Well, already I got my basic question answered, so no harm done. It's usually less confusing if you don't mix up threads, though.
I was about to ask dieseljeep if he knows of any other 17" tubes that can sub for a 17HP4, when I thought to check this one, just in case. After about 90 seconds on the tester, the needle cruised up into the happy zone.
It may not be the greatest tube on Planet Earth, but I'm betting it will make a picture. I'll set the brightener aside and try the set bareback when its turn comes.
Phil Nelson