Tubes are closely matched for Gm testing within 8% (see below) and internal structure - they are identical.
Black 4 hole staggered plates with bottom square D getters.
Please note: The tube with the date code KG has the tube number scratched into the backside of the base by a previous owner - see photo 4. The wording is also slightly faded on this example, but still legible.
Tested on a calibrated Hickok TV-10 military tube tester with no shorts and no leaks with the following results:
Gm Test = 2625-2750 and 2875-2625 micromhos
Minimum pass/good = 1650-1650
Further tested under real operating conditions on a Class A amplifier and exhibiting no microphonics.
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