[OZ4]

For another test you might try connecting  a .1uf cap from the control grid of the output tube to ground. That should tell you where the hum is coming from. There shouldn't be high voltage on the grid side of C22 so a low voltage cap should OK. This will short the signal to ground.

If it's quiet then it's the 2nd det or before.
And if not then it's the output section and or power supply.

I just had one with a bad output tube that was shorted from the filament to the cathode. The AC hum could be seen on the grid side feeding back to the det. And it was a brand new tube.