I’ve read this entire thread. And the 421a/5998 comparison thread. And multiple other sources including tube specification sheets. I qualified my answer with “short story long” because there isn’t a short answer. There is conflicting information, various opinions, a multitude of pictures and no one has a universal understanding or answer to the history of these tubes. That being said, a search is a step toward forming a better understanding. Yes, there are dimpled/domino versions of the 6520 that many say are a 5998. That doesn’t mean they are a 5998. That doesn’t mean they’re not. Spec sheets would argue they are not but humans made these things and things can be mislabeled. This thread is miles long. I don’t expect everyone to read it. But it has a LOT of useful information.
