I think you're getting two sources of noise: the inherent hum from the field coil, and hum pickup from the long test lead that you have on the grid of the 47. Run your test the opposite way: tune the radio to no signal, clip your lead to the chassis and then touch the other end to G1. The audio stage may not be producing much hum.

I almost hate to bring it up, but if it were my radio and there were room around the field coil, I'd try making a hum bucking coil. Wind a few turns of insulated solid hookup wire around the coil and then connect it in series with the voice coil. One way would increase hum, switching the bucking coil wires the other way would reduce (buck) it. I don't know how many turns would be required. I guess I'd start with about four and go from there.