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The input capacitance of amplifier tubes decreases with increasing grid bias, thereby detuning the input circuits of intermediate-frequency stages in a superheterodyne receiver. The phenomenon is caused by a shift in space charge and also by feedback through the grid-plate capacitance. The change in input capacitance is of the order of 1.5 micromicrofarads for each of the two causes. A simple form of feed-back coupling compensates for this change of capacitance. The grid-cathode capacitance and an unbypassed resistor in the cathode lead provide the feed-back coupling.