[NJARC] How to spot super rare audio tube

[Pete O]

While working at HP/Boonton Radio decades ago, the "selected" tubes in most cases were merely stamped "Selected", without testing, and a premium was charged.  In the case of tubes for the 260A Q Meter, the ones that DIDN"T meet specs were stamped "selected" and sold at a premium for use in other models.   
A few years ago I used an HP distortion analyzer to test a wide variety of 12AX7's at all sorts of signals and voltages within its' curve. The differences were so slight amongst the 12AX7"s that it couldn't be measured.  Since then I was advised that I hadn't measured it's audio "blackness";  oop, foolish me.

Pete AI2V