Vacuum-Tube Amplification Techniques: A Practical Guide to Load Lines, Gain and Biasing
A valve amplifier stage is not defined only by its tube type. Its actual behaviour depends on the selected supply voltage, load resistance, bias point, screen supply, and the way the cathode is treated for AC signals. This article explains a practical procedure for analysing and designing the most common voltage-amplifier stages: the triode RC amplifier, the pentode RC amplifier, cathode-degenerated stages, and cathode followers. The approach follows Keats A. Pullen’s G-Curve design method, in which tube parameters are read at several points around the operating condition rather than treated as fixed catalogue values. The Design Workflow Before calculating component values, define what the stage must do: Required input and output voltages. Required voltage gain. Expected signal swing. Input and output impedance requirements. Acceptable distortion. Frequency range. Maximum permissible plate and screen dissipation. The basic design sequence is: Select a suitable trial tube. Choose a...