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A better question might be why are you trying to test a 7JP4? The phosphor on these usually wears out long before the emissions go bad.
All I know is the only tester I have that supports a 7JP4 has a switch that says (Scope -10V bias). I would think that adds negative 10 volts to the grid with respect to the cathode, but have never measured it.
I would try the 440 as is before modifying it.
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