Tube tester...anyone own their own?

I own two. 

My advice is to buy one that is already working and calibrated. Also, make sure that it can test the kinds of tubes that you want to test. Because not all testers are equivalently capable. This will mean that you'll pay quite a bit for a good tester.

I've owned a Hickok 600 for about 12 years now. It was freshly calibrated when I bought it and I haven't had it updated, but it doesn't appear to have drifted too much. I test every tube I get and record the readings, so that I can both compare them after a period of use and compare them to other tubes of the same type. That system has worked well, and I believe mitigates the effects of any change in calibration. I rarely resell tubes so absolute accuracy just isn't a priority for me.

That said, markus is correct; if you're buying one it should be calibrated and completely functional, and that doesn't come cheap these days.

I’ve had one for many years. I get it re-calibrated every so often. One of the best investments I’ve made. I have drawers full of tubes, so it comes in handy quite often...