Wow...interesting. I've had 2 JJ Tesla KT88s from the The Tube Store fail at ~400 hrs and ~700 hrs. When I wrote them about the first failure, they indicated expected life was >1000 hrs and I must have gotten a bad tube. I thought it was just my bad luck.
I also paid extra for matching replacements from The Tube Store only to find the "matched" replacement wasn't "plug 'n play" as advertised. Give that biasing is not a big deal on the 502B JoLida, not a problem BUT something other than advertised.
Holos- Thanks for your post. "You don't know what you don't know"!
Order fulfillment from Doug's Tubes was very fast but I can't yet comment on quality.