[R-390] R-390 filament voltage

[THOMAS SMITH]

My Collins R-390 is continuing to throw me curves. I’m in the process of realignment and noticed V505 glowing brightly yesterday. I pulled the tube and checked the pins for filament voltage. Got 0 vac on pin 3 and 26 vac on pin4. I’m assuming the filament voltage check is with tube in place to complete the circuit but I’m confused about the tube glowing brightly all of a sudden. It’s what I might expect with 26 volts on it. I know V505, the two HV rectifiers, heaters etc get 26v but assumed 505 was supplied 12 through circuitry.

Any suggestions??
Thanks,
Tom N5AMA