Valves of the Future

Radio Record, Volume II, Issue 24, 28 December 1928, Page 32

Valves of the Future

Development of MultiElectrode Valves : A few years ago it appeared as if there was little further progress to be made in the design of. thermionic valves beyond the development of tubes with lower filament energy consumption. At that time interest was centred almost completely on the three-elec-trode valve, which was then, as it still is to-day, the most widely used of all types. But makers are now paying more attention to the undoubted advantages which accrue from the use of valves with more than one grid, and the advance notices of the new type

shown at the Radio Exhibition, which was held at Olympia on September 2229, demonstrate this very markedly. Sereened Grid Valves. for some time double-grid or four electrode: valves have been on the market, but rapid strides have not been made until the Osram 8625 appeared on the market, This gave the needed impetus to what appeared was going to be a revolution in RF reception. This valve includes an extra grid situated between the normai grid and the plate, the function being to eliminate the capacity between these two electrodes and thus ensure stability in the HF side of the receiver without recourse to the usual neutralising condensers. The 8625 valve, however, required a special kind of valve socket, and its introduction into an old set required considerable modification of the high-fre-quency side. The same makers now announce a new valve of this type which will fit into the standard fourpin socket, the fifth electrode being brought out to a small terminal on the top of the glass bulb. As the type name indicates, the new $215 runs on two volts with a current of 0.15 ampere, as compared with, the six volts and 0.25 ampere required by the 8625. There is therefore a gain in filament efficiency. Other screened grid valves; requiring little filament current are ‘the Radiotron UX222 with a filament consumption of .182 at 3.8 volts, Philips 442 requiring .06 amperes at 4 volts and PM12 (Mullard) sereened grid are characterised by extremely high imredance.

Pentode. (THE introduction of still another electrode, making a valve with five electrodes, was first shown to adtage in Mullard’s PM22, and the advance announcements of the new valves show that other makers are realising the -importance of this type of pentode valve as an efficient output valve. Philips have produced a_ splendid pentode A448, which, when used in the last stage adds tone, volume and quality to output. PXHE penode may be regarded as a sereened valve with an additional earthed screen grid between the screening grid and the anode. By the introduction of this third grid an undesirable interchange of electrons, bombarded out of the anode and caught by the screen grid, is prevented. Such an interchange takes place in an ordinary sereened-grid tube with only four electrodes. With this pentode a remarkably wide grid range of undistorted output is obtained, the characteristic being substantially linear for a total grid swing up to about 15 volts. (THE introduction of extra electrodes in the ordi.ary three-electrode valve has thus led to improvements in both high-freyu-ncy amplification and lower-frequency amplification. and there seems little doubt thet before long a considerable’ number of the tubes in standard multi-valve receivers will be of the four or five electrode type.