Audio Engineering

360 Chapter 11

The required grid drive voltage for typical pentode or beam tetrode output valves, at
Va  300 V, will be in the range of 20–50 Vp–p for each output valve, and whether or not
the valve is triode connected has little effect on this requirement. The triode connection
does, however, greatly affect the anode current impedance, which is reduced, in the case
of the KT88, from 12 kΩ to 670 Ω , and the need for a lower turns ratio greatly simplifi es
the design of the necessary, load-matching, output transformer with low half-primary to
half-primary and primary to secondary leakage inductances.

11.7 Output (Load-Matching) Transformer ..................................................................

This component is probably the most important factor in determining the quality of the
sound given by a valve-operated audio amplifi er, and the performance of this component
is infl uenced by a number of factors, both mechanical and electrical, which will become
of critical importance if an attempt is made to apply negative feedback (NFB) over the
whole amplifi er. However, for a low power system, such as might be used as a headphone
amplifi er, it is possible to make a quite decent sounding system without the need for

Input

0V 0V

0V

TR1

200 V

Vcc

V2b

1/26SN7

V1A

1/26SN7 V1B

1/26SN7

1/26SN7

V2A

R 6

820 R

R 3

1M0

C 6

220 μF

C 4

220 μF

C 2

0.1μF

C 3

0.15μF

R 8

220 K

R 7

220 K

R 1

47 K

R 5

47 K

R 10

27 K

R 2

R 9

270 R

220 K

R 4

220 K

RV1

1M0

0.4 7μF

C 1

C 5

Output trans

Output

0.15μF

Figure 11.14 : A simple headphone amplifi er.