I was using a pair of 5814 black plates in 2019 and when I pulled them to test they were dying. Then amother pair of RCAs went in. I forgot whether those became weak. But Recently I ran a pair of Sylvania 6189 black plates 1958 manufactured with beautiful internals; labeled for Bendix Eclipse Pioneer. They were known to be good when they went in; several months later I pulled them and they tested weak, well into the red. So I am wondering how often to check the inputs. I am running strong testing GE 5814s as inputs now.

As the tubes age the change is subtle and occurs over time so you do not notice it. But if you swap those out for a newer pair I bet you would hear a difference.

When I swapped out the [unbeknown to me] dying 6189s with the good strong 5814s the change in sound was immediate.

I've got to assume Steve looked over your amp thoroughly when you had it upgraded to anniversary specs, so likely not some defect. Your MT is running the inputs hard for some reason.

It would be nice to get a comment to this thread from another MT user with a tube tester, as I do not have one. It would be good to know if this common to MTs in general, or to certain models, or to anniversary mod units.

Did you have similar wear on your inputs prior to the upgrades? I know you have used an MT for a long time yet your first post on this was not quite a year ago. That suggests that something changed. It may simply be that you started testing tubes more frequently or did your amp start running them harder somewhere along the way?