I've done a bit of searching and can't seem to find an answer. I recently got a 70s Fender Bassman 100, I like it a whole lot, but I've noticed that sometimes it produces a hum (sounds a lot like a rumbling oscillation) I've checked to make sure it's nothing in cable (it does it without anything being plugged in) and I've noticed that tapping or moving it causes it to stop for a bit. Sometimes it will stop for 10 seconds, others for hours. The previous owner mentioned that the power cable wasn't properly grounded, could that be the issue? If not, where do I start checking for problems? I'm a newbie to tubes, so I don't know much about them. Thanks!