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LF Vacuum-Tube-Amplifier Parasitic Oscillations

February 23rd Fact-of-the-Day

An unwanted oscillation below the normal operating frequency of an RF amplifier is referred to as being a low-frequency parasitic oscillation. Parasitic oscillations at relatively low frequencies are nearly always caused by radio-frequency choke resonances. DC grid and plate currents normally are supplied to a vacuum tube amplifier via radio frequency chokes that prevent significant amounts of RF current from flowing into power supply circuits. The tuning inductors in RF amplifier input and output tank circuits usually have such low impedances at low frequencies that they can be considered essentially as short circuits that connect the grid and plate to their respective RF chokes. Those chokes typically resonate with their own distributed capacitances and various circuit capacitances at relatively low frequencies. If grid and plate RF chokes resonate near the same frequency, the result can be a tuned-grid, tuned plate, low-frequency oscillator. ©2005 Martek International All rights reserved.

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