[Test-Equipment] tube input impedance

[Arthur Shulman]

I suspect the grid will go negative to cutoff, due to electron bombardment
from the cathode, and no path (or too high a resistance path) to ground.
If a substitute resistance value is attempted, it would be a good idea to
monitor the plate (anode) voltage (if resistance coupled) or, preferably,
plate current, to determine the effect of increasing the grid to cathode
resistance.
Those grid return resistors are commonly called grid leak resistors, for
cause!

Arthur Shulman