Found in old white box stamped 6681 12AX7.
Mullard Japan 12AX7.
From a testing standpoint this is the best one of these I've ever had.
Tests 1760/1800 for gm where 1000 is good. Also, essentially identical plate current on each side at 1.7/1.7 ma. All at 250 volts in a Triplett 3444/Westmore 501 laboratory tube analyzer restored and upgraded by Paul Hart.
Listening tested for excessive noise and microphonics in a Maxi-preamp 2 noise tester.
Quiet, Zero Microphonics when tapped on.
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