I'm hoping someone can get me the correct information regarding whether black plate 6L6GCs by any manufacturer (but especially RCA) can cause red plating in Mac amps (no other cause other than the tube having a black plate)? The friend who told me this said that there is no explanation for it, but, because of the red plate danger, I should always avoid using black plate 6L6 tubes in Mac amps. I know that McIntosh-labeled tubes were mostly black plate, so this puzzles me. I also searched the internet and could not find anything on this unexplained phenomenon. Has anyone else heard of this (or experienced it) or it just some unsubstantiated rumor/urban legend floating around?