Is it a series filament? Start bisecting.
Also check the tube pins, heat can cause them to expand.
Talk about not tight. I had a 30 KW electron beam power supply that kept breaking. Finally it became my responsibility and I said, I'll fix this for good. Locktite 222 to the rescue. There were a few 625K resistors that were out of tolerance, a 1 meg 200 W resistor that was out of tolerance and a LOT of loose screws, particularly on the divider. This thing was shunt regulated by a vacuum tube. to like 15 kV at 1.5 Amps. Incidentally, we ended up getting three of these things and all suffered from the same issue. One became a "parts mule" because I was "not allowed" to fix it by management. One also fell off a truck. Someone forgot that it was on casters, so it rolled off the lift gate.
Some thing with a 1940's 100 kV, 0.1A power supply used for an X-ray goniometer. Lots of loose screws. The researcher said, that the signal was the quietest that it has ever been.
I did the same thing to a precision vapor drier, but this time I had to replace and upgrade the wiring. All annoying.