A.C. Valves

Radio Record, Volume III, Issue 3, 2 August 1929, Page 29

A.C. Valves

Use of Heater Type on Those reading descriptions of A.C. valves will have noticed that.there are two types, directly heated filaments (226) and indirectly heated (227). Manufacturers of alternating-current sets are now employing the heater-type valves more extensively because, primarily, of the greater ease in building sets with these valves. Now that proper means for regulating supply-voli-age variations are being provided, and experimenters are becoming more familiar with the methods of handling unusual line yoltages, this valve is considered as a very very good all-pur-pose one. The construction of sets with these involves less work and fewer parts than the use of the directfilament A.O. valve, because of the simpler methods of balacing to reduce the "hum." In sets:employing the direct-filament valves a number of tapped resistances with adjustable taps are required to obtain the centre points of the filamént circuits, and extreme care must be exercised in wiring the sets. When the heater type is employed, no balancing resisters .are required and the sets are much easier to contruct for this reason. The inter-electroed eapacity of the 227 type, too, is much lower than the capacity of the 226 type, and this makes it a better radio-frequency amplifier. ‘The lower capacity also tends to made the valve more stable and, because of the construction, slight changes in the filament voltage do not cause a noticeable change in the output. .

Brilliancy of the Valves. FRADIO enthusiasts who have expertmented with the heater-type valves are frequently puzzled by the fact that these valves do not always glow with the same brillianey, even when the operating voltage’ remains unchanged, The contrast with the battery-type valve is quite marked; the latter showing quite uniform brilliance. The reason for the difference is quite simple and it will be seen from this explanation that such differences in the brilliancy of the A.C. valves do not affect the performance of the valves in any way. The filament which carries the heating current is made of pure tangssten, which is threaded through a cylinder of insulating material. The filament is exposed to the top of the insulation and any slight change in the contact and space left at the top will result in a change in the apparent glow of the tube. It is interesting tc note that the filament is not operated close to the melting point; so that the voltage on the filament can be increased a great deal over the rated value without burning it out. The. use of too high a voltage for grent lengths of time, however, will reduce the life of the filament, and for this reason it is advisable to measure the filament voltage from time to time with an A.C. voltmeter. If the filament is operated a voltage higher than that specified by the manufacturer, the glow will naturally be increased.

Low Filament Voltages. HE reasons for the .use of very low voltages for thé filaments of alternating-current: valves may be unknown to many experimenters, but they will quickly be appreciated. The first is that a low potential difference between the ends of the filament resulfs in a weaker electrostatic field and 2 corresponding reduction in the tendeney to produce an A.C, bum. The second important reason is that the use of a low voltage permits the use of a heavier filament and heavier current, which reduces the effect on the plate current of temperature changes (due to the current variations created by the alternations) and, consequently, the tendency to hum. The different types of valves have different’ filament characteristics because of the difference in ‘their construction and operation. The heatertype valve requires a longer filament than the direct-filament type, because of its insulating sleeve. On the other hand, the method of reducing the hum in the direct-filament valve requires a very low-voltage high-current charac- . }

teristic; so in order to keep both valves operating at their highest efficiency, different filament voltages are employed.