How do they test? A Ferrari 328 with 200,000 miles on it is probably worth a bit less than one with 2000 miles on it.
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Also- check between the pins for a diamond, molded into the glass envelope’s bottom. Some other companies manufactured tubes for Telefunken. Those with diamonds are worth more, if they test well, as mentioned above. Some of the smooth plate Teles are quite rare/desirable. You may have struck gold! https://www.tubedepot.com/products/telefunken-ecc83-12ax7-smooth-plate Lots of info here, regarding the variants from Tele: http://www.audiotubes.com/12ax7.htm
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.