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CRT is likely gassy...the gas conducts the heat away from the heater, so it remains fairly cool (neck of CRT gets warm quickly)... resistance of the cool heater remains quite low, causing the tube tester power supply to current limit at a low voltage.
If you applied 6.3 volts to the heater from a supply that doesn’t current limit, it likely would blow the heater quickly.
jr
Last edited by jr_tech; 02-16-2022 at 03:24 PM.
Reason: correct spelling
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