Lighthouse Electric Tutorials

Vacuum Tube Theory

Part 5.
Operating Classifications of Tube Amplifiers and Maximum Tube Operating Conditions

The classification of amplifiers by operation is based on the percentage of the time that the tube conducts
when an input signal is applied and is directly determined by the bias voltage of the tube.
Amlifiers are divided into four main classes:A, AB, B and C.

Class A Operation
An amplifier biased into class A operation, is one in which conduction through the tube occurs continuously
throughout the cycle of a-c voltage applied to the control grid.
The grid bias and alternating grid voltage are selected so that the entire input signal is reproduced at
the load resistor.

Class AB Operation
The class AB amplifier is one in which the tube conducts for more than half, but less than a complete cycle
of a-c grid voltage.

Class B Operation
A class B amplifier is one in which the plate current flows for only one half of each cycle of a-c grid voltage.

Class C Operation
Class C amplifiers are biased below cutoff point, so that plate current flows for less than half of each cycle
of a-c grid voltage.

Maximum Tube Operating Conditions
Certain limits must be imposed on the operating conditions of all tubes if destruction of the tube is to be
avoided and reasonable life obtained. Factors that limit vacuum tube operation are:
Plate Dissipation
Maximum Emission Current
Peak Voltage Rating
Plate dissipation, is calculated by multiplying (quiescent) d-c plate voltage by d-c plate current.
The maximum dissipation rating for a particular tube is always listed in the tube manual. Maximum emission
current and peak voltage rating are the other factors that should be kept within the recommended limits.
For rectifying tubes there is one additional factor:
Peak Inverse Voltage. This is the maximum voltage that can be applied to a tube in reverse (AC) with respect to cathode. Exceeding this will cause arcing from plate to cathode and damage the tube. --END--

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