Wireless Receiver Developments

Radio Record, Volume II, Issue 35, 15 March 1929, Page 2

Wireless Receiver Developments

Progress of 1928 "THE exhibition of wireless receiving apparatus at Olympia in September was of great use to the broadcast listener who doe; * follow the technical journals closely in assisting him to realise the trend of progress in both components and receivers, states the wireless correspondent of the "London Times." Although at the time even the experts were unable to follow in detail all .he lines of development illustrated at the exhibition, the interval of the last three months has made it possible to view the material in better perspective, so that certain outstanding features now emerge. Just as 1927 had its special valve novelty in the shape of the screened- _ Brig valve, so the year 1928 has given

Se e ~ us the pentode, the new valve for the loudspeaker stage which has been intro duced into many standard receivers, The pentode, as its name implies, is a five-electrode valve, and it is best described as being similar to a screenedgrid valve with the addition of an extra earthed electrode between the Screening grid and the anode. By the addition of this third grid an undesirable transference of electrons (initially bombarded out of the anode) from anode to screen grid is prevented. With the pentode a remarkably wide gride range of undistorted output is obtained, For a grid swing of 15 volts a change of anode current of 16 milliamperes is obtained, which is ample for average loudspeaker purposes.

valve as a high-frequency am#itier 4 _ the pentode as a power valve has ' to the prediction, in some quarters, that | the standard three-valve set of the future will be one incorporating both these features with a detector stage of normal type in between the two. At present, however, there are some excellent new three-valve sets built on what one might call straightforward lines, which can be recommended to the listener who, originally a erystal user and thus accustomed to faithful reproduction, wishes to obtain good reception from local stations with the possibility of hearing some Continental stations. Such sets should not be judged by the number of stations they can bring in, but by the standard of reproduction of the local station with a good loudspeaker. The general adoption of an "all-from-the-mains" policy will undoubtedly y come in time, and there have appeared during the year some interesting sets for which the only external connections are those to a power socket, aerial, earth, and loudspeaker. It is difficult to say at present what will be the ultimate trend of this development -ég., whether unrectified alternating current will be used for the filaments of the amplifying stages together with an indirectly heated cathode valve for the detector, or whether the tendency will be to use a rectifier to produce direct current to run filaments in the ordinary way. The problem is an . interesting one, and the year 1929 will ihe success of both the d

be one of intensive experimenting in both these directions. At present the tendency seems to be to go on using existing sets with a high-tension eliminator and a trickle-charger for the layytension accumulator For bath devices the copper-oxide rectifier Ss been found to be eminently suitable and reliable. A year ago there were many who were of opinion that the "moving-coil" loudspeaker would gradually replace the old "moving-iron" type, because at that time the best moving-iron instruments could not compare with the moving-coil types either in quality or ‘in the power they could handle. Bxtensive research on the part of a small band of workers has, however, produced an instrument which has shown such an opinion to be quite erroneous.