358 Chapter 11
phase-splitter circuits, it has the disadvantage that the available AC output swing, at
either anode, is reduced greatly by the fact that the cathode voltages of V 2 and V 3 are
considerably positive in relation to the 0-V line, which will almost certainly require an
additional amplifi er stage between its output and the input of any succeeding triode or
beam-tetrode output stage.
This disadvantage is shared by the circuit layout shown in Figure 11.13 , in which a
direct-coupled triode amplifi er is operated with identical value resistive loads in both its
anode and cathode circuits. Because of the very high level of negative feedback due to
the cathode resistor, both the distortion and the unwanted phase shifts introduced by this
stage are very low. Signifi cantly, this was the type of phase splitter adopted by D. T .N.
Williamson in his classic 15-W audio amplifi er design.
11.6 Output Stages ........................................................................................................
The basic choice of output valves will lie between a triode, a beam tetrode, or a pentode.
If large output powers are required—say, in excess of 2 W—triode output valves are
unsuitable because the physical spacing between the control grid and the anode must
be small, and the grid mesh must be relatively widely spaced, in order to achieve a low
anode current resistance and a high practicable anode current level. This closely packed
0V 0V
Vcc
Output 2
Output 1
R 4
R 5
27 K
R 7
27 K
R 7
27 K
R 3
27 K
22 K
C 1
0.1μF
V 3
1/26SN7
V 2
1/26SN7
V 1
6J5GT
C 3
25 μF C 4
1 μF
R 2
C 2 470 R
25 μF
1M0
R 1 R^6
1M0
Input
Figure 11.12 : A long-tailed pair circuit.