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Siemens & Halske ECC83/12AX7 Code mC6 Long Ribbed Plate Double Getter Support, Made in Munich West Germany from 1959 to 1960
Does not come with the original box
From the internet: This is the rarest long plate 2 getter support audio tubes ever made and overall perfection in a 12AX7 / ECC3 class of tubes
triode 1 = 1400 micromhos
triode 2 = 1325 micromhos
Tested with my Hickok 600A. A typical new 12AX7 will test between approx 1100 and 1400, with 1250 being considered "nominal" or average for a new 12AX7.
Please see the images of the actual item above for a better idea of the condition.