Ratings/numbers on tubes

[JCM900MkIII]

I have noticed that the ratings/numbers on 3 of my EL84's are very far apart.

The one in my Class 5 says 43,6 (JJ)
I have a spare Sovtek EL84M which says 18
And the original tube that came with the amp has a label "I 30" (JJ)

Do those ratings on the tubes/boxes give an indication about possible headroom? (the sovtek has the most headroom but lowest rating)
Do they give an indication about lifetime?

[SteveD]

Different brands have different systems and criteria for matching valves and the label usually (though not always) indicates a nominal transconductance figure. This is kind of like gain for a power valve. So for a given input voltage, the measured 'output' current flowing through the valve will be stated on the label. If you don't know what the 'given input voltage' is, the figure doesn't tell you much except for matching purposes.

[JCM900MkIII]

Thank you Steve.

Ahhhh, so comparing ratings for the same type but different brands doesn't make any sense at all.
Comparing same type/same brand could be usefull for matching tubes, but nothing besides that.
And even shops can put their own measurements on those tubes/boxes, disregarding whatever the manufactures has writen on them.

Different tubetesters, different readings as well then I presume?

I think it makes sense that it doesn't make sense to look at those numbers.

Cheers!