WIRELESS NEWS

Press, Volume LXVI, Issue 20038, 20 September 1930, Page 9

WIRELESS NEWS

(By "MAX.") Last week it was stated that mutual conductance was simply the amount of change in plate current, in milliamps, for a change in one volt on the grid. Now there is one other way in which a rise or fall in plnte current can be obtained, and that is by making the plate volts higher or lower. By finding how many volts change on t-ne plate have to be made to have an effect equal to one grid volt, we have another most important feature when dealing with those apparently fearsome things, valve characteristics. # * Suppose that dropping the grid volts by one volt makes the plate current drop two milliamps, and that the plate current can be brought up again to the orginal value by raising the plnte volts by ton volts, say from 90 to 100 volts, then we sa? that the amplitieation factor is 10. Therefore a change in grid volts of one has the same effect as a change in plate volts of ten volts. c * To avoid delving too much into the peculiarities of different types of valves, it is sufficient to state that valves suitable for high frequency work have a high amplification factor, whilst general purposes valves have a medium factor. Power tubes, except the special screen grid ones, have a very low amplification factor * * The signal which is received on the grid of the first valve is usually of the order of a minute fraction of a volt, so there is no need of a valve, in the first H.F. socket, which has to handle a large grid swing. This ability to handle a big swing is most necessary in the last stages, and we find that this is provided for in power valves. It is the position and size of mesh of the grid which determines type ot valv». to a great extent. * ♦

It is one of the peculiarities of the ordinary three electrode valve, 'with filament, plate, and one grin, that it is impossible to make it so that it can handle a large signal voltage at the same time give a large amplification. Therefore, before the development of the types with extra grids it took a number of valves to work a loudspeaker decently. Now we have clectro-dynnmic speakers which require a larger amplified signal to operate them, bo that a big number of valves, oven though they are more efficient than the three electrode types, is still necessary. * * Loudspeaker® are very inefficient things, as a whole, 'when we come to consider the electrical eneriry delivered to them and the sound energy which thev put out. The older horn type converts only 1 per cent, into acoustic (sound) energy, the magnetic cone "ives 2 per cent, whilst the dynamic has an efficiency of 10 p«r cent. * * A racehorse in the shaftß of a heavy dray would seom an absurdity, but this is a good parallel of what happens in a radio receiver when a wrong type of speaker is coupled to it. It is ni'ceßsfiry to match the units, and some of the older sets will not work modern speakers for this reason. Uemember this if" you happeri to hear a modern dynamic loudspeaker and then wish to purchase a similar one to operate from a three-year-old receiver, for results will certainly not • t*> what were expoc-tod. - . 7 * Si"

The writer tried out a Western Electric cone speaker of large dimensions some two or wore years ago, when they first appeared on the market. The receiver was a Bremer-Tulley power six, but- it needed a full 135 volts of B battery and a valvn in the last socket, as well as the re quired C bias. It was found that a 201-A valve also worked almost as well, provided the full plate voltage and, a most important point, more bias than if usually [riven for that plate voltage, with a 201-A. ♦ ♦ A 201-A will stand quite a lot of hard work, but it will not deliver undistortcd output if pushed to any extent. However, as a low power transmitting valve one has been used with a plate voltage of 300 and six volts on the filament, for some months, but then the emission suddenly dropped oft, which was not to be wondered at, * * A short wave transmitter at Washington, U.S.A., shortly to be completed, will be the last link in a chain of aural radio beacon stations covering the Boston-Atlanta air route. The importance of these stations cannot be overestimated, and it is considered that they will enable the aeroplanes to take the air with safety in spite of fogs, etc. A double signal is sent out and tells the pilot when he is straying to either side of the course or is fairly in the middle of the beacon, the character of the'signal picked up being different in the three positions, left, right or centre of his route. « * The Federal Radio Department, U.S.A., is taking further steps in its control of broadcasting. A big problem had to be faced because there were so many applications for permission to broadcast, the position being acute when the Department was created, and something had to be done to prevent the overcrowding of* the already limited channels. 4 * It is reckoned that it will take a year's survey of the existing stations to classify them according to merit. The idea is not only to check for wavelengths and keeping to frequencies, but to obtain the views of public bodies and organisations in different places as to the value of the various stations and their- usefulness. As almost anyone could obtain a license a few years ago, the result has been overcrowding, there being over six hundred stations, and the latest idea will be a great help in placing the position on a workable basis when it comes to granting new licenses or revoking any already existing. * * Considerable space is devoted* in "Badio Kecord" covering' "B" class stations In New Zealand, and the position which has arisen now that the Australian Performing Copyright Association is taking steps to protect their into lests regarding the use of gramophone records, etc. Some time ago the "A" class stations were in tlie same position, and the matter was settled by mutual arrangement and probably after consultation with the New Zealand Gov- j emment. If the writer V memory is correct, the question was reduced to what waR a reasonable amount to pay the A.P.C.A. and an agreement was t . reached. It is claimed that the "B" class stations, as they receive no portion of listeners' fees, are not able to meet the demand mnde upon them to pay royalties. As the outcome of a meeting held in Dunedin. branch of a Now Zealand Listeners' League has been formed 16 investigate the subject with the object of finding some means of assisting the "B" class stations to meet the tlon. It was also stated that the League, or rather the branch, supports the

"Notes From Far and Near

"B" stations in their endeavour to obtain tlie right, to broadcast advertising matter. . ' The sting appears to be in the tail of the last" paragraph. With the first, part of the League's object the writer has every sympathy, but the question of radio advertising is quite a serious one. We all know the results in America, where chains of broadcasting stations are utilised for advertising, purposes. So far, in this country, we have been free from direct advertising, though & certain amount of indirect advertising took place. Fairly recently restrictions were made to prevent even this, with rather bad results to some of the class "B" stations.

"Radio Record" states that the New Zealand B.oadcasting Company has limited rights in the matter of permitting advertising, but has so far considered it inadvisable to make use of the privilege. The subject opens up a most important question which requires very careful consideration before radio advertising should be'eome ft recognised addition to our ordinary broadcasts.