Audio Engineering

346 Chapter 11

Both pentodes and beam tetrodes can be used with their screen grids connected to their
anodes. In this mode their characteristics will resemble a triode having similar grid–
cathode and grid–anode spacings to the grid–cathode and grid–screen grid spacings of
the pentode. The most common use of this form of connection is in power output stages,
where a triode connected beam tetrode will behave much like a power triode, without the
need for a directly heated (and hum-inducing) cathode.

11.1.6 Valve Parameters

In addition to the anode current resistance, Ra , and the amplifi cation factor, μ , mentioned
earlier, there is also the valve slope or mutual conductance ( gm ), which is a measure
of the extent to which the anode current will be changed by a change in grid voltage.
Traditionally, this would be quoted in milliamperes per volt (mA/ V or milli-Siemens,
written as mS) and would be a useful indication of the likely stage gain given by the valve
in an amplifying circuit.

This would be particularly helpful in the case of a pentode amplifying stage, where the
value of Ra would probably be very high in comparison with the likely value of load
resistance. (For example, in the case of the EF86, Ra is quoted as 2.5 M Ω and the gm
is 2 mA/ V .) In this case, the stage gain (M) can be determined, approximately, by the
relationshipM – gm · RL , which, for a 100 k anode load would be  200 ̆.

The various valve characteristics are defi ned mathematically as

RVI

gIV

aaa

mag

d d at a constant grid voltage,

d d at a const





/

/aant anode voltage, and

μdVVag/nd at a constant anode current.

In these equations the negative sign takes account of the phase inversion of the signal.
These parameters are related to one another by the further equation,

gRmaμμ/ or gRma.

11.1.7 Gettering

Preservation of a high vacuum within its envelope is essential to the life expectancy and
proper operation of the valve. However, it is diffi cult to remove all traces of residual gas
on the initial pumping out of the envelope, quite apart from the small but continuing gas