Hi, like the title says, I was digging through my spare tube box and came across a 6cg7 tube (?). wondering if I can use it to replace a 12ax type tube? Or? (I 've never had an ampeg or Silvertone, btw, so I'm sort of baffled where it came from, also have a jan phillips 5814a? ideas what i can use them for. thnx!