I.R.E.

2YA IN THE DAYTIME This station has been coming in at I quiet strength during the daylight transmissions and without fading. This reception, according to modern accepted theory, .is due wholly to the earth currents, which gradually die out as they get further from the broadcast station. At night the main transmission is by way of the aerial waves, which are reflected from the heaviside layer, but at such distance that these two sources of energy a.rc about equal, then interferences between the two takes place and fading j results. The fact that daylight transmission is mainly one of earth waves explains the fact that 2YA can be got without fading here in Auckland in the daytime. PROGRESS IN AUSTRALIA On his return to London, Sir William Noble, one of Britain’s most notable radio authorities, expressed his surprise and pleasure at the pa-o-gress of broadcasting in Australia. He was particularly struck with the arrangements for speakers at 2FC. Instead of speaking in the ordinary studio, the speaker was accommodated in a private room where he sat in a comfortable easy chair with the microphone on a table conveniently at hand. Even liquid refreshment is provided to s*uit. the taste of the speaker. Perhaps this incident lingers in Sir William’s memory, because he is Scotch and when he spoke from 2FC the liquid refreshment deemed most suitable to his taste was clear water and in abundance. INTERVALS IN PROGRAMMES A striking feature of the complaints from many countries is that the one about intervals in the programmes is one of the most common. Wireless listeners appear to be naturally impatient (do we hear Mr. Harris acclaim this, too?) and if they are kept waiting for an appreciable time at the end of one item before the next is begun, they do not hesitate to protest against the delay. Observations on the programmes of the famous 2LO, London, showed that in a period of two hours of broadcasting the intervals totalled 17i minutes or about 50 seconds between items. This compares very favourably with 3LO, Melbourne, upon which a careful check was kept unknown to the management and the average of the times lost between items and announcements during one week of transmission worked out in the neighbourhood of five seconds. ANNOUNCERS, BEWARE Some persons who listen regularly to the broadcasting stations become surprisingly interested in the announcers, but no case is on record in Australia or New Zealand approaching one just reported from the United States. A woman of 52 years of age committed suicide because a broadcasting artist who had taken her fancy did not reply to her letters of appreciation. Coming nearer home we hear of extraordinary cases. One lady was so fascinated by the announcer that she wrote for his autographed portrait. Failing to get a reply after repeated applications she finally communicated with the manager and among other things declared that she would not listen to that station again. Another correspondent persisted in signing herself as the announcer’s “radio wife.” To return to the American case the disappointed lady sealed the door and the windows, put the phones on and turned on the gas. COMPLETE A.C. OPERATION A PRACTICAL REALITY

The principal trend in radio of the present season is toward the elimination of batteries. Many satisfactory plate supplies have been developed, but the problem of filament supply has offered more difficulty. Larger currents are required for filament than for plate supply, and this means greatly increased expense in the rectifier and filter circuit. Then too many of the plans proposed for batteryless filament lighting required rewiring of the tubes in series. A competent analysis of the filarrfent supply problem seems to put the onus on the valve manufacturers rather than on the manufacturers of accessories. Several manufacturers have now produced an A-C. valve. This does not mean immediate obsolescence of present receivers. The new valves will have plate characteristics similar to those now in use and though the present valves will continue the trend will undoubtedly be toward the A.C. filament valves. Two types of A.C. filament valves are to be supplied. If alternating current is supplied to the filaments of ordinary valves a hum will result due to several sources. As the current through the filament changes during the alternating current cycle, varying from zero to maximum, the temperature of the filament will follow’ accordingly which results in a change in the valve characteristics producing a hum at double the frequency of the supply. A certain amount of hum is due to capacity effect between the electrodes and to voltage drop along the filament. When an alternating current is applied to the filament one end becomes increasingly negative with respect to the other end and the emission from that end is reduced. Every half-cycle the two ends are reversed and the effect repeats and so a hum at double the frequency is produced. In one type of valve the design follows the ordinary type, except that the filament is very short and heavy. 'l’llis filament retains its heat and so the temperature does not vary through the cycle. In fact, it will continue to function for some time after the filament current is cut off. The grid effect is also very slight, but the valve takes some little time to arrive at its working temperature. Using a low voltage across the filament also reduces the hum due to voltage drop. The characteristics of this type of valve, the UX 226, to mention one type, are: Filament voltage. 1.5 A.C.; filament current, 1.05 amperes; plate voltage, 90-135; amplification factor, 8.2; plate impedance, 7,000 to 9,400 ohms; plate current, 7.5 to 3.7 mils. This valve may be used for radio or audio amplification, but some of the makers do not recommend it for detector work, but in this respect use a valve which has a cathode separately heated and a separate connection for the positive C and negative B which do not go to the filament direct as in the case of the direct current valve. In rewiring a set to take these new valves it must be remembered that the current consumption is heavy and heavy wire not less than IS gauge should be used, especially in a multivalve set. The characteristics of the separate heater type of valve are very similar to those mentioned above, except that the filament voltage is 2.5 and the filament current is 1.75 amperes. A useful table for rewiring and other purposes is given below: CURRENT CARRYING CAPACITY OF RUBBER-COVERED WIRE Current Gauge. in Amperes. 12 20 14 11 IK 6 15 3 20 1.3 Special sockets are necessary for

the heater type of valve, also special centre tapped resistances and rheostats capable of handling the large currents used, and in this connection it is interesting to note that the resistance wire used is made of brass which is very unusual. ' In addition, a small transformer is required with a load capacity of about 70 watts. For a four-valve set the changeover, in addition to the valves, would cost approximately 15 dollars in the LTnited States. The English valves of this A.C. class cost approximately 30s each in England. So it would cost somewhere about £l2 to change over a four-valve set and the question is whether it is worth while immediately crops up, although the convenience is undoubted.