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OK, lets think this problem through, back up and test.
Take ten watt 10 Ohm resistor and connect it to the filament leads of the tester.
(This closely simulates the filament load of a typical vintage B&W tube, say a 10BP4.)
Turn your filament rheostat all the way down and turn the tester on.
See what the meter on the tester reads and at the same time read the voltage across the resistor with a normal meter and see if both meters have the same voltage adjustment range.
This test should give you enough info to determine where to start look for the problem.
James.
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