I just picked up a used Dr. Z Maz 18 Jr. that I want to replace the crappy GT EL-84 power tubes. I'm going to try a set of JJ's, as I heard that they are pretty good for a new tube. My question is, I have a pair of old Phillips El-84's that were in my Vox Berkeley(that I no longer own) when I bought it back in the early 70's! How can I tell if they are still good? I was thinking about trying them in the Z.