Valuable Tips and Jottings

Radio Record, Volume I, Issue 27, 20 January 1928, Page 13

Valuable Tips and Jottings

(BY

MEGOHM

VOLUME CONTROL. Dimming the R.F. filament is one of the ‘best means of volume control, as by this means the overloading of the detector and consequent distortion are avoided. Of course, reaction must first be reduced if the set is not a neutrodye. A variable high resistance across the speaker leads answers as a volume control within a certain range, but if it is used to give a big reduction of volume, tone is inclined to be scratchy, SHORT-WAVE SUPERHETS. Superhets for short-wave reception are iinding favour in America. For the very low wave-lengths a good type autodynes the incoming signals to 80,000 cycles instead of the usual 1000 cycles. A two-stage amplifier, oscillating slightly, amplifies the 30,000 cycle signal component and a second detector makes the signal audible. ‘The intermediate frequency amplifier oscillates weakly, say on 81,000 cycles, so that these two components combine in the second detector to give a 1000 cycle note. This method effects a great reduction in power aud motor ignition interference. DOUBLE-GRID TUBES. American factories have started making the double-grid tubes. Mr. H, Gernsback, in a ‘‘Radio News" editorial, points out that the importance of. this tube has been brought under’ the | manufacturers’ notice for two years, and no notice was taken of the matter, although during that time such tubes have been freely sold in Britain and Holland and, says Mr. Gernsback, ‘‘The chances are that the European tubes are every bit as good as the new Ametican one.’ THE SHIELDED GRID VALVE. The new Marconi shielded grid valve is finding favour in Britain. Similar valves wll shortly be produced in Americ?. The valve is used for R.F. amplification and makes external neutralisation unnecessary. This is effected by interposing a fine mesh secondary grid or shield between the usual grid and plate, the inter-electrode capacity being thereby overcome within the valve. There are five connecting prongs, three at one end, two at the other. ‘he valve is mounted horizontaHy, the centre resting in a correctly sized hole in a copper screen in the plane of the screening grid. This leaves the grid and plate circuits separated by the screen. Normal filament current 0.25 amp. at 6 volt, plate volts 120, 80 volts on sereening grid, 9 volts on control grid, amplification factor 110, plate impedance 75,000 ol:ms.

AUDIO FREQUENCIES, The normal human ear detects frequencies between 16 and 20,000 per second, but between the extreme ranges of the transmitter and reveiver, audio frequencies are in practice usualIv limited within 100 to 5000 cycles. The lowest note of the piano is about 27 cycles. A 82-foot organ pipe gives 82 cycles, and a 64-foot 16-cycle tone The highest note of the piano is about 4100 cycles. The highest speech frequencies make th, f, s and z, about 6400 evcles per second. A 5000-cycle limit distorts these sounds. LOUDSPEAKER COUPLING. The condenser in a loudspeaker fil-. ter coupling should not be of too small. capacity, owing to the fact that the reactance of this fixed condenser varies according to the cycle frequency. At 30 cycles the reactance of 2 mfds, is 2650 ohms, and at 5000 cycles only 31 ohms. It will thus be seen that at low frequencies high resistance is introduced into the circuit, so there should be not less than a 2 mfd. condenser employed. High capacity will reduce the a.c. resistance. OUTPUT TRANSFORMERS. A push-pull amplifier stage is at the same time an output transformer, so that a choke filter is not necessary. If an output transformer secondary winding has an impedance to match the loudspeaker winding at the lowest frequency used, maximum power will be delivered to the speaker at that audio frequency. As the transformer impedance rises more rapidly than the loudspeaker impedance. The output transformer compensates for some of the defects of the loudspeaker. In cases where an output transformer does not seem to be satisfactory, it is probably owing to the secondary not matching approximately the speaker winding in impedance. RECTIFYING TUBES. The Raytheon BH double-wave rectifying tube for B eliminators delivers 65 imilliamps at 200 volts, with a suitable transformer. ‘This tube rectifies without a filament, is guaranteed by the manufacturers for at least 1000 hours’ service, which is, roughly speaking, twelve months’ use. Larger sizes of the tube will pass 125 and 850 milliamps respectively,

FIXED RESISTANCES, Fixed resistances may vary as much as 10 per cent. above cr below their stated value, so where accurate restl are required, two or three shonld be tried in turn and the best one: deter. mined for the particular position, WAVE-TRAP. Constructors of the selective cryst; circuit as a wave-trap only, should connecting the aerial to the end of. coi Opposite to earth, as under some com ditions this may be found to be th best position. HIGH CAPACITY FIXED CON. DENSER. "Zhe ordinary type of high-capacity paper dielectric fixed condenser stand up very well to Jigh voltazes for elim inator smoothing. The case of a ne one of Dubilier make breaking down has been investigated and showed that the cause was an accidental tear in the dielectric paper, which was turned back, leaving only one thickness of paper where there should be two. This shows that the svstem as carried out is good, apart from accidents of a mechanical nature. This type of condenser is made from two long strips of thin tinfoil, between which are two strips of very thin waxed paper. Outside each is another strip of waxe paper, so that when the whole is rolle up, two thicknesses of paper evetys where separate two adjacent tinfoils. "MOTOR-BOATING." "‘Motor-boating"’ m resistances coupled amplifiers occurs between zero and a value just above 80 cycles, usual ly because the filter systems of elemin= ators are resonant within that band. To eliminate this without external aps paratus, the amplification over-all mus¢ not exceed approximately twenty. PLATE CURRENT. The average general purpose valve tequires from 1 to 8 milliamps plat current, depending npon whether used aS a detector or ampliher. A small power. valve takes from 3 to 10 milliamps, and a super-nower anything up to 2 milliamps, The higher the plate volt age the greater the milliamps passed. Resistance-coupled audio amplifiers con sume considerably less H.T. curren than transformer-coupled. A 16:FOOT LOUDSPEAKER.

| An American exnerimenter in the search for sweet and mellow music, ha constructed a 16-foot wooden loud speaker horn. Jt is made from ‘tw 16-foot boards 16 inches wide, each sawed diagonally from corner to corner. In order to accommodate the horn in a erdinary house, the wide portion rest upon the rafters of the ceiling, and th narrow end comes down between the walls. The sound is conducted through an ornamental grating in the centre of the ceiling. It is said to give really, good reproduction.

SECRET TRANSMISSION, The latest idea for ‘secret transmission of radio messages is to arrange two beam ‘stations some distance apart, arm range for the message to be broken up into fragments to be alternately transmitted by the two stations. ‘The beams are arranged to intersect at a given point, and in that zone both portions of the message are received as though sent ont in the ordinary way. The area of the receiving zone would depend upon the sharpness of definition of the beams. (End of Construction.)

A Hawera correspondent (‘Buzzy writes asking whether power-line interference can be stopped if its source is located. The trouble is due to the generation of radio frequency currents at spots where there should be no radio frequency currents and the broadcasting of these volunteer energies with the power lines acting as an aerial. As one Writer puts it, "What more natural, then, than to seize upon these emanations at their source and block them from the lines? There are various ways of doing it. In some instances they are allowed to wander into a condenser which acts as a sort of sump tank and loses them until their ambition is all gone. In others they are actually choked out of the line. Occasionally they are grounded."