[Radioroslyn]

Just put a jumper between the center and "ground"
connections of the pot. If you still hear sound, the
"ground" really isn't ground or there is some leakage
in your 75 tube between the grid and diode. This
is common manufacture issue that has to do with
the placement of the internal parts. Won't show
on the tube tester as it's capacity coupled in side
the tube.
Terry
ps
As usual GREAT WORK from Bob! Keep it up!