I fiddled around for about 3 minutes tonight on my Kustom Coupe 36 tube amp when I had a loud pop followed by a steady hum. I turned it off. Ten minutes later I got the nerve to turn it on again and had no pop, no hum after 10 minutes of playing. If I have a bad power tube, can it be hit or miss like this? I've got another set of 6l6's at the ready. Probably the oldest tube that I have in it is the 12ax7 phase inverter tube. Does any of this sound like a bad phase inverter tube?