ENGL Savage 120 with weird grounding issue

[kka]

This amp makes an annoying 60 or 120Hz noise when guitar is plugged in. When Guitar volume is turned OFF the noise is GONE. When I remove the input tube (V1) the noise is gone. I checked the shielded wire from input jack to V1 tube and is fine. I checked all ground connections of input jack and all seems fine. Different guitars do the same thing.
So I can't measure the frequency of the hum because when I hook up the ground wire clip of my scope to any ground connection of the amp, this noise is eliminated. When I unplug my scope from outlet (scope ground wire still connected to amp's ground) the noise is back. Anyone can help me figure this out ?

The amp is grounded. I measured continuity from any of the amp's ground connections on PCB or chassis to ground pin of power cord. But somehow through the ground connection of the scope I can eliminate this noise.

Thanks for the help (attached is schematic)

[g1]

How about if you plug the amp into the outlet that the scope is plugged into?

[drewl]

Sounds like the amp's ground connection is bad.
I assume it has a detachable cord.
Measure the ground pin of the socket to the chassis.
Could be the jack, the cord or the chassis ground.