ACTION OF THE SCREEN-GRID

Radio Record, Volume I, Issue 34, 9 March 1928, Page 12

ACTION OF THE SCREEN-GRID

THE UX 222 VALVE

(By

Megohm

TLE = coustruction of the UX222 screen-grid valve is fairly complex in comparison with the ordinary receiving valye. ‘Ihe extra grid consists of two circular screens of wire, one encircling the outside of the plate, the other inside the plate, between the latter and the ordinary control grid, the lead for which comes out at the top of the valve. In the very centre of all is the filament. The two screen-grids ate connected together, and the lead brought out to the ordinary grid connection of the valve base. The sensitivity of the control exerted by the grid (and hence the amplifying value of the valve) is definitely limited by the cloud of electrons surrounding the filament. If an extra grid is interposed between the filament and the regular control-grid, and charged positively, it will, by virtue of its proximity to the filament, break up the cloud and increase the control range of the regular grid. The extra grid may also be located between the control-grid and the plate. Here it will require a higlier positive charge than before, but it now also serves another purpose-namely, to reduce the troublesome capacity effect between the grid and plate. It acts as an earthed centre plate, splitting the

"fixed condenser’ formed by the grid aud plate into two smaller condensers in series. | As already mentioned, the outer screen encircles the plate completely. ‘The inner section acts both as a spacecharge disrupter and as a capacity re-

ducer, while the outer section (which has nothing to do with the stream electrons, because they stop at the plate), serves merely to reduce the capacity between the outside of the plate and the comnecting leads, ete. It should be

noted that the inner screen-grid is chiefly concerned in the following remarks. The "space-charge"? spoken of is the negative charge of tle cloud of electrons hovering round the filament, and which have been unable to travel to the plate through being insufficiently heated, and so haying too low a velocity, and therefore easily retarded by the space-charge. Increased plate voltaze decreases the space-charge, which means that more electrons reach the plate. As the control grid is nearer tu the space-charge than is the plate, variation in the grid voltage has a greater effect, in proportion, than changes in plate voltage, so that small grid voltage variations make large plate-current variations, which gives the amplifying effect. Now, this space-charge is actually detrimental to the working of the tube, and the cause of its low efficiency, because its repelling effect is added to the repelling effect of the grid, and so any change in the repelling potential of the grid is only a partial change in the whole repelling potential. If we could wipe out the space-charge altogether, and leave the grid with the only negative charge, then grid variations would have a much greater gurcentage effect upon the whole negative charge, and the amplification factor would at once rise from 6 or 8 to per-. haps 80. ‘This because the change in repelling effect of the grid is complete in itself, and no energy is wasted in ‘‘gingering up’? the drowsy encumbrance known as the "spacecharge,’?’ which has now been cut out by the interposition of the positiyelycharged extra grid between the plate and grid. In ordinary valves, 85 per cent. of the plate yoltage is used in overcoming the space-charge, and the remaining 15 per cent, in establishing plate current, so that by removing the space-charge the 85 per cent. of high-tension voltage needed to overcome it may he in future saved, and the yalve will operate with only the 15 per cent, required to establish current through the valve. This means that where 100 volts was formerly applied to the plate, with the sercen-grid 15 volts, would give an equal result. On the other hand, instead of thus reducing plate voltage, and getting equal results, we may in practice keep the plate voltage as before, and thus obtain increased amplification from the greater electron stream from filament to plate that is affected by the grid voltage variations. Tt is by placing a positive charge at the point where the negative charge acenmulates that the extra (inner) grid obliterates the space-charge, The extra grid is really doing part of the work of the plate, but by its position near to the source of the trouble it is able to do this part of the work more efficiently than.the plate can, This is a brief description of the two main points in the action of the new yalve,