What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
Which raises another question- why would one side of a 12AX7 (or any
other similar twin triode) test markedly different from the other
side? I've seen big mismatches in 12AX7 tubes. I can see where used
tubes that have different "roles" in the circuit could "wear" unevenly
depending on how their used (driver, phase splitter, etc).