I have a '66 blackface Champ that has two good old RCA tubes in the power tube slots. The preamp tube is an Electro Harmonix 12AX7eh. I have on hand a JJ ECC83 and a NOS Phillips Jan 12AT7WC. Are all three of these interchangeable? I'm thinking either of the other two would be better quality than what's in there now but I'd like to swap them all out and figure out which one I like best. I just want to make sure I don't screw anything up.