[Radioroslyn]

Something else came to mind if you still can't find it check your RF coils for (green primaries) corrosion. You may have a sightly bad coil that isn't giving you a good ground return for the grid of the tube. Also you could have a bad 24 tube. It could have a short from the cathode to grid 1 or a heater to cathode. If you've got a spare I'd swap it and see if that does the trick. Either 24 could be at fault. The pilot lamp may affect the overall amount of hum. It may have a bit more of less with or without it in the circuit.
Just a thought.
GL
Terry