Radio age (May 1922-Dec 1923)

RADIO AGE— "THE MAGAZINE OF THE HOUR" Photo-Electric Detector Tubes By H. A. BROWN and C. T. KNIPP, University of Illinois A YEAR ago the writers completed an investigation of the effect of various residual gases and various degrees of vacua upon the characteristics, constants and efficiency of detector tubes. The investigation showed that in the case of a low vacuum the optimum plate voltage for detector action decrease with the ionizing potential of the gas in the tube. The vapors of certain alkali metals have ionizing potentials of 4 volts and less, and some of these were experimented with. It was found that the vapor of potassium-sodium alloy, having an ionizing potential of 4 volts, when present in the ordinary threeelement vacuum tube or Audion caused it to function as a very sensitive detector of high frequency oscillations at a plate potential of 5 to 10 volts. Tests in this laboratory have shown that this tube is from 3 to 5 times more sensitive on weak signals, with 8 or 10 volts plate potential, than is the same type of tube containing any of the commonly used gases, such as argon and helium, and which require 18 to 25 volts. This latter is the widely used "gas content" or "soft" detector tube. In spite of the extremely low plate voltages needed for this alkali vapor filled tube it is not "critical" in adjustment of plate voltage as is the conventional "soft" detector. Fig. 1 shows this clearly, curves A representing three different tubes each primed with alkali vapor, and curve B for the conventional "soft" detector tube. Users will appreciate this advantage. /{Iko// \/oPor Deiec/'ars >Q ■is Professor Charles T. Knipp, University of Illinois FJ\ y i/re I. H. H. Brown, Associate in Department of Electrical Engineering, University of Illinois The most astonishing discovery about this tube is the fact I hat it operates efficiently at zero plate voltage. Fig. 2 shows the characteristic curves for one of these tubes, the lower curve being taken at zero plate voltage. To do this the plate circuit return was connected to the negative filament terminals. The plate current flows through the vacuum from the plate to the filament in spite of the opposing effect of the filament drop. This curve shows the plate current to be about 1 milliampere at zero grid voltage. As is well known, the potassium-sodium alloy is used as the sensitive coating in the photo-electric cell, a device which furnishes a source of feeble electric current when light shines upon it. In all probability the source of plate current in these tubes at zero plate voltage is the photoelectric effect of the alkali vapor, the luminous and non-luminous radiation from the filament being the source of energy. These photo-electric detector tubes function very well as detectors of damped and undamped waves and of radiophone modulated waves. As a test several of the tubes were tried out. Using one tube as a detector together with a "variometer" type of regenerative tuner, and an antenna 45 ft. high, the broadcasting stations at Schenectady, N. Y., Detroit, Pittsburgh, and Chicago were heard in this locality without an amplifier and with a directly measured audibility of about 30. When a plate voltage of 6 to 10 volts was applied the audibility increased to 150, this corresponded with the results of the carefully made laboratory tests shown in Fig. 1. At zero plate voltage the tube was used to receive the 17,000 meter station at AnnapoHs by the beat method, the tube oscillating very easily and steadily. It is equally efficient on short wave amateur C. W. reception. The foregoing features in addition (Continued on page 24) A/Aa// \/apor /~/aivre 2