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