Different valve tester settings for ECC83 and 12AX7?

I have a "Precision Model 650" valve tester and I've been asked to test a dodgy sounding JJ ECC83S by a friend. I've always thought that the ECC83 and 12AX7 were electrically identical. However, according to my valve tester handbook, they require different settings on the tester. Sure enough this ECC83S tests weak using the ECC83 settings but strong using the 12AX7 settings. :scratch:

Can anyone shed any light on this?
If it helps it's the sensitivity and voltage/load settings that are different.

Cheers. :)

What are the settings you're testing them on? Maybe one of the settings you are using for one or other tube type accesses a slightly different part of the tube's curve? Frankly I thought they were supposed to be interchangeable. Or maybe it's just a squiffy tube. How does a known good tube (bog standard ECC83 or 12AX7) rate using the two settings? If they are still different then its possibly variance within the valve tester - an out of spec resistor or somesuch.