An Interesting Valve

Radio Record, Volume V, Issue 11, 25 September 1931, Page 12

An Interesting Valve

American Pentode QNB of the features of American radio this season is the development of the output pentode. A similar valve was developed in Continental three years. Briefly, the pentode Vis an output valve with an extra grid like the sereen-grid Valve, and in addition — xn third grid attached to the filament. It then contains five elements, the three normal ones, and the two extra elecments in the form of a grid. The addition of these grids results in the valves having a high amplification factor and being able to deliver a big power output. The a.c. resistance of the pentode is many times higher than that of an ordinary power valve and the result that a very brilliant and pleasing tone canbe obtained. Furthermore, owing to the unusual amplification, this valve can eliminate an audio stage. The new. American 247 type of valve delivers 23 watts undistorted power output with 11.7 volts a.ec. input. This compares with 1.6 watts output and 34.5 volts input with the ordinary 245 power valye. The American / type valve uses the UY socket whereas the English and Continental use the ordinary UX socket with an extra terminal on the side.. This extra terminal is connected to a high B+ voltage. The American valves operate on 2.5 volts, whereas the Wnglish and Continental employ 2, 4, and 6 volts. The new pentode is almost four times as sensitive as the 245 power valve, 2 property of no small economic importance when considering output, detector overload and: amplifications. The Continental manufacturers make available a series of pentodes varying not only in the filament voltage, but also with the plate, there being available pentodes that will take from 150 yolts to 550 on the plates. It is interesting to note that a Continental pentode with characteristics very similar to the new American valve has been available for some considerable time.