Mono records: build-in hum?

I have a major problem since i set up my dedicated mono tonearm (second arm on my TT), namely Denon 102 on Acos Lustre. The problem is background hum, but sort of unusual one.
When I switch my amplification on and crank the volume about all the way up, I hear no hum, just usual very light tube hiss. When I drop the needle in the lead groove, still, no hum. But, when the record begins, prononced 50-60 hz hum on the music background just drives me mad. The hum is largely inconsistent from record to record, it varies from utter silence on old Melodias and DG up to absolutely unbearable on Classic Record Library and most of Columbias. My stereo arm on the same TT hooked into the same phono stage, produces absolutely no hum whatsoever. Any guess?