Hairpin threaded through parallel tubular holes in a ceramic

insulator. Tubes having cathodes of this type are referred to as heater type tubes.

The heaters may be operated on either direct or alternating current. The one disadvantage of using alternating current

FILAMENT

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FILAMENT

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A.C. SOURCE

Fig. 9.—Diagrani of connection to a triode employing a filament transformer.

for the filament of tubes used in audio-frequency circuits, is that it introduces objectionable hum in the output.

This hum may be lessened by connecting the plate and the grid circuits to the midpoint of the secondary of the trans former, as shown in fig. 9. Generally, however, it is not possible to use alternating current in the filament of tubes used in the early stages of high-gain amplifiers.

Classification of Tubes.—Tubes are usually classified according to the number of electrodes present, so for example a two-element tube is called a diode; a three-element tube a triode, and so on to tetrodes and pentodes. A pentode therefore is a tube having five elements. See pages 228 and 229.

Tubes may also be classified according to whether there be high vacuum, gas or an element which vaporizes in the bulb.

Diodes.—From the foregoing it is evident that electrons are of no value in a tube unless they can be controlled or made to work according to a pre-determined schedule. The very simplest form of tube consists of two electrodes—a cathode and a plate, and is most often referred to as a diode, which is the family name for two-electrode tubes.

In common with all tubes, the electrodes are enclosed in an evacuated envelope with the necessary connection projecting out through airtight seals. The air is removed from the envelope to allow free movement of the electrons and to prevent injury to the emitting surface of the cathode. If the cathode be heated, electrons leave the cathode surface and form an invisible cloud in the space around it. Any positive electric potential within the evacuated envelope will offer a strong attraction to the electrons.

In a diode, the positive potential is applied to the second electrode, known as the anode, or plate. The potential is supplied by a suitable electrical source connected between the plate terminal and a cathode terminal. See fig. 10. Under the influence of the positive plate potential, electrons flow from the cathode to the plate and return through the external plate-

battery circuit to the cathode, thus completing the circuit. This flow of electrons is known as the plate current and may be measured by a sensitive current indicator.

BATTERIES

Fig. 10.—Connection diagram for a two electrode tube.

BATTERIES

Fig. 10.—Connection diagram for a two electrode tube.

The Diode as a Rectifier.—It is obvious that under no conditions can the current flow from the plate to the cathode, i.e., the tube is as far as the direction of the current is concerned a one-way proposition. Increasing the positive potential will of course increase the flow of electrons from cathode to plate and consequently increase the current flow in the plate circuit, but if the plate is made negative instead of positive it will repel the electrons and no current will flow.

TRANSFORMER

TRANSFORMER

The diode therefore acts as an electrical valve that will permit current to flow in one direction, but not in the other. It is

this characteristic of the diode that has been utilized as a means of converting or rectifying an alternating current into a direct

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