A brief summary of the characteristics of valve cathodes in present-day general use.
Most mains operated radio valves are indirectly heated because it is most convenient to use an AC heater supply and because the use of a directly heated valve with this type of supply will probably lead to the introduction of hum. Directly heated valves are normally used for battery operation because the heater power required by a small directly heated valve is less than that required by a similar indirectly heated valve.
All small radio valves have 'oxide coated' cathodes, but there are two other types of valve cathode which are suitable for certain purposes, namely pure tungsten and thoriated tungsten.
Electrons are emitted from pure tungsten only with some difficulty and a. very high temperature (about 2,300°C) is required in order to obtain a reasonable anode current. Tungsten cathodes are therefore used only in directly heated valves, as it would not be practical to raise the whole of an indirectly heated cathode to such a high temperature. The heater power required is comparatively high and the valve gives out as much light as a small incandescent lamp.
Tungsten filaments are the only type which are suitable for use in valves operating at very high anode voltages (over a few kilovolts), all other types of cathode are destroyed if a high anode voltage is used. Tungsten filament valves also find an application in noise diodes which generate a very small amount of radio noise this noise can be used to find the noise factor of a receiver. Tungsten filaments can also be used in valves for experiments on saturation anode currents.
The addition of 1 to 2% of Thorium to the tungsten used for making valve filaments enables the filament to emit electrons more easily. See also Use of Thoria. Temperatures of about 1,600°C are suitable, but thoriated tungsten filaments are not satisfactory at very high anode voltages. The performance of thoriated tungsten is somewhere between that of pure tungsten and that of the oxide coated cathode.
Oxide Coated Cathodes
Oxide coated cathodes are the type which are most widely used because electrons are emitted from them very easily. The operating temperature is a red heat (about 780°C) but, if the anode current is small, satisfactory operation can be obtained at slightly lower temperatures.
Either directly heated tungsten filaments or indirectly heated nickel cathodes can be coated with a mixture of barium and strontium oxides. The electrons are believed to be emitted by a single layer of barium and strontium atoms on the surface of the cathode, but this layer can be easily damaged by impurities or by an incorrect operating temperature.
Oxide coated cathodes should not be used at anode voltages appreciably above one kilovolt or the cathode will probably be damaged. Oxide coated cathodes generate a type of noise known as 'fluctuation noise' and are not therefore suitable as noise diodes.
See also The Cathode.