GB874939A - Improvements in or relating to apparatus employing electronic valve devices - Google Patents

Abstract

874,939. Valve supply circuits. COMMUNICATIONS PATENTS Ltd. June 3, 1960, No. 19705/60. Class 40(6) The filament or heater current of a valve is limited to a safe valve during the warming-up period, when the filament or heater resistance is low, by means of a resistor 21 with adjustable tappings in series with the primary of the transformer 10 feeding the filament or heater. When the filament or heater becomes operative its resistance increases and the voltage across it and thus the voltage across the transformer primary rises to such a value as to energize the relay 17, contacts 19 of which close to short-circuit the limiting resistor 21. A further pair of contacts 20 prevent the application of H.T. to the valve before the filament or heater is fully operative. The working current of the filament or heater may be set by means of an adjustable resistor 25. In an alternative arrangement (Fig. 2, not shown) the relay is provided with two windings, the one in series with the transformer primary and opposing the second which is connected, in series with a resistor across the supply. In this arrangement the current in the second winding tends to close the relay, the contacts of which are held open initially by the heavy starting filament or heater current passing through the first winding. D.C. relays may alternatively be used, connected in series with dry-contact rectifiers.