International projectionist (Jan-Dec 1946)

RCA's New Blue-Sensitive Phototube NEED has recently arisen for a phototube that could be interchanged with the type commonly used in theater soundheads. The necessity is the result of an apparent trend toward an increasing use of color in motion picture film productions. Also, there is the possibility that a larger percentage of these color films will have dye sound tracks instead of the usual silver tracks. The desired tube should be interchangeable with the red-sensitive type, therefore, and must perform as well when used with films that have the regular silver sound tracks. Such a phototube has been developed in anticipation of these needs, and is known commercially as the type 1P37. To date the observed performance of this tube indicates that it accommodates this changeover very well. Mechanically and electrically the 1P37 is interchangeable with the type 868 phototube which has been used in RCA theater sound equipment for more than a decade. In order to evaluate the merit of this new blue-sensitive phototube for theater use, comparisons of performance have been made in the laboratory with the type 868 red-sensitive tube. In addition a number of the blue-sensitive phototubes have recently been distributed to various theaters throughout the country with instructions to use them in place of the 868 tube, and report their relative behavior. Too short a time has elapsed since EXCITER LAMP VOLTS AC OUTPUT WITH 1000 CYCLE FILM HUM OUTPUT HUM OUTPUT BELOW 1000 CYCLE FILM OUTPUT IP37 868 IP37 868 IP37 868 75 17.7 19.1 -0.4 -34 131 22.5 8.0 20.0 204 +2.0 -1.6 18.0 220 8.5 21.8 21.8 +3.8 -0.2 18.0 22.0 9.0 23.3 22.7 +5.7 + 1.1 17.6 21.6 9.5 24.8 23.8 +74 +2.6 174 21.2 10. 26.5 24.9 +10.0 +4.0 165 20.9 FIGURE 1. Relative film and hum output variations in db — IP37 versus 868. Film removed for hum output measurements. these phototubes were placed in the field for observation, so reports on performance are not complete at present. Laboratory tests have been made, however, t J. Soc. Mot. Pict. Eng., May 1946. By J. D. PHYFE RADIO CORPORATION OF AMERICA A new phototube designed to provide optimum performance when used in reproducers having standard black-andtvhite silver sound tracks, or with color films having either dye or edge-treated sound tracks, has been developed by RCA as a result of the rapidly increasing color footage now being used. Some results of laboratory tests and field observations when this new tube is substituted for the standard red-sensitive 868-type phototube are detailed herein. to determine how well they would function in theater soundheads with standard release prints, and if there were any adverse operating characteristics which might preclude their being substituted for the 868 tube. Some of the characteristics investigated were: (1) Relative gain as compared to the 868 tube when used with films having the usual silver sound tracks. (2) Relative hiss level and microphonics. (3) Ionization or glow point. (4) Relative distortion. (5) Relative hum level when used with raw (unfiltered) a-c on the exciter lamp. (6) Gain variation with changes in exciter lamp brilliancy owing to line voltage fluctuations. (7) Ease of balancing output between sound heads by varying the anode voltage. (8) Frequency response relative to the 868 tube. (9) Refocusing of the optical system because of different spectral sensitivities of blue-sensitive and red-sensitive phototubes when interchanged. Results of Laboratory Tests The results of these tests are reported in the order enumerated previously: (1) Gain Check. — This was made by running a 1000-cycle loop of standard silver sound track through a regular theater soundhead and amplifier channel, and noting the levels. Both types of phototubes were used. A sufficient number of both types were checked to represent an average cross section of sensitivity variation. Exciter lamp and phototube anode voltages were held constant at 8% and 75 v, respectively. Results of this check showed that the sensitivity of the two types of phototubes is practically the same, with a slight superiority of the 1P37 tube. It was noted that the 1P37 tube showed somewhat less variation in output from one tube to another than tube 868. (2) Hiss Level and Microphonics— ■ This test was made by checking the relative outputs of a group of red-sensitive and blue-sensitive phototubes using a 1000-cycle film loop as a signal source. The film was then removed, the amplifier gain was raised to approximately 130 db and the hiss level was measured. The 75 8 85 9 9.5 10 EXCITER LAMP VOLTS A.C. FIGURE 2. Relative signal-to-hum response. hiss level was checked both on an output meter and a sound-level meter used in conjunction with a standard 2-way theater loudspeaker system. Microphonics was then checked by starting the projector motor and noting the output level, as was done in measuring hiss. The results of these tests revealed no apparent difference in hiss or microphonics when the blue-sensitive phototube was used. (3) Ionization or Glow Point. — No changes in voltage supply are necessary when changing from the type 868 phototube to the 1P37 type. The same maximum supply voltage limitations exist for both types. (4) Distortion. — A constant-frequency film having 80 per cent modulation was run through a standard theater soundhead and theater amplifier channel using both types of phototubes, and the r.m.s. harmonic distortion measured by means of a distortion factor meter. The measured distortion was found identical for the 1P37 tube and the 868. (5) Relative Hum Level. Using a.c. on the filament of a standard 10-v, 7.5-amp exciter lamp, the 1P37 tube showed 4 db more hum when the exicter INTERNATIONAL PROJECTIONIST • September, 1946