How to know if there is no grid current in an electron tube

At your own risk. If listening tests are used, all nessesarily ear protection should exist in the signal path for example a limiter.


When there is no grid current if the grid leak or return resistor is disconnected there would be no change in anode current [Victoreen].


Would very careful and ear protective listening tests be to ultimate test? Absence of click. (All nessesarily ear protecting precautions should be followed).


Reference:


Electrometer Tubes for the measurement of small currents - Victoreen - Proc. IRE