The old, original 3-500Zs from Eimac should have been bronzed, the freakin' things lasted forever. I built one in the early 70s and sold it in 1988 and the "new" owner is still using the same tubes which are now about 45 years old. It's evidently a great design if everything is done just right. My lil' AL-80B has an "RF Parts" (Chinese) tube in it for 16 years now and still working fine, so they can get it right at least sometimes.
