Ever had a tube die on the tester?

[RitchieMars]

I recently decided to check some of the tubes in my Zenith K731. It's a solid all-original that I've had for years and continued to use it long before I knew anything about tube radios. It's never hummed with it's original capacitors still in place, and most of the tubes are probably the original yellow-lettered Zenith tubes. I decided I'd check these tubes recently and started with the 35C5. The tester is a simple emissions tester, an Accurate Instruments 257. I set up the tester for the 35C5 and let it warm up, checked for shorts, and pushed the quality switch over. The needle jumped up and then sharply fell-off, starting to nosedive into the "bad" area of the meter and stayed there. I doubled checked my settings, all correct. The radio had just been working fine before this, so I put the tube back in the radio and gave it a try. Sure enough, it warmed up and started to hum so I switched it off. I replaced the 35C5 with another out of my H274 and the set warmed right up, no hum, and sounded better than ever. In fact, I could hear the tweeter working which I hadn't noticed before.

So, apparently the old Zenith 35C5 died on the tester. Was it just the tube's time to go, or is there something about a tube tester like this that can prematurely kill a weak tube? I tested the other 35C5 on the same tester, same settings, and it tested fine. I'm rather curious as to why my Zenith hummed with the bad tube in place. Is that what a bad 35C5 sounds like, or is it the electrolytics? If so, why doesn't it hum with a good tube?