Audio Engineering

142 Chapter 5

into operation and start drawing current. Using an indirectly heated rectifi er would avoid
this voltage surge and would allow lower working voltage components to be used with
safety in the rest of the amplifi er. This would save cost. However, the directly heated
rectifi er would have a more effi cient cathode system and would have a longer working
life expectancy.

Although there are several other reasons for this, such as the greater ease of manufacture,
by the use of modern techniques, of large value electrolytic capacitors, or the
contemporary requirement that there shall be no audible mains hum in the amplifi er
output signal due to supply line AC ripple, it is apparent that the capacitance values
used in the smoothing, decoupling, and reservoir capacitors in traditional valve amplifi er
circuits are much smaller than in contemporary systems, which operate at a lower output

(g)

C1

C2

D1

D2 C3

0V

V out

Mains input

TRI

(h)

C1 C2

D1

0V

V out

TR1

Mains input

Figure 5.1: (Continued).