GB623430A - Improvements in modulating and control arrangements for valve oscillators - Google Patents

Abstract

623,430. Valve generating, modulating, and relay circuits. NEUBER, H. K. Dec. 23, 1946, Nos. 37754 and 37755. Convention dates, Dec. 22, 1945, and Dec. 11, 1946. [Classes 40 (v) and 40 (vi)] [Also in Group XXXIX] A valve oscillator has, in addition to the normal feed-back, an auxiliary feed-back comprising a coil with a centre-tap connected to the anode (or grid) and inductively related to the grid (or anode) circuit, the two halves of this coil thus providing negative and positive feed-back respectively, and causing a fall or rise in oscillation amplitude when the current in the two halves is unbalanced (e.g. by a differential microphone). In Fig. 1, the oscillator comprises a valve 12 with a tuned anode circuit 25 back-coupled to the grid circuit by virtue of the anode-grid capacity. A further back-coupling is afforded by the connexion of the anode to the mid-point of a coil 41 loosely coupled to the grid circuit. The ends of the coil 41 are connected through twisted cables 45, 46 enclosed in an earthed shield 47, to capacity plates 55, 56 between which is pivoted a movable plate 57 operated by the stylus of a gramophone pick-up or other movable member. In the central position of the plate 57 the coil 41 is balanced in its effect on the grid coil 17, but as the plate 57 vibrates, the normal feedback through the valve is assisted and opposed alternately, and the amplitude of oscillation correspondingly increased and decreased. Owing to rectification at the resistance-condenser unit 19, 20, the grid voltage correspondingly falls and rises, giving rise to corresponding fluctuations in anode current. Voltage variations across the anode resistance 33 may be fed to an audio amplifier 98 ; or high-frequency voltages may be fed from the anode through a condenser 37 to amplifiers and a detector. By arranging that the grid circuit is tuned to a somewhat lower frequency than the anode circuit 25, the modulating effect may be enhanced. As the oscillation amplitude rises, the grid becomes more negative, and owing to the Miller effect the reflected capacity 18 falls and the tuning of the grid circuit moves towards resonance with the anode circuit, giving a further rise in oscillation amplitude. Corresponding action takes place when the oscillation amplitude falls. The capacity pick-up 55 ... 57 may have its fixed electrodes set at an angle to each other ; or it may be replaced by an inductive pick-up comprising a pair of stationary inductance coils and a conductive ring or a movable core supported by the stylus and movable between them, (Figs. 5 and 7, not shown). A pair of reversely-variable resistors may also be used, (Fig. 6, not shown). The oscillator valve may be enclosed in the same envelope with the first amplifier valve of the audio amplifier, (Fig. 8, not shown). In a modified circuit, Fig. 11, the feed-back coil L3 is close-coupled to the anode coil L2, and its centre tap is connected to the live end of the grid coil L1. The coils L2, L3 may be enclosed in a screen connected to the earthed shield 203, and these may form a separate unit with socket connexions to the remainder of the circuit, (Fig. 14, not shown). In a further modification (Fig. 13, not shown), the grid coil L1 and the grid leak and condenser 177, 178 of Fig. 11 are omitted, the centre tap on the coil L3 being directly connected to the valve grid. In addition to its use in gramophone reproduction, the invention may be used as a microphone sound pick-up, as a detector of the movement of bodies, for the protection of safes, windows, or doors, or for detecting foreign matter in flowing liquids.