Patent GB-687557-A (GB)
Title: Improved electro-mechanical transducers
IPC: H05H1/00
Filed: 1951-09-18
Abstract: 687,557. Gramophone pick-ups. RADIO CORPORATION OF AMERICA. Sept. 18, 1951 [Sept. 20, 1950; Sept. 20, 1950], No. 21899/51. Class 40 (iv). [Also in Group XL (a)] A phonograph pick-up comprises a gas-filled discharge tube of the type in which an auxiliary electron discharge is used to ionize the gas plasma which carries the main load current; a stylus passing through a flexible part of the envelope is attached to an electrode inside the tube, so that external mechanical movement of the stylus results in corresponding movement of said electrode, the load current being thereby modulated. This modulation is continuously controllable, even when the tube is ionized. In Fig. 2, 24 is the auxiliary cathode, surrounded by a shield 25; the ion-producing electron discharge is formed as a narrow stream directed through slot 27. The shield 25 may be biased with respect to cathode 24, or at the same potential, or left " floating." The conducting plasma is established between the main cathode 15 and a pair of anodes 16, 17. A control electrode 18 surrounds the anodes with a shield portion 21 between them. The plasma passes through a gap in the control electrode, which gap is partially blocked by the movable shutter 20 leaving two slots 22 corresponding respec. tively to each anode. The shutter 20 is attached to the stylus which passes through a flexible metal insert in the glass envelope, as shown at 14 in Fig. 8. A transverse vibration of the stylus will move the shutter so as to widen one slot 22 and narrow the other, the arrangement being suitable for utilization in a push-pull circuit. In the modification of Fig. 4, the control electrode 18a has several gaps instead of one, which are overlaid by the movable grid shutter 20a. The slots in 18a and 20a are so arranged that in the rest position of the stylus, equal currents are received by the anodes; and a transverse displacement of 20a increases all openings providing current paths to one anode, and decreases those providing paths to the other. A simpler form with only one anode is also described. In the arrangement of Fig. 5, instead of the control electrode comprising a stationary and a movable part, the entire electrode is a movable unit. It consists of a base-plate 36 carrying two rows of grid wires 37 parallel and closely adjacent to the respective anodes 16a, 17a. This grid is negatively biased. In the above-described devices, modulation is obtained by varying the useful crosssection of the conductive plasma in openings of the control grid of the load-carrying group of electrodes, as a result of mechanical movement of said grid. In Fig. 8, the mechanical movement is applied to the control grid of the auxiliary ionizing electrodes, in this case a triode, and thus the conductivity of the plasma is modulated by varying its density. The auxiliary triode consists of cathode 24, slotted anode 42, and grid 41, the latter attached to stylus 14. This triode is operated with a low anode potential, and the grid-cathode spacing is varied by mechanical movement of the stylus. The resulting variation in the current escaping through the anode slot towards the rest of the tube controls the production of plasma therein. A suitably polarized screen grid 46 inhibits any tendency for positive ions to drift back into the triode section. 47 is an accelerating screen which may be at the same potential as the main anode 49. The control grid 41 of the auxiliary triode may be omitted, and the cathode-anode spacing varied by moving either electrode. The pick-up of Fig. 8 may be modified for push-pull operation by duplicating the whole arrangement round a common auxiliary cathode. Another embodiment using modulation by plasma-density variation is shown in Fig. 15. In this case the shield 25a surrounding the auxiliary cathode 24 is extended into a lip 52 at one side of the slot 27a. At the other side of the slot is a movable shutter 51 attached to the stylus, so that movement of the latter varies the slot width. A satisfactory gas filling is helium at 750 microns, though other gases and pressures may be used. Suitable circuits for the different pick-ups described are given in Figs. 10-13 (not shown) and Fig. 16 (not shown). The principles of the invention may be applied to other electromechanical transducers, e.g. microphones.