Audio Engineering

Valve (Tube-Based) Amplifi ers 359

type of construction will lead to the almost complete stripping of the space charge from
the region between the cathode and the grid. Experience shows that the life expectancy of
cathodes operated under such conditions is short, and the only way by which this problem
can be avoided is by the use of a directly heated (fi lament type) cathode construction,
which is much more prolifi c as a source of electrons, and this leads to other diffi culties
such as hum intrusion from the AC heater supplies, and the awkwardness of arranging
cathode bias systems.

So, if it is required to use a triode output stage, at anything greater than the 50-mA anode
current obtainable from a parallel connected 6N7 double triode (the 6SN7 has a smaller
envelope and, in consequence, a lower permissible anode dissipation), a directly heated
valve such as the now long obsolete 6B4 or PX25 would need to be found. Therefore, in
practice, the choice for output valves will be between output beam tetrodes or pentodes.
Although a fairly close simulation of a triode characteristic can be obtained in both of
these valve types if the anode and G 2 are connected together, this approach works better
with a beam tetrode than a power output pentode because the presence of the suppressor
grid in the pentode somewhat disturbs the anode current fl ow.

0V 0V

Vcc

Output 1

Input

Output 2

CAP.NP

R 6

27 K

R 5

R 3 27 K

27 K

V 1

1/26SN7 V 2

1/26SN7

R 2

470 R

R 1

1M0

C 3

25 μF

C 2

25 μF

C 1

R 4

22 K

C 6

C 7

0.1μF

0.05μF

Figure 11.13 : A split load phase splitter.