344 Chapter 11
capacitance, typically of the order of 2.5 pF, between the grid and the anode. In itself, this
latter capacitance would seem to be too small to be troublesome, but, in an amplifying
stage with a gain of, say, 100, the Miller effect will increase the capacitance by a factor
of 101, increasing the effective input capacitance to 252.5 pF, which could infl uence the
performance of the stage.
When triode valves were used as RF amplifi ers, in the early years of radio, this anode–
grid capacitance caused unwanted RF instability, and the solution adopted was the
introduction of a “ screening ” grid between the triode control grid and its anode, which
reduced this anode–grid capacitance, in the case of a screened grid or tetrode valve, to
some 0.025 pF.
A further effect that the inclusion of a screening grid had upon the valve characteristics
was to make the anode current, in its linear region, almost independent of the anode
voltage, which led to very high values for Ra and μ. Unfortunately, the presence of this
grid caused a problem that when the anode voltage fell, during dynamic conditions, to
less than that of the screening grid, electrons hitting the anode could cause secondary
electrons to be ejected from its surface, especially if the anode was hot or its surface had
been contaminated by cathode material, and these would be collected by the screening
grid, which would cause a kink in the anode current/voltage characteristics. While
this might not matter much in an RF amplifi er, it would cause an unacceptable level of
distortion if used in an audio amplifi er stage.
Two solutions were found for this problem, of which the simplest was to interpose an
additional, open mesh grid between the anode and the screening grid. This grid will
normally be connected to the cathode, either externally or within the valve envelope,
and is called the suppressor grid because it acts to suppress the emission of secondary
electrons from the anode.
Since this type of valve had fi ve electrodes it was called a “ pentode. ” A typical small-
signal pentode designed specifi cally for use in audio systems is the EF86, in which steps
have also been taken to reduce the problem of microphony when the valve is used in the
early stages of an amplifying system. The EF86 also has a wire mesh screen inside the
glass envelope and surrounding the whole of the electrode structure. This is connected
to pins 2 and 7 and is intended to lessen the infl uence of external voltage fi elds on the
electron fl ow between the valve electrodes.