Audio Engineering

Valve (Tube-Based) Amplifi ers 341

It is this quality that is the most useful aspect of a valve in that a quite large anode current fl ow
can be controlled by a relatively small voltage applied to the grid, and so long as the grid is not
allowed to swing positive with respect to the cathode, no current will fl ow in the grid circuit,
and its effective input impedance at low frequencies will be almost infi nite. This ability to
regulate a large current at a high voltage by a much smaller control voltage allows the valve
to amplify small electrical signals, and since the relationship between grid voltage and anode
current is relatively linear, as shown in Figure 11.3 , this amplifi cation will cause relatively
little distortion in the amplifi ed signal. The theoretical amplifi cation factor of a valve,
operating into an infi nitely high impedance anode load, is denoted by the Greek symbol μ.

Although there may be several grids between the cathode and the anode in more complex
valves, the grid closest to the cathode will have the greatest infl uence on the anode current
fl ow, which is therefore usually called the control grid.

The effectiveness of the grid in regulating the anode current depends on the relative
proximity of the grid and the anode to the cathode, in that, if the grid is close to the
cathode, but the anode is relatively remote, the effectiveness of the grid in determining
the anode current will be much greater and will therefore give a higher value of μ than
if the anode is closer to the grid and cathode. Unfortunately, there is a snag in that the

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Ia (mA)

Figure 11.3 : Triode valve characteristics.