GP12SMX COMBO

[LERUM HEVITT]

Recently my GP SMX combo developed a serious hum earth loop noise and all sorts of crackles and noises if you tapped various components ? Could not see anything obvious no apparent earth from the preamp to the chassis ground. I did realise that the board had an earth track on the outside perimeter but could not see where it connected to an earth.On attaching an earth wire from the guitar socket ground to the chassis earth all is now back normal and working fine. 
Why has this fixed it . What had caused it in the first place? Is it safe to use like this I have just done hours of testing and seems okay.
All comments appreciated 
cheers LERUM HEVITT