Humming problem with my Blues Junior

[Priebe109]

I recently bought a used Blues Junior II from 07, which unfortunately came with a bit of a humming problem. When I turn the amp on and let the tubes warm up it has a weak and steady hum which I suppose is normal for tube amps. After a few minutes of playing however, it starts making a sort of rumbling/surging/pulsating hum about twice the volume (sometimes higher) of the normal weak hum the amp makes when it is turned on. Unplugging the guitar and turning the volume knobs up or down on the amp doesn't change the sound at all. It sound a bit like a freezer. Other than that the amp sounds great, not microphonic or popping/crackling. I noticed that lifting the amp from the ground greatly or entirely removes the hum. Tilting it at 15+ degrees also reduces the hum. It is a US amp. I run it through a converter to EU mains, but my converter doesn't have the ground pin in the output so my amp isn't grounded. I don't know if this can cause the weird sound since it only appears when the amp has been turned on for a few minutes. Any ideas?

[Reeltarded]

Issues:

50 cycles on PT maufactured for US domestic only 60Hz. This is a strain on the PT that works just fine if going the other way. (60Hz to 50Hz)

No ground. Absolutely unsafe. Not good.

I think you might have a bad output tube and the overbias they are seeing at converted voltage is also not good.