Valve (Tube-Based) Amplifi ers 343presence becomes of particular importance when a negatively charged control grid
is introduced into the system, in that the electron cloud will effectively fi ll the space
between the cathode and the grid and will act as the principal source of electrons.
The presence of this electron cloud—known as the space charge—has several important
operational advantages. Of these the fi rst is that, by acting as an electron reservoir, it
allows larger, brief-duration, current fl ows than would be available from the cathode on
its own and that it acts as a measure of protection to the cathode against the impacts of
positive ions created by electronic collisions with the residual gases in the envelope, as
these ions will be attracted toward the more negatively charged cathode. Finally, left to
itself, the electronic emission from the cathode suffers from both “ shot ” and “ fl icker ”
noise, a current fl uctuation that is averaged out if the anode current is drawn from the
space charge.
This random emission of electrons from a space charge-depleted cathode is used to
advantage in a “ noise diode, ” a wide-band noise source that consists of a valve in which
the cathode is deliberately operated at a low temperature to prevent a space charge from
forming so that a resultant noisy current can be drawn off by the anode.
In the case of a triode used as an output power valve, where large anode currents are
needed, the grid mesh must be coarse and the grid–cathode spacing must be close. This
limits the formation of an adequate space charge in the grid–cathode gap, and, in its
absence, the cathode must have higher emission effi ciency than would be practicable with
an indirectly heated system, which means that a directly heated fi lament must be used
instead. Usually, the fi lament voltage will be low to minimize cathode-induced “ hum ”
and the fi lament current will be high because of the size of the fi lament (2.5 A at 2.5 V in
the case of the 2A3 valve).
Directly heated cathodes are also commonly used in valve HT rectifi ers, such as the 5U4
or the 5Y3, because the higher cathode emission reduces the voltage drop across the valve
and increases the available HT output voltage by comparison with a similar power supply
using an indirectly heated cathode type.
11.1.5 Tetrodes and Pentodes
Although the triode valve has a number of advantages as an amplifi er, such as a low
noise and low distortion factor, it suffers from the snag that there will be a signifi cant