i71r11K41. d 11W%r w,?9,P No. 401. WEDNESDAY, MAY 4TH, 1927. VoL. XX. No. 18. Editor : HUGH S. POCOCK. Assistant Editor : F. H. HAYNES. Editorial Offices : 139 -4o, FLEET STREET, LONDON, E.C.4 - - Editorial Telephone : City 4011 (3 lines). Advertising and Publishing Offices : DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. Telephone: City 2847 (13 lines). Telegrams: "Ethaworld, Fleet, London." COVENTRY: Hertford Street. BIRMINGHAM: Guildhall Building3, Navigation Street MANCHESTER 199, Deansgate. Telegrams: "Autopres., Binnin than." Telegram.., "1 le, Manchester. Telephone: 0210 Coventry. Telephon::: 9970 and 2971 Midland. Tclep,,one- 8970 and 9971 City. Subscription Rates: Home, 17s. 4d.: Canada, 17s. 4d.; other countries abroad, lets. 6d. per annum. As many o,' the circuits and apparatus described in these par05 are 'covered by patents, readers are advesed, before making use of them, to sattsly themselves that they would not be in;ringi n; patents. EMPIRE BROADCASTING. We lo not, perhaps, hear enough from our readers over - seas, and we take this opportunity of extending to all of N our issue of List week we put forward the them the invitation to write to us on this subject and give case for a central short-wave station for us their views on the establishment of a short -wave honte Empire broadcasting. By a coincidence, broadcasting station specially erected for the purpose of within a day or two we received a letter from overseas broadcasting. If our Colonial readers show their a reader in Western interest in sufficient numbers it will Australia, of which ^4-4-4-fN1 help us in our endeavour to impress we publish the text the authorities with the need for under Correspondence " in this CONTENTS such a station. issue. pAOE 0 0 0 0 Our reader deplores the fact that EDITOR I1L VIEws .... .. 545 they have but one broadcasting sta- POWER AMPLIFICATION POWER AMPLIFIER FOR 40 -VOLTS H.T. tion in Western Australia, and goes WITH ... 546 on to ask ie When are the English- By A. P. Ca.stellaitl LOW VOLTAGE H.T. men going to broadcast on short PRACTICAL IIIN'rs AND 'l'IPS ... 552 waves ? " He reports reception of In the design of receivers where \VG1'''s programme relayed on B.B.C. QUALIrV FOUR" ... 554 considerable amplification is re- quired at low frequency it has cus- 32.79 metres from 2XAF, and also CURREN "r TOPICS ... 559 reception of the concerts from Hol- tomarily been recognised that it is B R O A D C A S T IiEI.'FIVERS- THE land, the only telephony heard requisite that the plate voltage from England being from the ex- " DAVENSET " PORTABLE II 551 available should be of the order of 120 volts or more, since modern perimental station of Gerald ]\ lar- VALVES WE 1lAVE TESTED ... 563 cuse, G2NJI. power valves are not designed to This fetter no doubt expresses INVENTIONS OF WIRELESS INTEREST 565 function on voltages below that the feelings of almost every repre- BROADCAST BREVITIES ... 567 ligure. Considerable interest is sentative of the British Empire therefore likely to be aroused in NEW ArlADA-rus 569 abroad, who must feel very MANUFACTURERS' the power amplifier for 4o volts strongly that we in the mother I.EFIERS TO 'rilE EDITOR 570 H.T., which is described in this issue. By the employment of country are neglecting a wonderful HEADERS' PROBLEMS ... 572 opportunity which short -wave specially designed 4- electrode valves it has been found possible broadcasting offers for strengthen Ñi4r11-1,14- HrN1N1N+kr.++Ñ ing the, bonds of Empire unity, for, to obtain an output for the opera- quite apart from the fact that listeners wv-ouhl be hearing tion of loud- spcake s using approximately one -third of the concerts and other programme items emanating front this plate voltage requited with a three -electrode valve giving country, the station could also be employed as a means equivalent output. The four -electrode valve has hitherto of broadcasting events of national, and therefore Empire, been very much neglected, and this application is calcu- interest, and the usefulness of the station could well he lates] to stimulate new interest in its development in the extended, at some future date, to direct communication very' useful direction of reduction of high -tension voltage, to the assembled parliaments of the individual sections of which, from the point of view of compactness or the Empire. portability is so important 13 3 www.americanradiohistory.com WApigIlt00 D1.41' VII, 1927, Wad tl An Efficient Four =electrode Arrangement Using Power Valves. By A. P. CASTELLAIN, B.Sc., A.C,G.9., D.I.C. rIL quite recently the idea of using only forty fully the advantages of push -pull amplification when low volts high -tension supply for a low- frequency H.T. valves are used. In Fig. r is shown the usual amplifier to give large volume output with good plate current -grid voltage characteristic curve for an quality reproduction would have been dismissed as absurd, amplifier valve for one particular value of plate voltage. and yet the amplifier illustrated in this article gives really This curve may be considered as consisting of a straight good strength and good quality on as little as 3o volts portion AB, followed at its lower end by a curved por- high- tension supply. tion, the straight part corresponding to a range of negative A short time ago' the writer described an amplifier grid voltage from D to C, and the curved part to grid using a four- electrode power valve embodying certain of voltages greater than that corresponding to C. his suggestions which gave sufficient loud- speaker results for the average room when operated from a crystal set Permissible Grid -swing. giving loud signals in telephones. The straight section AB is the only portion of the After the tests on this single -valve amplifier had been characteristic curve which is of use for distortionless completed, further experiments were carried out using amplifying purposes since it is only in this region that two stages of amplification with a view to obtaining suffi- the plate current varies exactly in accordance with the cient volume output for a very large room or small hall. variation of grid voltage, so that the maximum permissi- the high -tension supply having an absolute maximum of ble variation of grid voltage -or grid- swing, as it is 6o volts. usually called -is from C to D. In order to take full advantage of the low H.T. it was decided to ' make the last stage a push -pull one, since suitable push -pull transformers were already on the market. A push -pull amplifier may briefly be described as one in which the input voltage is divided equally between two valves, and in which the amplified voltages are combined in a special trans- former to pass on to the loud- speaker or on to the next stage of amplification. In this way an input voltage which is too great for one valve to handle without dis- tortion is split up between two similar valves, each dealing with half the- applied input voltage. Before proceeding with the de- sign details of the amplifier it will be as well to discuss more ' The Wireless World, March View from back of instrument 9th, showing method of supporting 1927. grid bias battery. E 4 www.americanradiohistory.com Wfizamo :71:1 Y 4111, 7927. Woacl 547 Four -electrode Power Amplifier.- characteristics for actual valves on two different plate In Fig. I (also in Figs. 2 and 3) the curves showing voltages. It will be seen from these figures that increas- the relation between grid voltage and corresponding plate ing the plate voltage effectively increases the available current are only shown for negative values of grid volt- length of the straight portion AB of the characteristic age, for the reason that when the grid of the valve be- curve, and therefore increases the permissible grid -swing comes pósitive electrons will beattracted from the stream CD. At this stage it should be clear that, for any given going from filament to plate, amplifier valve, the maximum permissible grid -swing is thus causing grid current to fixed when the voltage on the plate of the valve is fixed. How -which will cause seri- ous distortion... It is not here proposed to go into the ANELOY A.P. 425 'IMOMMOMMMEM reasons why grid current in OM% an amplifier produces distor- Ì has been MO AMMO tion as this subject ME III dealt with many times in the pages of this journal -so M I is sufficient to say that M that it /1 Ia! I it is only the straight por- omIIII tion of the characteristic r!IIMEI E curve corresponding to nega- EI'I.. I I, tive values of grid voltage IIIMMOMMOMMEMMAMMVMMOMME/.U Fig. 1.- Typical grid volt - which is permissible for dis- MMEII plate current characteristic. amplification. MA/a tortionless OMMIIMMIaI,a The next point to consider is the effect of varying the MOUr1 plate voltage on this straight part of the characteristic MMI.a//i/JIi curve. If this voltage is increased the relation between AMIIU w grid volts and plate current will he shown on the diagram EI H MMILI.a%/i J by a new curve lying to the left of the old curve a.nd of iAa a substantially the same shape, while reducing the plate old Mi/ the Ma to the right of MMMEW voltage gives a corresponding curve OM¡A one. In effect, then, increasing the plate voltage may EEEEEEEEMEE be regarded as moving the characteristic curve bodily -to 20 18 16 14 12 10 8 6 4 2 This point will be the left, and reducing, to the right.