GB600496A - Improvements relating to thermionic valve oscillators - Google Patents

Abstract

600,496. Automatic control systems. FERRANTI, Ltd., and WOOD, H. Sept. 25, 1945, No. 24794. [Class 38 (iv)] [Also in Group XL (c)] The loading of a valve oscillator is maintained constant by adjusting means under the control of a voltage derived from the difference between voltages developed across separate resistors in the anode-cathode and grid-cathode circuits, respectively, of the oscillator. In an application to a dielectric heater, the material 7 is placed between the plates of capacitor 3 in the tuned circuit of a push-pull oscillator 1, 2, and the separation of the plates is controlled automatically by the split-phase motor 17 with oppositely connected field windings 16, 21. The potential at tapping 15<SP>1</SP> relative to earth controls the current delivered to winding 21 by valves 23, 24 and equals the difference between voltage across resistor 18 due to anode current flow and the voltage across part of potentiometer 15 and field limiting resistor 20 due to substantially constant grid current flow. Tapping 15<SP>1</SP> is adjusted initially to balance the currents in the two windings 16, 21 for the loading conditions determined by the setting of potentiometer 14. A change in loading alters the anode current through resistor 18 and the fields of windings 16, 21 become unbalanced, operating motor 36 to restore the original conditions. Spring-loaded multiple switch S1-ABC enables the plates of capacitor 3 to be adjusted by the motor when the oscillator is de-energized, and switch S3 allows the oscillator to function without control. The common grid circuit may include a barretter and the armature may be braked by eddy currents, viscosity or from a coupled D.C. generator supplying negative feedback to the grids of valves 23, 24. Alternatively, the motor may control a variable capacitor in series with capacitor 3, an inductive component in the tuned circuit or the H.T. voltage for the oscillator.