This is important, much lower values would have been measured due to voltage drop if the voltmeter input impedance is say 10MΩ. The potential difference was measured between cathode and grid or anode when the electron tube is indoors or outdoors. The digital multimeter used is the YFE YU FUNG YF-3200, made in Taiwan.
On the other hand for measuring the high internal negative bias of the grid when heater voltage is applied (due to electron cloud) no special voltmeter should be needed. The electron tube becomes a low impedance voltage source between grid and cathode and even a needle voltmeter should show the potential difference.
The Pleiades bias counteracts on this voltage to make the electron tube operate at even 3V anode potential. For example the Pleiades V6 amplifier.
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