Audio Engineering

342 Chapter 11

anode current resistance of the valve, Ra , is also related to the anode/cathode spacing and
becomes higher as the anode/cathode spacing is increased. The closeness of the pitch of
the wire spiral that forms the grid also affects the anode current resistance in that a close
spacing will lead to a highRa , and vice versa.

The stage gain (M) of a simple valve amplifi er, of the kind shown in Figure 11.4 , is given
by the equation

M

R

RR





μ

a

so that a low impedance valve, such as a 6SN7 (typical Ia  9 mA, Ra  7.7 K, μ  20),
which has close anode–grid and grid–cathode spacings, and a relatively open pitch in
the grid wire spiral, will have a high possible anode current but a low amplifi cation
factor, while a high impedance valve such as a 6SL7 (typical Ia  2.3 mA, Ra  4 4 K ,
μ  70) will have a low stage gain unless the circuit used has a high value of anode load
resistance ( R), which, in turn, will demand a high value of HT voltage.

11.1.4 The Space Charge

Although a cloud of electrons will surround any heated cathode mounted in a vacuum
and will act as a reservoir of electrons when these are drawn off as anode current, their

V

R

0V

Eout

Ein

Figure 11.4 : A simple valve amplifi er.