Generic Tube types

TRIODES

When a third electrode, called the control grid, or simply grid, is placed between the cathode and the plate, the tube is known as a triode. The grid usually consists of a wire mesh, spiral, or grating, the appearance of which suggests its name.

When the grid of a triode is made positive or negative with respect to the cathode, the plate current correspondingly increases or decreases. This action makes possible the use of a triode as an amplifier. The electrical impulse to be amplified is applied to the grid of the tube and thus controls electro statically the flow of electrons from the cathode to the plate. The energy required to draw the electrons to the plate comes from a high-voltage d-c supply in the plate circuit. The power required to vary the electron stream from the cathode to the plate ordinarily is only a fraction of the power Bowing in the plate circuit. Therefore, the action of the tube is that of a valve, the d-c power of the high-voltage plate supply being converted by the grid-voltage variations into a-c power in the plate load circuit. The efficiency of this energy conversion is never 100 per cent, and some power is dissipated by the plate of the tube.

Triodes are used in transmitters as oscillators, frequency multipliers, r-f power amplifiers, a-f amplifiers, modulators, and for various special purposes. Some types are especially designed for audio power-amplifier service, but most types can be used in either r-f or a-f applications.

The grid, plate, and cathode of a triode form an electrostatic system, each electrode acting as one plate of a small condenser. The capacitances are those existing between the grid and plate, plate and cathode, and grid and cathode. These capacitances, as well as those of tubes having additional electrodes, are known as interelectrode capacitances. Generally, the grid-plate capacitance is the most important. In radio-frequency amplifier circuits, this capacitance may act to produce undesired coupling between the input and output circuits and cause uncontrolled regeneration or oscillation.