1. I play a note it sounds correct but then fades with a litte fizzle into silence. It seems to get better as the amp warms up. 2. Whenever I play a D cord or a single D note the amp takes off into crazy feed back and high pitch squeal. 3. The tube has a florescent purple/blue small circle on each side of it. Yes I replaced the tube and that fixed it. Is there a technical name for these problems other than just saying it was a bad tube.