Audio Engineering

Valve (Tube-Based) Amplifi ers 355

11.5 Phase Splitters .......................................................................................................

In order to drive a pair of output valves in push–pull it is necessary to generate a pair of
AC control grid drive voltages that are equal in magnitude but in phase opposition. The
simplest way of doing this is to use a transformer as the anode load for an amplifi er stage,
but with a center-tapped secondary winding.

Figure 11.9 shows a typical center-tapped, transformer-coupled 20W audio amplifi er,
of the kind that would have been common in the period spanning the late 1930s to
early 1940s. Because there are two coupling transformers in the signal path from the
input and the LS output, which would cause substantial phase shifts at the ends of the
audio spectrum, it would be impractical to try to clean up the amplifi er’s relatively poor
performance by applying overall negative feedback (from the LS output to V 1 cathode) to
the system.

Input

0V 0V

C 1

0.1μF

V 1

L63

C 2

25 μF

C 3

25 μF

R 2

1K0

R 3

22 K

100 μF

120 R

R 1

1M0

100 R

100 K

R 6

R 5

R 4 C 4

C 7

C 8

C 9

50 μF

C 5

C 6

R 8

R 7

100 R

100 K

0.05μF

0.05μF

0.05μF

0.05μF

KT63

V 2

KT 63

V 3

LS output

3R

TR2

O/P trans

LS 0 V

300 V.

100 mA

TR1

1:5

Figure 11.9 : A simple 20-W amplifi er.