Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 311

12.1 Tubes

In 1883, Edison discovered that electrons flowed in an
evacuated lamp bulb from a heated filament to a sepa-
rate electrode (the Edison effect). Fleming, making use
of this principle, invented the Fleming valve in 1905,
but when DeForest, in 1907, inserted the grid, he
opened the door to electronic amplification with the
audion. The millions of vacuum tubes are an outgrowth
of the principles set forth by these men.^1

It was thought that, with the invention of the tran-
sistor and integrated circuits, the tube would disappear
from audio circuits. This has hardly been the case.
Recently tubes have had a revival because some
“golden ears” like the smoothness and nature of the tube
sound. The 1946 vintage 12AX7 is not dead and is still
used today as are miniature tubes in condenser micro-
phones and 6L6s in power amplifiers. It is interesting
that many feel that a 50 W tube amplifier sounds better
than a 250 W solid-state amplifier. For this reason, like
the phonograph, tubes are still discussed in this hand-
book.

12.1.1 Tube Elements

Vacuum tubes consist of various elements or elec-
trodes, Table 12-1. The symbols for these elements are
shown in Fig. 12-1.

12.1.2 Tube Types

There are many types of tubes, each used for a partic-

ular purpose. All tubes require a type of heater to permit

the electrons to flow. Table 12-2 defines the various

types of tubes.

12.1.3 Symbols and Base Diagrams

Table 12-3 gives the basic symbols used for tube

circuits. The basing diagrams for various types of

vacuum tubes are shown in Fig. 12-2.

Table 12-1. Vacuum Tube Elements and Their
Designation

Filament The cathode in a directly heated tube that heats and

emits electrons. A filament can also be a separate

coiled element used to heat the cathode in an indi-

rectly heated tube.

Cathode The sleeve surrounding the heater that emits elec-

trons. The surface of the cathode is coated with

barium oxide or thoriated tungsten to increase the

emission of electrons.

Plate The positive element in a tube and the element

from which the output signal is usually taken. It is

also called an anode.

Control grid The spiral wire element placed between the plate

and cathode to which the input signal is generally

applied. This element controls the flow of electrons

or current between the cathode and the plate.

Screen grid The element in a tetrode (four element) or pentode

(five element) vacuum tube that is situated between

the control grid and the plate. The screen grid is

maintained at a positive potential to reduce the

capacitance existing between the plate and the con-

trol grid. It acts as an electrostatic shield and pre-

vents self-oscillation and feedback within the tube.

Suppressor

grid

The gridlike element situated between the plate

and screen in a tube to prevent secondary electrons

emitted by the plate from striking the screen grid.

The suppressor is generally connected to the

ground or to the cathode circuit.

Figure 12-1. Tube elements and their designation.

Table 12-2. The Eight Types of Vacuum Tubes

Diode A two-element tube consisting of a plate and a cath-

ode. Diodes are used for rectifying or controlling the

polarity of a signal as current can flow in one direction

only.

Triode A three-element tube consisting of a cathode, a control

grid, and a plate. This is the simplest type of tube used

to amplify a signal.

Tetrode A four-element tube containing a cathode, a control

grid, a screen grid, and a plate. It is frequently referred

to as a screen-grid tube.

Pentode A five-element tube containing a cathode, a control

grid, a screen grid, a suppressor grid, and a plate.

Hexode A six-element tube consisting of a cathode, a control

grid, a suppressor grid, a screen grid, an injector grid,

and a plate.

Heptode A seven-element tube consisting of a cathode, a con-

trol grid, four other grids, and a plate.

Pentagrid A seven-element tube consisting of a cathode, five

grids, and a plate.

Beam-

power

tube

A power-output tube having the advantage of both the

tetrode and pentode tubes. Beam-power tubes are

capable of handling relatively high levels of output

power for application in the output stage of an audio

amplifier. The power-handling capabilities stem from

the concentration of the plate-current electrons into

beams of moving electrons. In the conventional tube

the electrons flow from the cathode to the plate, but

they are not confined to a beam. In a beam-power tube

the internal elements consist of a cathode, a control

grid, a screen grid, and two beam-forming elements

that are tied internally to the cathode element. The

cathode is indirectly heated as in the conventional

tube.

Filament Cathode Grid

Plate

Beam

forming

plates

Eye-tube

deflection

plate

Photo

cathode

Cold

cathode

Gas

filled