Interestingly, in this tube I was not able to turn the secondary emission effect off. Grounding the third grid only succeeds in reducing the effect, not eliminating it. That means that the tube produces secondary emission even during normal operation as a pentode. The effect begins when the plate potential reaches about 5 volts and then ends when the plate potential reaches about 125 volts. Apparently, for the typical applications of the 5763 tube, secondary emission does not adversely affect the operation.
Eliminating secondary emission in the 6AU6 and 6CL6
Secondary Emission Page
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