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Boosters usually raise the filament voltage from 6.3 to about 8 volts. If it is on there, someone somewhere must have thought it was too dim. Some boosters are a transformer, and this would isolate the cathode from the heater power supply, possibly fixing something. (Some boosters I think are autotransformers that wouldnt isolate anything.) If the tube doesnt have a short this is probably not why they did it. I have a couple of roundie boosters that have a switch on the top. If the switch is off, it only provides isolation and doesnt boost the voltage.
The tube is probably fine. There are plenty of other things that cause low brightness.
John
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