Blue Glow in Pentodes

Radio Record, Volume V, Issue 27, 15 January 1932, Page 13

Blue Glow in Pentodes

Proof of Quality STATEMENT issued by the Cable Radio Tube Corporation, Brooklyn, N.Y., denies the assertion that the blue glow in pentodes is proof of poor quality-rather is it the reverse. The statement reads: ; There seems to be some considerable misunderstanding in regard to what should be the appearance of the Pentode type of valve. We have, had a number of correspondents state that the Pentode type has the appearance of having gas, as evidenced by a blue glow, and in consequence there was some doubt as to the quality. We want to remove any misconception of this gaseous content in the Pentode valve. his phenomenon is not gas, but a cathode ray bombardment of the glass, and is proof of high quality, This condition is caused by the electrons passing through the grid either ubove or below the plate, receiving an accelerating energy from the plate, but not being stopped by it, then striking the glass and causing the getter deposits on the glass to floresce with a hlue colour, This phenomenon is only present in yalves which are gas-free, as a slight trace of gas disperses the electrons in such a mauner that their energy and path is not concentrated enough to proGuee this effect. One must bear in mind that the phenomenon caused by the bombardment is proof of a gas-free valve. It appears only on the surface of the glass and not throughout the bulb. The real gas glow is most in evidence immediately surrounding the plate, between the filament and. plate, or sometimes filling the "whole bulb volume. When this phenomenon is not present in the Pentode it is an indication of poor quality, not the reyerse,