RADIO NOTES

Rotorua Morning Post, Volume 1, Issue 103, 22 December 1931, Page 2

RADIO NOTES

Kathode)

(By

Owners of radio sets, especially receivers of the A.C. types, should be in possession of a pick up and turntable. On evenings when static and interference are bad, records may be played through the receiver and the benefits of electrical reproduction obtained. Most receivers are adaptable to a pick-up and usually only two valves or two stages of amplification are necessary to provide ample volume. When buying a pick-up, choose a good make, as there are still some on the market which have poor frequency response, and therefore, poor tone. The hetrodyne type of interference has apparently become somewhat acute in England. It is understood that the British Broadcasting . Company have prepared a scheme with the idea of economising in wavelengths, and to allow the national pro grammes to be broadcast on .one wave-length. In effect, this would be •the same as if the main broadcasting stations in New Zealand all transmitted on one frequency. For instance, if the frequency of 2YA was used all receivers tuned to that wave would receive 1YA, 2YA, 3YA and 4YA, but as each of these stations would be transmitting the same programme from a central studio, the effect would be the same as if only one station was being received. Stronger reception would naturally result, but there is a "nigger in the woodpile." The slightest change in frequency of any of the stations would result in a bad hetrodyne whistle. Of course, crystal control would almost wholly overcome this, though elaborate precautions would have to be taken. Another drawback to such a scheme would be probable large areas of "mush" or blurred reception. Two transmitters in U.S.A. have been working on the same frequency for a number of years though the scheme has not proved wholly successful. When moving an A.C. receiver or battery charger from one locality to another in Rotorua, precautions should be taken to ascertain the mains voltage used in the. new locality, as some parts of the town are supplied with 230 volts. To connect a 110-volt receiver or charger, or in fact any electric consuming device made for that voltage would be disastrous when connected to 230 volts. | The sereen-grid valve, though in common use to-day, is little understood by the average listener. It is a tube having high amplification properties, unheard of a year or two ago. It is also perfectly stable in operation and does not require neutralisation. The very high emplifying of this valve arises from two functions of the extra grid known as the screen grid. When electrons are given off from the heated filament, they do not all leave the filament with the same velocity. Some of them are too sluggish to accomplish their journey across to the plate. When this occurs they group themselves in the space between the fiament and the plate and hinder the progress of electrons not so sluggish. Some, of course, have sufficient impetus to push through this .electron cloud and reach the plate, while others are forced to join the lazy ones. It is the presence of this electron cloud which reduces the amplifying properties of the ordinary three electrode valve. Now let us place an open-work grid in the space where these sluggish electrons gather in greatest number. Connect to this grid a positive potential higlier than tbe filament but lower than the plate. This fourth element will serve to att;.act this cloud of electrons and clear up the space, so that the energetic electrons are not impeded. Not only does this extra grid accomplish this, it also serves to attract electrons from the filament and give them high impetus. It might be asked here why the screening grid does not absorb the useful electrons as well, but when we remember that the plate has a very much higher voltage imnressed on it we will see that by the time the energetic electron has reached the open meshed screen grid, it will be rntering the influence of the plate, and so will pass on readily. It is on this free flow of electrons that the control or signal grid has its influence. The past week has been exceptionally bad for local and distant reception. Static has almost completely wiped oqt overseas stations, and the YA stations have not been sufficiently strong to over-ride the interference.