Hot-Cathode Gas Tube

an ion device with two electrodes that is used as a valve with uncontrolled electric discharge.

Hot-cathode gas tubes are used mainly in the high-voltage AC rectifiers of radio transmitters. The anode of the tube is made of nickel, steel, or graphite. The cathode is made of an oxide and has a direct or indirect heater. The electrodes are surrounded by a medium of inert gas or a gas mixture at a pressure of 0.1-0.25 mm Hg (1 mm Hg = 133.322 newtons per sq m), or mercury vapor at a pressure of 0.001-0.01 mm Hg. The cathode is usually placed in a metal (heat) shield to improve the thermal regime during operation. The rectifying action of a hot-cathode gas tube is caused by the fact that during the positive half-period of the cycle at the anode, when its voltage exceeds the striking voltage of the tube, a nonindependent arc discharge occurs, maintained by a small ignition voltage (10-30 volts); during the negative half-period, the anode is exposed to maximally rectified voltage and the flow of current through the tube virtually ceases. The ignition voltage only slightly depends upon the amperage, which ranges from 0.01 to 0.5 amperes for low-powered tubes and from 15 to 150 amperes for high-powered tubes. Because of the insignificant voltage drop (ignition voltage) during arc discharge, rectifiers using hot-cathode gas tubes are highly efficient (95-99 percent). The permissible temperature range of the surrounding medium during operation of hot-cathode gas tube rectifiers filled with mercury vapor is 15°-50° C; the range for tubes filled with gas is 60°-100° C. Hot-cathode gas tubes are differentiated according to the type of gas (or gas mixture) or metal vapor used to fill them (argon, helium, mercury vapor, and so on), the construction of the anode (open, half-closed, closed); the amplitude of the voltage being rectified (low-voltage, or Tungars, with an anode voltage up to 300 volts; normal, up to 15 kilovolts; and high-voltage, up to 70 kilovolts).