Valve (Tube-Based) Amplifi ers 339It is possible to use a plain tungsten fi lament as a cathode, but it needs to be heated to
some 2500°C to be usable, which requires quite a substantial amount of power and leads
to other problems, such as fragility. Virtually all contemporary low to medium power
valves use oxide-coated cathodes, which are made from a mixture of the oxides of
calcium, barium, and strontium deposited on a nickel substrate.
In the manufacture of the valve, these chemicals are applied to the cathode as a paste
composed of a binding agent, the metals in the form of their carbonates, and some small
quantities of doping agents, typically of rare-earth origin. The metal carbonates are then
reduced to their oxides by subsequent heating during the last stage in the process of
evacuating the air from the valve envelope.
In use, a chemical reaction occurs between the oxide coating and the heated nickel
cathode tube (or the directly heated fi lament), which causes the alkali metal oxides to be
locally reduced to the free metal, which then slowly diffuses out to the cathode surface
to form the electron-emitting layer. The extent of electronic emission from the cathode
depends critically upon its temperature, and the value chosen for this in practice is a
compromise between performance and life expectancy, as higher cathode temperatures
lead to shorter cathode life due to the loss through evaporation of the active cathode
metals, whereas a lower limit to the working temperature is set by the need to have an
adequate level of electron emission.
When hot, the cathode will emit electrons, which form a cloud around it, a situation in which
the thermal agitation of the electrons in the cathode body, which causes electrons to escape
from its surface, is balanced by the growing positive charge that the cathode has acquired as
the result of the loss of these electrons. This electron cloud is called the “ space charge ” and
plays an important part in the operation of the valve; a matter that is discussed later.
11.1.2 The Anode
In the simplest form of valve, the diode, the cathode is surrounded by a metal tube or box,
called the anode or plate. This is usually made of nickel and will attract electrons from
the space charge if it is made positive with regard to the cathode. The amount of current
that will fl ow depends on the closeness of the anode box to the cathode, the effective area
of the cathode, the voltage on the anode, and the cathode temperature. For a fi xed cathode
temperature and anode voltage, the ratio of anode voltage to current fl ow determines the