Here's a tip if you find a set that hasn't been powered up in a long time.
This 10BP4 showed 0 emission at first. That's pretty much how all my 10BP4s have tested at first. Let them run for an hour at least before considering them dead or weak. Running for a while @7 V can help too. Try this before trying to rejuvenate them. Sitting around for a few decades seems to put them to sleep.
Here's this one after 30 min @. After ran hour it was into the green
