Most old production 12ax7's test as new, or near new, until they immediately test as well used (same with old production 5ar4's).
Testers used (over the years) have been a 539C and 2 TV7's.
Difficult tubes to test, though as somewhat mentioned balanced sections are a must if required by the circuit it's used in.
Testers used (over the years) have been a 539C and 2 TV7's.
Difficult tubes to test, though as somewhat mentioned balanced sections are a must if required by the circuit it's used in.