Hi. I just got my SV20 head. Sounds great, but when I jump the channels i get a loud hum. Even when not guitar is plugged in. Have tried different cables. But the only one that I could get to stop hum is a longer cable where I could get the cable itself up in the air awau from the amp. As soon as I drop it on the floor it’s noisy like the short patch cables. Sounds like some sort of grounding noise. Any ideas what it could be?