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The change in average current flow due to the presence of 2nd harmonic distortion in the output tubes operation. In the absence of a input drive voltage, the average and the idle current draw are the same in a SE amplifier. Because of the strong 2nd harmonic distortion inherent in the triode's operation in a SE amplifier, the average current will increase with a strong drive voltage swing. A quick thought experiment is to imagine an output tube with a bias voltage of -40 volts on its grid. Now if the grid were to see a voltage swing of +100 volts and -100 volts, the output waveform would be monstrously distorted. At one extreme the peak current draw would be several times that of idle and at the other, the tube would be completely turned off and conduction would be zero. In this case the average current would be probably twice the idle current. This difference between average and idle current can pose troubleshoot problems for the SE amplifier designer, as the power transformer or filter choke or output transformer that did not saturate at idle might at full output due to the rectification effect.