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The negative supply makes this seem confusing but with the voltage's shown on the schematic the grid is basically 1 volt more negative than the cathode, which seems reasonable to bias that tube on. Another way to look at it is that the cathode resistor is dropping 1 volt, which makes the cathode 1 volt positive relative to the grid being pulled to -85V . Very similar to a cathode biased class A audio stage.
As Tim mentioned maybe we are chasing a red herring here. Possibly when the video signal is present it biases the grid more negative which would make the tube conduct less, bringing the plate to the positive voltage shown on the schematic.
Last edited by Kevin Kuehn; 08-13-2022 at 09:08 PM.
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