Patent GB-488859-A (GB)
Title: Improvements in or relating to photoelectric cell and amplifier circuits
IPC: H01J40/14
Filed: 1937-01-11
Abstract: 488,859. Optical sound - reproducing apparatus; valve amplifying circuits. WESTERN ELECTRIC CO., Ltd. (Electrical Research Products, Inc.) Jan. 11, 1937, No. 850. [Class 40 (ii)] [Also in Group XL] Sound-reproducing apparatus is adapted to be used either with a single-track sound film or a multiple-track film (e.g. a " pushpull " record), by employing photoelectric cell apparatus having two cathodes permanently connected together and two anodes, with switching means for connecting the anodes alternatively directly together or in separate circuits, so that the apparatus may in effect function either as a single photoelectric cell or as separate cells, as required. Fig. 1 (Prov.) shows an arrangement in which a photo-electric cell has two anodes 12, 13 connected respectively through resistances 14, 16 to a source 15 of positive potential, the negative side of the potential source (not shown) being connected to the double cathode 10, 11 through earth. A condenser 17 is in parallel with 16. The cathodes are connected through a resistance 24 to the conductor 23 of a cable having an earthed shield, coupling the cell to an amplifier, and to the earth lead 25 through a resistance 26. A switch 19 having three pairs of contacts 18, 22, 43 is connected as shown. When the switch is in the position shown, the anode 12 is connected to the conductor 23 through a condenser 21, the contacts 18, 43 being open. In this position, the apparatus is in condition to reproduce from a push-pull record, since the potential variations produced in the resistances 24, 26 by the effect of the modulated light beams on the cathodes will be in phase. When the switch 19 is in the alternative position, contacts 22 are opened, and contacts 18, 43 are closed. The anodes 12, 13 are then directly connected, and a resistance 27 is connected between 23 and earth. The apparatus is then in condition to reproduce from a singletrack record. A negative feed-back arrangement is provided, to reduce the input impedance of the amplifier, by coupling the control grid 29 of the first amplifying valve through a resistance 40 and condenser 39 to a point between a resistance 37 and a condenser 38 which are shunted across the primary winding of a transformer 36 in the output circuit of the valve. In a modified feed-back coupling, Fig. 2 (Prov.) (not shown), the grid of the valve is connected directly to the anode through a resistance and a condenser. Fig. 5 (Comp.) shows a modification of the circuit shown in Fig. 1 (Prov.) in which switches 60, 63, which may be interconnected, serve the same purpose as the switch 19. A source of D.C. supplies through suitable networks the operating potentials for the amplifier and the photoelectric cell system, the output of the latter being applied through a suitable network to the control grid 29 of the first amplifying valve. In this arrangement, voltage developed across a resistance 67 in series with the primary of the transformer 36 is fed back through an impedance 40 and a condenser 39 to the grid 29. An alternative circuit arrangement, shown in Fig. 1 (Comp.) embodies a switch 138 for enabling the anodes 136, 137 of a double photo-electric cell to be connected to the. primary of a transformer T (in the input of a sound-reproducing device) for either push-pull or single-track reproduction. The switch blade 139 co-operates with two sets of contacts 150, 151. In the position shown, for use with a single-track record, the two anodes are connected in parallel to one half of the transformer primary. The alternative position of the switch adapts the apparatus for push-pull operation, by connecting the separate anodes to opposite sides of the transformer primary and the mid-point of the primary to the positive terminal of the potential source A. A filter circuit for the polarizing potential for the cell is provided by suitable components 158-162. The switch 138 may be combined, as described in Specification 490,985, with a masking device which is moved into the beam of light modulated by the sound record, when the switch is set for push-pull operation, in order to mask a small area in the centre of the beam and so to ensure that light passing through each track falls only on the appropriate cathode. The operating arm of the switch projects through a sleeve which is connected to the mask movable over the slit and is moved longitudinally when the arm is rotated. Figs. 2-4 (Comp.) (not shown), illustrate the combined mask and switch, and show details of the optical sound-reproducing device in a sound- and-picture film apparatus to which the invention is applied, all of which features are described and shown in Specification 490,985. Specification 462,195 also is referred to.