Can you use it in battery's only mode? Without problems?
If the 1L6 is the culprit then a substitution is needed with another 1L6. If that solves the issue, then it's possible the grid is shorted. The next thing to look for is a shorted wire around that socket. Or one of the pins isn't making solid contact inside the socket, if that's the case then use a scribe to very carefully bend the contact that is causing trouble. It could also be a dirty pin or contact.
If you have a signal tracer, start at the plate on the audio, and temporarily short out the speaker output terminals. Then test the grid next, Do this with each stage backward.
Check the volume control for opens or shorts.
The biggest culprit is a floating grid. That is where the RF or IF transformer probably has an open winding.
An open AVC or screen bypass capacitor will cause Hum.
NOTE: whatever you do, DO NOT BYPASS ANY CAPACITOR WITH ANOTHER WHILE THE SET IS TURNED ON, YOU WILL BLOW OUT THE TUBE. ALSO DO NOT REMOVE OR REPLACE ANY TUBE WHILE THIS RADIO IS ON. YOU WLL BLOW OUT THE FILAMENTS. Make sure any capacitor you do a bridge with is shorted first to bleed off any stored voltage.
Hum usually doesn't start at the RF Section except for the antenna section, with one of its leads open or disconnected. That radio has a built in swing up antenna that is connected by hinges, they may need to be checked for intermittent contact. Also look at the whip antenna for the same condition.
Sometimes reversing the power plug will reduce the hum.
One thing would be to check is the tuner capacitor for shorts to ground.
Check the 117V bypass caps. If they are open, that could cause the hum.
Make sure you also check the D.C. PLATE Voltages meet specs. Sometimes the power is reversed and supplied from the input rather than the output filter which is wrong.
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