Correlation between induced grid noise and tube noise

The correlated and uncorrelated parts of the induced grid noise of a triode can be measured by plotting the equivalent saturated diode current of a tuned stage as a function of its input capacity after neutralizing the anode-grid capacitance. Measurement results on 6AC7, 7AF7, and 6V6 tubes are presented. The large uncorrelated part found with the 6V6, which has a relatively large grid-cathode distance, indicates that the uncorrelated part cannot be explained by a breakdown of the Llewellyn-Peterson theory at close grid-cathode spacings. The large uncorrelated part may be explained by considering the tube as made up of a large number of small independent diodes in parallel. The percent of correlation increases when the grid is made more negative since the spread in electron paths is reduced. The noise figure of a stage cannot be improved by detuning if the correlated part is small.