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You might wire a floating external power supply to the heater in order to perform
the test. The tester may want to see some kind of load on its own filament leads,
but I guess you could achieve proper test results this way. A heater to cathode
short is normal when testing such a tube.
Of course, performing a rejuvenation would not be possible with this trick, but
is it feasible anyway? Good luck.
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