Audio Engineering

Valve (Tube-Based) Amplifi ers 351

audio frequency transformer. This is a diffi cult component to incorporate within a high-
fi delity system, and much thought must be given both to its design and the way it is used
in the circuit.

A very simple circuit layout embodying the structure outlined in Figure 11.6 , using
directly heated (battery operated) valves, is shown in Figure 11.7. This is the type of
design that might have been built some 50 years ago by a technically minded youngster
who wanted some means of driving a loudspeaker from a simple piezo-electric
gramophone pick-up.

For the maximum transfer of power from an amplifi er to its load it is necessary that both
of these should have the same impedance, and since the anode resistance ( Ra ) of the
output valve is of the order of 10 k Ω , and the most common speech coil impedance of
an inexpensive moving coil loudspeaker is 3 Ω , there would be a drastic loss of available
power unless some impedance converting output transformer was employed.

Input

C 1

C 3

0.1μF

0.1μF

C 2

100 μF

C 4

0.01μF

TR1

O/P

Trans

V 2

Output to LS

Vsupply

Vbias

Vf

0V

KT2

R 2

22 K

R 3

47 K

R 4

220 K

RV1

R 1

4M7

V 1

HL2

1M0

Gain control

Figure 11.7 : A simple valve amplifi er.