I purchased a Fender Blues Junior Laquered Tweed about three months ago. New, so it’s under warranty, but from a reputable store some 200 miles away (online purchase). recently, it’s started making a loud ‘bang’ when it powers up. Only once, about 30 seconds after I hit the on switch. All tubes glow Volume when playing isn’t noticeably less than normal. Pedal board is plugged in but NOT powered Master Volume on 2 is this normal? Anything to worry about?