THE WIRELESS WORLD

Otago Daily Times, Issue 21277, 6 March 1931, Page 5

THE WIRELESS WORLD

NEWS AND NOTES.

By Magna Vox.

Items of local interest are invited bj “Magna Vox” for publication in thi.column. It is necessary that such mattei should reach this office by Tuesday of each week for insertion on the following Friday. Metres. 7ZL, Hobart 535 3AR. Melbourne 484 4YA, Dunedin .. 463 2FO, Sydney .. 442 2YA, Wellington ~ ~ ~ 420 4QG, Brisbane .. .. .. .. 385 3LO. Melbourne .. .. .. 371 2AB, Sydney- 316 -. 2KY, Sydney. 280 ,

ANSWERS TO CORRESPONDENTS M. G. O.'D., Dunedin,—Very frequently a dry joint is surprisingly difficult to locate. Try testing each joint by starting it with a pair of pliers. Sometimes it is a component that is faulty, the poor contact being between the terminal and the part itself. “ Voltage,” Clyde.—There seems no reason why the set, if properly operated, should prove eminently suitable for local conditions, and 1 think you should have no difficulty in obtaining good distanpe reception on it, “Push-pull,** Oamaru.— (1) A push-pull output stage requires a relatively high signal voltage in its input. It is for this reason that the additional audio stage is generally used. (2) The only way to use the charger direct is to provide its output with a smoothing circuit of much the same type as is used in eliminators. The chokes must be of low resistance and inductance of about i henry, while the condenser should be of high capacity—say about 2000mfd. “ Multi-Speaker,” Dunedin.—Provided that correct impedance relationship is maintained, it would not matter whether a series or parallel, or a combination of both was used, that if the number were varied, it would be advisable to use compensating devices to preserve the circuit characteristics when one or more was removed. A choke condenser output filter, feeding to a single line with an earth returning, would be suitable. I would suggest that you write to the agents for the speakers. 420 CHILDREN’S HOUR. WELCOME.;' The following la the twenty-fifth Hat of new members who have joined Uncle Ben’s Happy Radio Family, and who will receive their badges and certificates Eileen Brown, June Austin • (Ashburton), Alexander Benson (Wakarl), Muriel Patterson, Elsa Patterson, Jack Bain, Tom Mangus, Jim Shepherd, Mary Allen, Olive Pergueson. Sydney Orant, Doreen Thomson, Edna Smith, Joyce Peterson, Lawrence Marshall, George Nlsbet, Philip Wilson, Pearl Wilson, Gerald Saunders, Harry "- Green, Edith M’Queen, Arthur M’tntyre, Peter Anderson, Alice Brown, Ruby Robinson, Lorna Reid, The following children of " The House That Jack Built’ Company ” have also Joined the Radio Family Marjorie Files, Sylvia Haines, Edna -Smith, Ruby Golgorth, Edna Busse, Isabel Mahon, Owen Tugwell, Dossle Darling, Florence Whitwam, Pet MWllly, Valma Clarke, Dorothy Bretherton. Grace Bmmerson, Dot Rankin, Pat Corrigan. Joyce Davies, Joy Hancock, Pet Wright, Gwen Burgoiae, Joyce Moore, Valda Rowles, Carrie Moore, Bonnie Darling. BIRTHDAY GREETINGS. Glen Hunter, 24 Forbury orescent; Lorna Reid, 27 Pllklngton street, Maori Hill; Mr Drake, 68 Malvern street, Woodhaugh; Mr Barnes, 4 Hay’s : terrace; . June Anderson. Tainul; Peter Vincent, Durham street, Christchurch; Tom Dosherty, 6 Reid road; Lawrence Black, care of Mrs Stence. LETTERS RECEIVED. Uncle Ben acknowledges with thanks receiving letters from the following members Arthur James, Peter Jones, Jim Barnes, Jack Drake, Eileen Moore, Bessie. Brown, Muriel Carter,-Alma Gordon, Jessie Murdoch, Dorothy Burns, Mary Burton, Elsa Greenwood, Bert Carruthers, Daisy Donald. Vernon Porter, Dot Simpson, George Yates, Jim Arthur, Dick Hughes, Amy Robinson, Charles Simm (Oamaru). Kia Ora, everybody. , UNCLE BEN. r .. !-;. CHOOSING A SPEAKER. A loud-speaker is not a' musical instrument. It is decidedly not the work of the loud-speaker to make up musical sounds—that is the task of the broadcasting artist or orchestra. The purpose of a loud-speaker is to transform electrical impulses into sound waves. It it deals with the different electrical frequencies unequally, or if it adds some vibrations of its own, then it is not performing its work properly. If a loud-speaker be high-pitched or low-pitched, that, too, is not a particularly commendable quality Neither of those phrases could be applied to the perfect loud-speaker. Therefore, when you are listening to one of these instruments being demonstrated, do not criticise “its tone,” but endeavour to visualise the original music or speech and then make a mental comparison between this and the form it assumes when it is thrown off the loud-speaker’s diaphragm. There is no loud-speaker available that will give you perfect results, and in selecting an instrument to buy, what you have to do is to locate that one which appears to you to provide a good balance between the high and low notes. That ia the first consideration, but there is another which is almost equally important, and that concerns definition. You should never have to listen hard to hear the words spoken by the broadcast announcer or a lecturer. These should come out crisply and clearly. . VALUE OF EXPERIENCE. In wireless matters it is often very difficult to give definite answers to the many questions which one is asked from time to time. This often seems rather surprising to non-techhical people! All this only goes to show that the answer depends to quite a considerable extent upon the opinion of the person giving it, and the opinion itself again depends upon circumstances , of the case. There are numerous points, which will occur to your mind immediately, where at least two (and sometimes more) different opinions can be given. For instance, take the old question as to whether anode bend or leaky-grid rectification is the more satisfactory. One person you will find is all in favour of anode-bend and will hear nothing to the credit of the grid leak, . whilst another will tell you that leaky-grid is more sensitive, better for reaction-control, . and generally more satisfactory and adaptable. The same sort of thing applies with regard to loudspeakers. Some people swear by the moving coil speaker, whilst other people complain that they can never get a tnov-ing-coil speaker to work on their set nearly as well as the ordinary permanent magnet type. Again, the pentode valve is the centre of a good deal ot controversy. According to some enthusiasts, there is nothing whatever to be said against the pentode (which does everything under the sun and completely supersedes power and super-power valves): others, however, will say that the pentode does not give good results with their particular speakers. Inwall these cases, there is something to be said on both sides, and when a definite opomon is expressed this opinion is based (possibly unconsciously) upon the particular conditions and experience which the giver of the opinion happens to have in mind. TESTING VALVES. There are so many inquiries as to how to test valves to see if there is something wrong, it is perhaps just, as well to deal with the matter in detail. A valve has three main elements, disregarding screen-grid valves and their brethren.. The filament of a valve is nothing more than a fairly fine strip of special wire arranged so that a current of electricity when it flows through it will make it hot.. When hot this special material from which filaments are made gives off electrons. These electrons go off to the plate except.for a few that fall by the wayside at the grid. The filament provides the life blood ol the valve. If it is heated too hot these electrons come off too fast, and the valve is ruined. Also after a certain length of time, usually estimated at 1000 hours ot service, the electrons get scarcer. In both ■ases the valve is said to have lost its emission. There is no satisfactory cure. This emission, as it is calle-’ may be measured with an instrument called a milliammeter. It is placed in the plate dreuit of the valve. The valvemakers •ndicate what current and how many milliatnps should pass normally under tpecified grid voltages. If the normal current does not flow, the valve has lost emission. If less than a milliamp flows with any valve with a plate voltage of 100 volts and with a zero grid bias it

is probable that the valve is worn out. If no current flows at all either the filament has lost its emission completely or the filament is broken. A broken filament may be discovered by connecting a voltmeter in 1 series with the filament. Place the one end of the . meter and the other end of the filament across the normal voltage of the valve, and see if any reading is obtained. In the case of alternating-current valves there is often no filament. The emission teste are all that are required. The grid and plate of a valve play a static part in the proceedings. They may become displaced as a result of jar. This displacement may alter the characteristics of the valve. The amplification factor may change. On the other hand, either the plate or the grid may make contact with some other Inside portion of the valve. If this occurs the valve is not only useless, but is also a danger in the set. Normally there should be no flow of current from the grid to the plate when connected in series with a milliammeter and the < local source of voltage .when the valve is not alight. * The grid of the valve, indeed, when the valve is not alight should have a resistance of some hundreds of megohms to either the plate or the filament. Modern valves rarely give trouble from defective grids or plates. They simply fade away from loss .of emission. Most radio shops have special instruments for testing the more important constants ol any valve.