You should read my post on the 10BP4 I posted a few months ago. The cathode lead was blown open. No longer a cathode connection. I think the previous owner tried to rejuvinate it and the fine wire to the cathode opened like a fuse.
If you carefully examine in the neck you might see it. I did in mine! See the photo after I gave up and smashed the tube to look more closely inside.
My guess is that in manufacture the very fine wire used has a tendency to melt, especially if you try and rejuvenate. I guess that is why I got the set that contained this tube cheap!
https://videokarma.org/showthread.php?t=276674