Tube testers have gotten crazy expensive compared to when I started out 20 years ago. Not really worth getting anything less than a TV-7 or TV-10 or Hickok. You guys probbably don't want to know that I bought my TV-10 for $50 in 1992 or so. I sold a TV-2 a while back for around $700.
Anyway, you have voltages in the manual for your Bottlehead gear. If a tube is getting old the operating point will shift, typically the plate voltage will drop. That is just as good an indicator as plugging it into a tube tester.