The circuits that we have discussed up to this point were chosen to show the general concepts
discovered by Edison and Fleming. They are not practical because they do no useful work. For now, only
the concepts are important. Practical circuitry will be presented later in this chapter as you learn specific
points about the construction, limitations, and other characteristics of modern diode tubes.
An ac voltage is applied across a diode. The tube will conduct when what alternation of ac is
applied to the plate?
What would be the output of the circuit described in question 5?
Diode tubes in present use are descendants of Flemings valve. There is a family resemblance, but
many changes have been made from the original. Diodes are both smaller and larger, less powerful and
more powerful, and above all, more efficient and more reliable. The search for greater efficiency and
reliability has resulted in many physical changes, a few of which will be covered in the next paragraphs.
Most of what is said here about construction and materials will be true of all electron tubes, not just
Modern filaments in ALL tubes last longer, emit greater amounts of electrons for a given size, and
many operate at a lower temperature than in the early days. Most improvements have resulted from the
use of new materials and from better quality control during manufacture.
Three materials that are commonly used as filaments are tungsten, thoriated tungsten, and
Tungsten has great durability but requires large amounts of power for efficient thermionic emission.
Thoriated-tungsten filaments are made of tungsten with a very thin coat of thorium, which makes a much
better emitter of electrons than just tungsten. Oxide-coated filaments are made of metal, such as nickel,
coated with a mixture of barium and strontium oxides. The oxide coat, in turn, is coated with a one-
molecule-thick layer of metal barium and strontium. Oxide coating produces great emission efficiency
and long life at relatively low heat.
A major advance in electronics was the elimination of batteries as power sources for tubes. Except in
electronic devices designed to be operated away from the ac power source, alternating current is used to
Voltage may be supplied by a separate filament transformer or it may be taken from a filament
winding that is part of a power transformer. The actual voltage may vary from 1 volt up and depends on
the design of the tube. Common filament voltages are 5.0, 6.3, and 12.6 volts ac. Filaments may be
connected in series with other tube filaments or may be in parallel with each other. This is determined by
the equipment designer.
As was mentioned previously, a more formal name for the electron-emitting element in a tube is the
Cathodes in all tubes, not just diodes, are of two general types, either directly heated or indirectly
heated. Each has its advantages and disadvantages.