Patent GB-696431-A (GB)
Title: Improvements in protective systems for electron valves
IPC: H03F1/54
Filed: 1950-08-31
Abstract: 696,431. Valve circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 31, 1950 [March 9, 1950], No. 21494/50. Class 40 (vi). A valve is protected by connecting across its anode supply source a normally non-conductive short-circuiting device which is rendered conductive by a fault-indicating voltage derived by combining voltages proportional to current through the valve and the A.C. output of the valve. An oscillator or amplifier valve 10 derives its H.T. supply from a power unit comprising a rectifier 28 and a transformer 29 and feeds part of its output through a coupling coil 54 to a rectifier to develop at the adjustable tap 60a on the rectifier load a positive D.C. voltage dependent on the A.C. output. This is neutralized by the negative voltage developed at the tap 62 of a resistor connected in series with the D.C. supply to the valve. The combined voltage is fed to the grid of a D.C. amplifier valve 68 (described below) arranged normally to feed a negative voltage to the grid of a gas-filled valve 70. On the occurrence of a fault due, for example, to arcing in the valve, the negative voltage due to the H.T. current rises and the positive voltage due to the A.C. output decreases so that a negative voltage is applied to the grid of valve 68. This causes the grid of valve 70 to become positive and thus the valve fires. This produces a positive voltage at its cathode which fires a further gas-filled valve 72 connected to the igniter electrode of a trigatron 33 which accordingly also fires, and as the trigatron is connected across the signal valve 20 it instantaneously diverts the anode current to itself, as described in Specification 696,430. The power unit is protected by a circuit-breaker 30 operated by a coil 32 coupled to the main A.C. supply and also operated by the voltage developed across resistor 100 and supplied through a resistor 116. In addition the voltage across resistor 100 fires a further gas-filled valve 115, the cathode voltage of which renders a D.C. amplifier valve 112 conducting, causing the grid bias on the rectifier 29 to change from a positive to a negative value and so to extinguish the rectifier. The auxiliary anode 38 of the ignitron and some of the gas-filled valves are supplied from a separate power unit comprising a half-wave rectifier 120 which provides an output with a high ripple content so that when the fault is cleared and the grid voltages return to normal the gas-filled valves supplied and the ignitron will extinguish. A switch 119 is provided to extinguish valve 115 and allow the rectifier 28 to fire again. A fault indicator 134 is coupled to the cathode lead of the ignitron and a further fault indicator 129 for testing the auxiliary circuits when the main H.T. is switched off is coupled to the auxiliary anode 38 and is operated when the test switch 84 is operated. The operation of the circuit is delayed by the smoothing capacitor 61 of the signal rectifier but in Fig. 2 the signal rectifier 152 coupled to the signal amplifier 10 does not comprise such a capacitor and its output which accordingly is of pulse form is applied to the second control grid of a gas-filled valve 158. The first grid of this valve is fed with the voltage pulses developed across the cathode resistor 150 of the signal amplifier. The two pulses act in opposition but on the occurrence of the fault valve 158 fires supplying a positive voltage through a rectifier 184 to operate the igniter control valve 172 as before. The rectifier 184 is provided to combine without interaction fault potentials from several similar circuits. The D.C. amplifier valves 68 and 112 have their H.T. supply sources effectively " floating." Valve 68 is normally conducting and the anode potentiometer is normally set slightly below the point on the potentiometer resistor at which the potential is that of earth so that the output voltage at the potentiometer tap is negative. When the current through the valve decreases the absolute output voltage becomes positive. Valve 112 is similarly arranged but is normally non-conductive. When rendered conductive it provides an output voltage which extinguishes rectifier 28.