I recently picked up a Mercury Model 1000 tester and it works and appears to provide coherent results. However, after checking a bunch of tubes in my collection, I still wonder about its accuracy and sensitivity. Bottom line is that these are test instruments and they need to be calibrated or they are worthless. Mine was manufactured in the early 1960s so I would expect it to require parts and be in need of calibration. My advice would be to search for a good condition, inexpensive unit (found mine on Craigslist) and then send it out to be serviced by a qualified technician.
I've had a few revelations with my uncalibrated tester. Some tubes test rather poorly and still sound good. Also, I have a pair of NOS 12ax7s that test as new according to the tube chart, but they have been in my preamp for about 4 years. This makes me wonder about NOS tubes being advertised as "testing strong", with the implication that they have seen little use. Maybe I'll get different results when I check again with the calibrated tester.
I've had a few revelations with my uncalibrated tester. Some tubes test rather poorly and still sound good. Also, I have a pair of NOS 12ax7s that test as new according to the tube chart, but they have been in my preamp for about 4 years. This makes me wonder about NOS tubes being advertised as "testing strong", with the implication that they have seen little use. Maybe I'll get different results when I check again with the calibrated tester.