TWO NEW VALVES.

Waikato Times, Volume 110, Issue 18407, 14 August 1931, Page 5

TWO NEW VALVES.

SET EFFICIENCY INCREASED. THEIR CHARACTERISTICS. (By A. Seccombe, IZH Technical Adviser). It will be the purpose of this paper to describe briefly two new vacuum tubes that are being used extensively in many of the new radio receivers nowon the market. These are the new type—47 pentode and the new type—--51 screen grid amplifier. The pentode tube, about which there has been so much discussion lately, is not by any means new. It has been used in Europe for a number of years, but it is only during the last few months that the pentode has been put In Its commercial and thoroughly practical form for American radio receivers. The pentode is essentially a five element output amplifier tube. The chief justification for its existence is in its ability to provide a given undistorted power-output with a much lower signal input voltage than that required for the three electrode output amplifier. It is because of this ability that the pentode is spoken of as having a. superior “power sensitivity.” The new type, 47 pentode is capable of delivering an undistorted output of 2.5 watts with a signal input of 11.7 volts, whereas the universally known type 45 tube requires an Input of 35 volts to provide an undlstorted output of 1.6 watts. The obvious advantage of this characteristic is that for a given output, the pentode requires less amplification ahead of it. This, in practice, means that the amplifier ordinarily placed between the detector and the output lube can be eliminated. The tube has another minor though sometimes important characteristic. For a given plate dissipation, the pentode is capable of producing a greater power output than the three element tube. It is, in other words, more economical of plate power. I From the broadcast angle, the tube J has yet another advantage. With the pentode it is readily possible to design the output circuits so as to give a. rising overall frequency response. Because of this it is possible to compensate for the side band cutting which results from the use of a highly selective radio frequency amplifier. The important disadvantages of the tube lie in its complexity from Hie manufacturer's view-point and in the fact that it cannot equal the three-element tube in fidelity of reproduction. A well designed push-pull type 45 amplifier is much superior to an amplifier using pentodes singly or in push pul.'. Should Hie broadcast listener have had a better opportunity of becoming ’thoroughly accustomed to the reproduction of a three element tube operating with a minimum of distortion, the fidelity consideration would be of greater consequence than it actually is. The pentode tube will undoubtedly be used more and more in receivers but it. is doubtful if it will ever supersede Hie versatile lyi 15 power lube. The Variable Amplifier Tube. Since the first days of broadcasting ihe trend of receiver design has been towards high amplification before the audio system. This was required at first by Hie need of greater sensitivity to obtain good reception from distant stations. Later, when the maximum practical sensitivity had been reached, slill higher amplifications obtained by improved vacuum tubes and boiler receiver design, permitted economically, the improvement in selectivity demanded by the ever-increasing number of broadcast stations. The screengrid tetrode, with its high amplification factors, made it possible to obtain more economically the high amplification and extreme selectivity required in the modern receiver. This advantage led to the almost universal use of the screen-grid tetrode ot the —24 type. Unfortunately, the very high amplification factor which resulted from the use of this tube had the effect of increasing modulation distortion, cross talk and other associated modulation disturbances. These limitations are of great importance when located close to a powerful broadcast station so hence the use of local-distance switches, antenna potentiometers, double and triple pro-selectors and better power pack filtering to overcome these faults. The type—sl screen grid ■tetrode was therefore designed to eliminate to a large extent the defects of the conventional— 24 type tube, without losing any of its desirable characteristics. The variable amplification factor of the new 51 tetrode has been accomplished by an entirely new prin ciple in a vacuum tube construction. In ordinary vacuum tubes the internal placement of the elements is uniform and hence a constant value of amplification factor for all operating conditions. In the new variable mu-tube the internal structure is such as to provide an amplification factor which varies from point to point of tht? cathode area. This means Hiat at a low grid bias—3 volts the amplification is at a maximum but with a grid bias of 30 to 50 volts the amplification factor automatically decreases. Such a lube reduces by a large factor, modulation distortion, cross talk, modulation hum, and associated modulation I troubles encountered with the 24 type tube, and permits partially or wholly I dispensing with the precautions necesi sary to overcome these faults. Tnc I type —5l has been designed for I volume control by variations of the control grid bias from —3 to —5O I volts. Also a marked reduction of hiss I is noticed when using these new tubes [and it is certain that the type —5l ■ variable mu tube will totally replace the old type —24. This article ns attempted to give a few of the salient features of these new tubes and <i later article will attempt to give further working details. HERE AND THERE. . The song of a lyre bird from his I forest home was sm’ressi’iilly bro.id east <i few days ago by station 3Ln Melbourne, and f layed to 2i'i; Syd 1 Officials of the Australian Broad-

casting Co. had previously placed a microphone in a hollow log near the home of Mr Tregallas, in Sherbrooke Forest, Victoria, ami made a connection to a telephone line direct to the broadcasting station. A mirror was placed in position so that the bird could see himself, and at 10.45 o'clock —his usual time each morning—he came on to the dancing ground. Seeing himself in the mirror, the lyre bird commenced to sing. He imitated the sounds of a stone-crusher, a sawmill, numerous birds of thd* forest, and even a farmyard fowl. In the opinion of many Canadians the radio in Canada is in danger of being over-Americanised, due to the preponderance of broadcasting from the United States, Mr Graham Spry, presilent of the Canadian Rauio i.eague, told the second annua’, institute for education by radio in Columbus, Ohio. This is one reason, he explained, why many Canadians are advocating Government ownership of Canadian broadcasting facilities. Canada, the speaker said, has 67 stations, only four of which are of 500 watts, which use the six exclusive and 11 shared wave lengths allotted by the United States to Canada. Tnc total power of all Canadian stations is less than 35,000 watts, while at. least 10 American citizens broadcasting into Canada have greater power. The gross revenue of the American National Broadcasting Company in 1930 was in excess of £4,500.000. aecording to the annual report of its president. Mr Merlin Aylesworth. The gross revenue in 1929 was £2,862,000. With the addition of six new trans mitters to the N.B.C. system in 1930 the permanent wire network now tolols 34,500 miles, linking seventy-four broadcasting stations, two of which are in Canada. The synchronisation of Stations, i.e., linking them together by short-wave channels or by wire so that a group can operate simultaneously on a single wave, is soon to be put to practical use. Radio appeals broadcast by WLS, Chicago, in connection with unemployment aid, have resulted in contributions of food and clothing from thirty-four States. The Supervisory Committee of Radio Transmission in Holland has issued a special note to Dutch amateurs forbidding them to transmit anything in the nature of a criticism of the Government's policy in broadcasting matters. As the outcome of a Iranscontin ental message relay contest conduct 'd recently by the American Radio Rela> League, it was shown that under per feel conditions radio telephony is the most rapid medium for the transmission of messages, while for all round reliability radio-telegraphy excelled. Some short time ago. whilst visiting the Stale radio station on Resolution Island, situated in the frozen wastes iContinued in next column.j

north of Canada, an official was carried away on a huge piece of ice, which broke off from the main field. After many hours, an Eskimo managed to manoeuvre along the icefloe and mount, it, only to find the unfortunate man exhausted and halffrozen. Having no medical equipment or specialised knowledge, the rescuers would have been helpless to save the man had it not been possible to contact Ottawa by wireless. Here medical advice and instructions were given over the air and the man’s lite was saved. Radio is also used on the Swiss \lps as .i method of obtaining medic 11 aid in case of accidents. The Swiss Alpine Club is in communication with an association of short wave amateurs, who keep in touch will all the shelters and rescue centres on the mountains.