[MrFixr55]

I had this once, due to a very weak 45 in a Gloritone 26.  B+ was about 310 at the Output transformer.  Replaced the 45 and all was well.  However, I don't know what you mean by tunable hum. However, after originally posting this, I looked at the schematic and noted that you have 2 45s in push pull.  Since it is possible but not likely that both 45 tubes are low emission, we want to look elsewhere. This is likely a type A biased output stage.  These draw a lot of current even under no signal conditions.  So we need to look for something else that can cause low current.

Assuming that the output transformer does not have an open leg, the other high probability failure is in the Cathode Bias resistor for the 2 45s.  This is R59 on your schematic.  It is likely part of a tapped wirewound resistor.  Actually, there are several parts of that resistor that can be bad.  Replace it with a string of 5 watt resistors.  R57 which is likely part of this tapped wirewound resistor will also cause very high screen voltage, as this resistor forms a voltage divider with the speaker field.  However, if this is open, the field strength will be very low, but there could also be hum.

In my office / shop, I can hardly operate an AM radio due to the computer, fax machine, LED lights and unshielded AC wiring.  It does not matter whether the radio has a loop or is hooked up to my outside long wire.  I have to turn off the computer, the monitor and the lights.  Even the ceiling light which is on a dimmer., and there is still some noise.
The symptom is a hum, stronger on some parts of the band than others.  If I take the radio outside, it is an entirely different story.