I've got a 2003 Vox AC30. I'm the original owner. I used to run it with a hotplate up until a few months ago. It only gets a few hours of use a week. Tonight while using it at very low volume as I have been for the last few months it started making an intermittent crackly popping noise. Any ideas on what's wrong? Does it need to go in the shop? I apologize if this is Tube Amp 101 stuff, been playing a long time but lacking in the technical dept.