Audio Engineering

Valve (Tube-Based) Amplifi ers

John Linsley Hood

Although the bulk of modern electronic circuitry is based on “ solid-state ” components,
for very good engineering reasons—one could not, for example, build a compact disc
player using valves and still have room in one’s house to sit down and listen to it—all
the early audio amplifi ers were based on valves, and it is useful to know how these
worked and what the design problems and circuit options were in order to get a better
understanding of the technology. Also, there is still interest on the part of some “ hi-fi ”
enthusiasts in the construction and use of valve-operated audio amplifi ers, and additional
information on valve based circuitry may be welcomed by them.

11.1 Valves or Vacuum Tubes .......................................................................................

The term thermionic valve (or valve for short) was given, by its inventor, Sir Ambrose
Fleming, to the earliest of these devices, a rectifying diode. Fleming chose the name
because of the similarity of its action in allowing only a one-way fl ow of current to that
of a one-way air valve on an infl atable tire, and the way it operated was by controlling
the internal fl ow of thermally generated electrons, which he called “ thermions, ” hence the
term thermionic valve. In the United States they are called “ vacuum tubes. ” These devices
consist of a heated cathode, mounted, in vacuum, inside a sealed glass or metal tube.
Other electrodes, such as anodes or grids, are then arranged around the cathode so that
various different functions can be performed.

The descriptive names given to the various types of valve are based on the number of its
internal electrodes so that a valve with two electrodes (a cathode and an anode) is called
a “ diode, ” one with three electrodes (a cathode, a grid, and an anode) is called a “ triode, ”
one with four (a cathode, two grids, and an anode) is called a “ tetrode, ” and so on.

CHAPTER 11