Showmen's Trade Review (Jan-Mar 1947)

E-18 SHOWMEN'S TRADE REVIEW, February 1, 1947 A New Photocell For The New Films need has recently arisen for a phototube that could be interchanged with the type commonly used in theatre sound heads. 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 theatre sound equipment for more than a decade. In order to evaluate the merit of this new blue-sensitive phototube for theatre EXCITER LAMP VOLTS AC OUTPUT WITH 1000 CYCLE FILM HUM OUTPUT HUM OUTPUT BELOW IO00CYCLE FILM OUTPUT IP37 868 IP37 868 IP37 868 75 17.7 19.1 -04 -3.4 18.1 22.5 8.0 20.0 20.4 +2.0 -1.6 18.0 22.0 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 16.5 20.9 Fig. 1. Relative film and hum output variations in db — IP37 versus 868. Film removed for hum output measurements. 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 theatres 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 these phototubes were placed in the field for observation, so reports on performance are not complete at present. Laboratory tests have been made, however, to determine how well they would function in theatre sound heads with standard release prints, and if there were * Condensed by permission, from the paper ent.i.ed: "Behavior of a New Blue-Sensitive Phototube in Theatre Sound Equipment," which appeared originally in the Journal of the Society of Motion Picture Engineers. By J. D. Phyfe Radio Corporation of America 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 (un filtered) a-c on the exciter lamp. (6) Gain variation with changes in exciter lamp brilliancy owning 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 redsensitive phototubes when interchanged. Results of Laboratory Tests The results of these tests are reported in the order in which they are enumerated above: (1) Gain Check. — This was made by running a 1000-cycle loop of standard silver sound track through a regular theatre sound head 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 sction of sensitivity variation. Exciter lamp and phototube anode volt*ages were held constant at 8V2 and 75 v, respectively. Results of this check showed that the sensitivity of the 2 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 1000cycle 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 hiss level was checked both on an output meter and a sound-level meter used in conjunction with a standard 2-way theatre 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 theatre sound head and theatre amplifier channel using both types of phototubes, and the rms harmonic distortion measured by means of a distor tion factor meter. The measured distortion was found identical for both tubes. (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 exciter lamp voltage was adjusted to a normal operating value of 8.5 v. This was based on equal signal outputs for both types of phototubes using a 1000-cycle film loop. Figs. 1 and 2 show relative signal output and hum levels between the 1P37 tube and the 868 when the exciter lamp voltage was varied between 7.5 and 10 v. Hum level was measured by removing the film after output measurements were taken. (6) Gain Variation with Changes in Exciter Lamp Voltage. — Referring to Figs. 1 and 2, a gain change of 5.8 db is observed for the 868 phototube and 8.8 db for the 1P37 tube, using a 1000-cycle film. This shows a 3-db increase in gain variation for +8r 1 1 1 1 ■ 1 , EXCITER LAMP VOLTS A.C. Fig. 2. Relative signal-to-hum response with and without film of the new IP37 and its predecessor, the 868. the 1P37 tube when the exciter lamp voltage is varied between 7.5 and 10 v. (7) Balancing Sound Head Outputs by Adjusting Anode Potential of Phototube. — The adjustments used for balance of the sound head outputs are the same for both the 1P37 tube and the 868. These are made by control of anode supply votage. (8) Optical System Focus When Using Both Types of Phototube. — A standard IVamil slit image optical system was focused for maximum output using a 7000-cycle loop of film and an 868 red-sensitive phototube. Output readings were then taken for both types of phototubes. No observable increase in output was obtainable with the 1P37 tubes by refocusing the optical system. This test was then repeated using a 9000-cycle loop of film. The results were identical. This observation seems to substantiate (Continued on Page E-31)