WIRELESS NEWS

Press, Volume LXVIII, Issue 20470, 13 February 1932, Page 9

WIRELESS NEWS

. (By "MAX.") Just a few months ago a new system was montioned in this column whereby a valve of low plate current consumption could be employed in an audio amplifier to give quite' a large output. This idea has been tried out in America and is being incorporated in more than one commercial receiver. It is called the Class "B" amplifier, as opposed to Class "A" for the ordinary type of amplifier. It is necessary to use a pair of valves in a push-pull output stage, and the negative bias is adjusted until the plate current of the two tubes is almost zero. The particular set wherein this system. is in use has a total of six valves preceding the output stage, and is a superheterodyne. Designed for dry battery operation, the drain is less than 20 milliamps when in use, and is considerably less when no signals aro being received, duo to the two pysli-pull valves drawing practically no current with no input. A pair of type -30 tubes will ordinarily give about onetenth of a watt output, but used as above will give a full watt, quite sufficient to work tlio average dynamic speaker. Experimenters should ecr- , tainly try out this method. * * ! In the ordinary amplifier the grids of the valves must not swing on to the positive side or distortion will occur, but in the "B" amplifier incorporating push-pull, grid current is permissible to a certain oxtent, and there ivill be no noticeable distortion. Turning to systems requiring large outputs, as in transmitting plants, another problem crops up.' This is the fact that the secondary of the input transformer must be designed with coarser wire, which will carry the grid current without serious voltage drop. November and December issues of "QST" delve into this aspect and give the necessary data for the construction of push-pull transformers. * « Nearly a quarter of a million receivers wero sold in Canada the year before last and the factory value was close on twonty-three million dollars. Of this nmount about twenty millions represented sets assembled or manufactured in Canada. Ontario wag responsible for the purchase of approximately fifty per cent, of the output. * • One of the largest British radio companies is E. K. Cole, Ltd., and the profits made by this firm are remarkable. Fancy paying a dividend of 100 per cent, and watering the capital at a similar rate. No wonder that when there was a recent allotment of shares i available for members of the trade these were oversubscribed within one and a half hours. In 1925 the firm started business and the floor space occupied was only ISO square feet, whilst at the present time the works cover some three acres, and are still growing. The added capital will be utilised in further expansion. Export trade will be one s of the features which will receive special attention ia the near future. In fact a number of the leading British manufacturers are concentrating onoversea trade. * * A new theory has been advanced by an observer who has every reason to believe that meteors are responsible for a good deal of static. Rather worse crackles than usual him to overhaul his aerial to ascertain whether a faulty connexion was responsible. He noticed that there seemed to be a larger number of meteors and he wondered if these caused the crackles. Using extension leads and placing the speaker outside he carefully watched the heavens. It was soon apparent that his surmises were correct. Ihis does not mean that electrical phenomena are not the cause of a good deal of static, but that meteors do make trouble of a similar character. Collisions with the gas atoms of the outer layer of the atmosphere are considered to set up ionisation and resultant electrical interference. "Radio News" gives details of a superhet circuit for use on the short waves where a penthode is in the first socket as an R.F. amplifier to feed into on autedyne detector. A special though not a new method is adopted to couple the two valves and to give satisfactory results free from certain disabilities which would otherwise occur. The usual coil and condenser receive the signal, and this is impressed between grid and cathode, the screen grid connexion going to a tickler coil for feeding back a negative reaction for stability if required. Tho place has an lI.F. choke to confine the radio frequency currents to a path through a coupling condenser to the grid side of the first detector tuned circuit. This system is stated to give excellent results, and the use of a penthode in this position is novel but most satisfactory. The first types of penthode valves seen in Christchurch were Milliard makes and were tried out by Mr C. R. Russell as audio amplifiers, this being the purpose for which they were designed. Mr Russell, however, in a lecture before the Radio Society of Christehureh. stated that thefio valves would function well in neutralised radio frequency stages, and suggested the Rice system for balancing out tho undesirable feedback which would tend to cause instability. This was before tho modorn screen grid valve was developed for high frequency purposes in screened boxes. It must be remembered that the recent types of penthof'es are great improvements on the earlv ones. # # The possibilities of the penthode have not been developed to any extent. For example ah R.F. oscillator self modulated can be built with the one tube. Just include a tuned circuit between plate and screen grid and we liavo in oscillator, a resistance being ii.eluded between control grid and "A" negative and the "B" positive going to a centre tap on the tuning cod. The resistance may Ik: from one-quar-ter to three megohms. By joining a condenser to screen and control grids a modulating frequency will be set up as well and this frequency will depend upon the values of the condenser and the associated control cirid rosiytance. Going a step further, the condenser can be removed :mk! a gramophone pick-np can be coupled to the control grid and filament, a radio frequency choke now being required nt tho grid end. * * The Radio Emergency Corps now being organised by the N.Z.A.Iv.T. is going ahead well, and amateurs m all the centres are extremely keen. In all probability there will be field days started very shortly, and there is a suggestion to give prizes for the best design for low-powered transmitters of compact construction and light weight. When organisation is completed, details of interest will be published in this coin inn. » # The radio pillow is an improved receiving device for people confined to their beds. The ordinary kapoc is re-

Notes From Far and Near

placed by a sponge rubber roi.ts the music irom the rep unit to travel in ali du'octions* tlie maximum strength is obtaitie • tiio centre. By moving the he . one side the volume can bo eontio t * * The annual meeting of the Kadio Society takes place nest Thursdnj. , ! ; : | j