If you connect the filament pins (2 & 7) together a 1B3GT, or similar vacuum diode makes an excellent HV capacitor capable or at least 15KV. Filament power is not applied. You can then use it as the upper cap in a high voltage capacitive AC voltage divider. The plate to heater capacitance is typically 3-4 pf. With a 100 pf capacitor in series you will have an approximately 34:1 divider. Note that the lower (larger) capacitor does NOT have to have an especially high voltage rating; a 100 pf 500 volt cap will work fine. If you use a 1000 pf lower cap for a 340:1 divider a 50 volt cap has sufficient ratings. The usual warnings apply; Don’t touch high voltage devices or associated circuitry when under test.