Hot cathode
[1][2] The cathode is heated to a temperature that causes electrons to be 'boiled off' of its surface into the evacuated space in the tube, a process called thermionic emission.
[1] There are two types of hot cathodes:[1] The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament, allowing vacuum tubes to use alternating current to heat the filament.
It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.
To improve electron emission, cathodes are usually treated with chemicals, compounds of metals with a low work function.
The untreated thoriated tungsten filaments used in early vacuum tubes (called "bright emitters") had to be heated to 2500 °F (1400 °C), white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes (called "dull emitters") produce far more electrons at a given temperature, so they only have to be heated to 800–1100 °F (425–600 °C).
[4] For manufacturing convenience, the oxide-coated cathodes are usually coated with carbonates, which are then converted to oxides by heating.
This layer has high electrical resistance, especially under discontinuous current load, and acts as a resistor in series with the cathode.
This is particularly undesirable for tubes used in computer applications, where they can stay without conducting current for extended periods of time.
A common type of directly heated cathode, used in most high power transmitting tubes, is the thoriated tungsten filament, discovered in 1914 and made practical by Irving Langmuir in 1923.
Thoriated filaments can have very long lifetimes and are resistant to the ion bombardment that occurs at high voltages, because fresh thorium continually diffuses to the surface, renewing the layer.
Metals from groups IIIB (scandium, yttrium, and some lanthanides, often gadolinium and samarium) and IVB (hafnium, zirconium, titanium) are usually chosen.
This is why older electronics often need some time to "warm up" after being powered on; this phenomenon can still be observed in the cathode ray tubes of some modern televisions and computer monitors.
The cathode heats 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 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.
One approach used in transformerless line-operated radio and television receivers such as the All American Five is to connect all the tube heaters in series across the supply line.
The final models of tube-equipped radio receivers were built with subminiature tubes using less than 50 mA for the heaters, but these types were developed at about the same time as transistors which replaced them.
The activated electrodes can be destroyed by contact with oxygen or other chemicals (e.g. aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements.
High-reliability tubes had to be developed for the early Whirlwind computer, with filaments free of traces of silicon.
Slow degradation of the emissive layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes.
