Tube Tester - Calibration? What does it mean? How should I know when it needs done?

[a_tubesupply]

Hi, I have three different tube testers, 1) Eico 666, bought as-is, works (un calibratied - I believe) 2) Heathkit TC-1, same story... 3) B & K 606 Dyna Jet, same story.

They all appear to be working fine for me, but when I have tested, a 12AX7 tube, on both the Eico, and the Heathkit the tube tested ok, but when I sent the tube to some one else, they tested on a Hickok TV 7, and told me the tub e was a reject, below emmissions level for acceptable. Why??

They tell me I should get my tube tester calibrated... Is this necessary??

What does this entail/mean?

Is any of these tube testers, (Eico 666, or the Heathkit TC-1 ) well thought of in the High end Tube (audio) world as a good tube tester?
Thanks