When replacing tubes in an amp, I'm guessing you can't just go out and buy any tube? There must be different types of tubes which the amp was made to house. For example, if I'm replacing tubes in an amp with "EL84" tubes, are there a few different types of EL84s to choose from to replace the tubes? Or are EL84 tubes one type of tube and the amp can accommodate a few other types of tube too?