The simplest type of vacuum tube operates as a diode: that is, it allows current to flow in only one direction. The cathode heater is used to raise the temperature of the cathode filament, permitting thermionic emission of electrons into the evacuated tube. The other element inside the tube is called the "plate", or anode. If the anode is positively charged relative to the cathode, the emitted electrons will be attracted to it, and current will flow. This exhibits the characteristics of a diode as current flow in the reverse direction is not possible (the anode is not heated, prevention thermionic emission.) More complex vacuum tubes operated as triodes (the predecessor to the modern transistor) or other circuit elements, but all tubes required some type of cathode heater in order to trigger electron emissions.
The purpose of the cathode heater is to heat the cathode to a temperature that causes electrons to be 'boiled out' of its surface into the evacuated space in the tube, a process called thermionic emission. The temperature required for modern cathodes is around 800-1000°C (1500-1800°F)
Construction
The cathode is usually in the form of a long narrow sheet metal cylinder at the center of the tube. The heater consists of a fine wire or ribbon, made of a high resistance metal alloy like nichrome, similar to the heating element in a toaster but finer. It runs through the center of the cathode, often being coiled on tiny insulating supports or bent into hairpin-like shapes to give enough surface area to produce the required heat. The ends of the wire are electrically connected to two pins protruding from the end of the tube. When current passes through the wire it becomes red hot, and the radiated heat strikes the inside surface of the cathode, heating it. The red or orange glow seen coming from operating vacuum tubes is produced by the heater.
There is not much room in the cathode, and the cathode is often built with the heater wire touching it. The inside of the cathode is insulated by a coating of alumina (aluminum oxide). This is not a very good insulator at high temperatures, therefore tubes have a rating for maximum voltage between cathode and heater, usually only 200 - 300 V.
Heaters require a low voltage, high current source of power. The voltage required was usually 5 or 6 volts AC. In older electronic devices this was supplied by a separate 'heater winding' on the device's power supply transformer that also supplied the higher voltages required by the tubes' plates and other electrodes.
power distribution transformers
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