The Screen-Grid Valve

Radio Record, Volume III, Issue 10, 20 September 1929, Page 45

The Screen-Grid Valve

Race Lectuine to Radio Society At the recént sapnthiy inating of the Amateur Radio Society Mr. L, Wright gave an, interesting and in‘structive lecture on the screen-grid valye. In order to be more: readily ‘inderstood when he came to the action of this latest type of R.F. valve, Mr. Wright led up to. the subject by explaining briefly the virious types of valves and their shortcomings when used in the R.F. stage. The lecture started with methods practised years ago when R.F. amplification was considered to be useless. Certa{ily in those days practically no amplification per stage was obtained, the reason. being that in order to prevent the valves from oscillating resistances Had to. be introduced, which completely damped any obtained. Sereening was introduced many years ago to prevent interaction between the variqus cdils and leads, bit this only partly eured the troyble. It was realised that the small capacity between the various electrodes of the valve was quite sufficient to cause instability. No gentine soliition was available until the neutrodyne method was introduced, in which the valve capacity Was rieuttalixed by an equal and opposite capacity working directly against the capdcity inside the valve. This is. the method still used on most commercial receivers. Even this has its drawbacks, as Mr. Wright explained. To neuttalise a receiver on its lower wavelength reduces the efficiency on thé highér wavelengths. Yet another method of stabilising was mentioned, in which a non-induttive resistance of 600 to 800 olims, is introduced in series with the grid, lead. This method, again, ig still used by. many manufacturers, About ‘two years ago Capt. Round in Hngland put an idea into practice. A fourth electrode or second grid was introduced betwee the control grid and the plate. This grid, although giving a positive potential, was used to reduce the eapacity between the control grid and plate. The result was the well-known S625 type of valve. In appearance it is of tubular shape, with the connections of the electrodes coming out at either end. Before discussing the various . Gcirenits adaptahle to this valve, Mr. Wright ex: plained, with the help of actual valves. the construction and advantages of later types of the sereen- -grid valve. These were adaptable to the standard four-pin Arerican and Pnglish sockets, and canh he incorporated in existing sets by making minor alterations to wiring, eta. As the valve had a very high impedance the tuned anode method of coupling was the only one to be used if the maxitnum amplification per stage was to be obtained. Unfortunately 4 tuned ariode circuit is not ultra-selec-tive, and to minimise this disadvantage a ciretit is available on which the plate cireuit of the screen-¢rid valve and the grid cireuit of the succeeding valve were tuned. This constitutes the well-known band-pass Alter or tpned plate tuned ‘grid coupling. There were, one or two points that Mr. Wright stressed most strongly. In order to get the very mast from a seréen-grid valve all inductances inust be of low-loss construction and per-

-- fectly screened. Poor screening, if not resulting in instability, gives poorer quality and is a sure method of introducing background™ nojses. This abserjce of background hiss is one of the main advantages to be obtained when using the screen-grid valve, and it can easily be marred by. introducing feedbacks due to poor screening. }