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You can date them accurately - https://mullard.org/blogs/news/mullard-valve-etched-codes-how-to-interpret-them
Good valves in both cases, if they work properly. If they came out of a working bit of audio kit and were in the signal path then it's a reasonable assumption that they will. If you have any amp which takes a standard double triode in the V1 position - it's a near certainty that you will! - then you can easily test the 12AX7, and if you're sure that the plate resistors are adequately rated then you can test the 12AT7 the same way. Poor gain or tone, noise or microphonics will be very obvious - you don't need a valve tester just for that, and the results from one aren't necessarily that useful anyway for guitar amp applications.
They're certainly worth enough to sell if you have no use for them yourself - but by the same token, if you do have a use yourself I probably wouldn't sell them, they aren't worth a fortune. I sold most of the fairly large number I had not long ago, I did the 12AX7s for £30 each and the 12AT7s for £20 if I remember rightly, after a quick check of Ebay selling prices.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein