I was on a camping trip with super limitted, read only, phone service. Lol
Where are your probes connected? If you havent already, You can pull the rectifier tube while you figure out the bias voltage.
amp off/unplugged caps drained, Black probe to chassis, red probe to powertube control grid, reading ohms, what do you have at full clockwise and full ccw?
I must have been brain farting as your voltage will not vary at the top of the divider. Youll have to remove the jumper, you can replace it with a resistor say 100k to achieve the initial thought. Appologies. I'm surprised no one else stepped in to point out my error.