338 Chapter 11
It helps to understand the way in which valves work, and how to get the best performance
from them, if one understands the functions of these internal electrodes and the way in
which different groupings of them affect the characteristics of the valve, so, to this end, I
have listed them and examined their functions separately.
11.1.1 The Cathode
This component is at the heart of any valve and is the source of the electrons with which
it operates. It is made in one of two forms: either a short length of resistor wire, made of
nickel, folded into a ‘ V ’ shape and supported between a pair of stiff wires at its base and a
light tension spring at its top, as shown in Figure 11.1(a) , or a metallic tube, usually made
of nickel, with a bundle of nickel or tungsten heater wires gathered inside it, as shown in
Figure 11.1(b). Whether the cathode is a directly heated “ fi lament ” or an indirectly heated
metal cylinder, its function and method of operation are the same, although, other things
being equal, the directly heated fi lament is much more effi cient in terms of the available
electron emission from the cathode in relation to the amount of power required to heat
it to its required operating temperature (about 775°C for one having an oxide-coated
construction).
Figure 11.1 : Valve cathode styles.(a) Directly heated(b) Indirectly heatedCathode
connection