THE WIRELESS WORLD.

Otago Daily Times, Issue 21265, 20 February 1931, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES,

By Maona Vox,

Items of local interest are invited by “ Magna Vox ” for publication in this column. It is necessary that such matter 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 2FC, Sydney 442 2YA, Wellington 420 4QG, Brisbane 385 3LO, Melbourne .. .. .. 371 2AB, Sydney .. 316 2KY, Sydney 280

ANSWERS TO CORRESPONDENTS. “ Aerial,” Dunedin.—Your aerial would appear to be quite satisfactory, though the trouble may possibly be due to a poor contact in it. Also, it may be blowing against some object, and being partially shorted to the ground. Examine carefully the entire aerial, all joints, and also the ground lead. D. O’C., Dunedin.—Probably you have not mastered the tuning with the new coils. You will have to make extremely careful adjustment with the detector oscillating, and you should not expect loud signals. If you fail to get any sound at all, I would suggest that you communicate with the dealer from -whom you purchased the coils. “ Amateur,” Mosgiel.—Reception of broadcast stations over appreciable distances is always poorer during the day than at night, this effect being the result of the sun’s rays causing heavy ionisation and absorption of the radio waves. The effect has nothing to do with your receiver, and is quite unavoidable at the present time. The humming noise you complain of originates in the alternating current supply from which your set is operated.. Most A.C. operated receivers produce some hum, although with the modern set it is so negligible as to be hardly noticeable. If, however, you get a fairly loud hum, I would suggest that you get in touch with the agent who sold you the set, with the idea of correcting a possible fault. A larger aerial would probably improve your reception results in general.

INDOOR AERIAL POINTS. With the great increase in efficiency in receiving sets and the increase in power in broadcasting transmitters, the indoor aerial is becoming increasingly popular. From time to time, though, you come across not a few that give receiving sets no real chance of showing what they can do. The beginner at wireless —and, perhaps, sometimes the old hand, too, does not always realise that walls are, from a radio point of view, earth. Hence you will find the down lead draped for yards and yards close to some wall or brought in through a door by a very long way round for the eake of neatness. REACTION CONTROL.

There are those who say that no receiver can possibly give its fullest efficiency unless reaction is used (says a writer in “ Wireless World.” There are others who maintain that it should be made a criminal offence to fit a reaction control to any set. Both views are extreme, but both contain a germ of truth. Reaction is usually associated with the use of a grid rectifier, where the grid current damps the tuned circuit to a very considerable extent. The effect of this damping, and of any other damping due to such causes as dielectric losses, can be nullified by the_ intelligent use of reaction. In a receiver of this kind there is therefore a certain setting of the reaction control that invests the tuned circuit with the same resistance that it would have had if dielectric losses had been carefully minimised and an anode detector, which is free from grid-current damping, had beeh used. If reaction is pressed no farther than this its use can hardly be regarded as criminal, for the tuned circuit has exactly the same resultant resistance as in many receivers in which reaction is not used at all. The use of reaction up to this point may therefore be regarded as perfectly legitimate. There is, however, the temptation to increase reaction beyond this point in an endeavour to improve the sensitivity and selectivity of the receiver. Apart from the fact that this increase is liable to result in oscillation, to the infuria.tion of every listener for miles round, the user of the set himself begins to suffer. Signals received when the set is on the verge of oscillation are always distressingly poor in quality, for the resultant resistance of the tuned circuit has been so reduced that the outer side-bands are not received at all. All high notes are therefore completely missing, music becomes just a noise, and speech is nearly unintelligible through lack of consonants. Such results indicate that reaction is being used to an illegitimate extent. CHOOSING A POWER VALVE; The whole object of a power valve is to turn electrical vibrations fed to them into current vibrations capable of operating a loud speaker. All the other valves in the set have been busy amplifying not current, but voltages. This highly desirable state of affairs ends, however, with the last valve. A volt without current to back it up is powerless. Indeed, power in the electrical work is a measure of the volt multiplied by the current. The duty of a power valve is to produce power. In order to be able to do its job properly a power valve must have a low alternating current resistance or impedance. Some power valves have impedances of only 1000 ohms. Valves used for high-frequency amplification may have an impedance of 1,000,000 ohms in certain cases. In order to obtain low impedance sacrifice has to be made somewhere. This sacrifice is made in the amplification factor. Many power valves do not have an amplification factor of more than two or three. This is really undesirable. If a valve could be made with an impedance of 1000 ohms and an amplification of, say, 100, radio experts would go mad with joy. The pentode is perhaps an effort to this end. Unfortunately, although it has an amplification of nearly 100, it also has to drag along an impedance of 50,000 ohms. A power valve is then the best of a bad job. Incidentally, compared to early efforts in this line, a modern power valve is a very good job indeed. But there is more in a power valve than that. The grid side of the valve must also be considered. A power valve has to handle grid swing. The valve before it impresses upon the power valve varying electrical impulses at varying voltages. There are definite limits to this grid swing voltage. It must not go beyond zero grid bias, and it must not go so far to the lowor end of the curve. A valve used for high frequency amplification can tolerate a swing at this point of perhaps half a volt, or, at the most, two volts. A power valve on account of its low amplification factor is designed to tolerate as high as 100 volts grid swing _in some cases. A Pentode, owing to its high amplification factor, can give the same output with only 20 volts grid swing. This means that the grid of the valve is biased to 20 volts or slightly more. It will be seen, therefore, that the mere inti'oduction of a so-called super power valve may not give increased signals. Signals may be less, not greater. The valve superseded may have had a higher amplification factor. Introducing a super power valve lowers the amplification factor. The same grid swing is still fed to the valve. If the amplification is only one-half, signals will fall proportionately. Before inserting a super power valve the circuit before it must be redesigned if results are to be improved. The aim is to fit a receiver with a power valve with the highest possible amplification provided it can accommodate the grid swing. Fitting a super valve to an existing sot may necessitate the addition of another valve before it, RECTIFYING MAINS SUPPLY.

Considering the fact that practically every radio set sold to-day derives its power from the mains, it is remarkable how the average purchaser takes the functioning of the part of the set that does the job for granted. Many people have a working idea about tuning, erecting aerials, and the like, but so far as the power supply is concerned a veil is drawn over the box of tricks tucked away inside the set. Every valve set requires a smooth supply of direct current for the anodes of its various valves. The filament supply may or may not be direct current. In most cases it is not. Direct surrent is a steady flow of controlled electricity moving in one direction only. Alternating current, or a.c. for short, is a to and fro motion of the self-same electricity, or its brothers and sisters. Dry

cells or accumulators give possibly the most perfect supply of direct current in the world. But nobody could deny that they are not expensive things. On a basis of electrical units dry cells deliver electricity at the rate of roughly £4 a unit. At one time there was no alternative. It was virtually impossible to rectify alternating current. To-day, thanks to the valve itself, we can take the to and fro motion of alternating current; stop the “to ” portion of the motion, retain the “ fro,” filter the result, and produce as good a copy of direct current as is wanted for radio purposes. As a'matter of fact, actual rectification, although fundmentally carried out as mentioned above, is more complicated, but that does not matter for pux-poses of explanation. It is curious that the valve that made radio broadcasting possible also gave to radio engineers a weapon whereby they might harness alternating currents to run their radio sets. But value does not stand alone as a rectifier. The problem has been attacked in all manner of ways. The homely crystal itself is capable of rectifying alternating currents. Unfortunately it can only do this in microscopic amounts, which although quite enough for feeding headphones, is hopelessly insufficient for feeding radio sets. Recently a distant relation of the crystal has been discovered which is capable of handling large amounts of power. More by good luck than anything else, it was found that certain oxides deposited on copper when clamped tightly with the oxide covering in contact with some other metal such as lead, formed excellent rectifiers. We see the result these days in the metal oxide rectifier. This rectifier has advantages over even special valves that have been designed for rectification. For one thfeg there are no filaments to heat. The whole thing closely resembles a crystal rectifier in that respect. It is practically unbreakable and so far seems to be extremely long lasting. Provided provision is made to dissipate the heat generated little trouble may be expected. Overloading, or too high a voltage, however, will reduce the life of an oxide rectifier considerably. Efficiencies up to some 70 per cent, are possible with this rectifier. _ But for the fact that the valves of rectifiers are customarily lumped in with the rest of the valves of a set (quite wrongly), it is not improbable that in a few years’ time they would tend to become obsolete for mains supply rectification. From a purchaser’s point of view there is little to choose between the two methods except that rectifier valves do burn out, whilst oxide rectifiers if properly installed last as long as the set itself. Whichever method is installed in your set the cost per unit of direct current for the set will work out at ruling prices with about 50 per cent, added 031 for losses in transformers, and components connected with rectification inside the get.