Tube 4 & 5 would make a nice pair. Maybe the best 6SN7 ever made. No cracked bases, chips, etc. - all are in exceptional condition. Same construction for all tubes.
I contracted testing to TC Tubes in MN who use an Amplitrex AT-1000 and stated a new/NOS tube should measure between 80-150%. Unlike many, the Amplitrex tests at actual voltage and current for the tube under test. The GM/transconductance result for this one:
Tube 4 [email protected]
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Exceptional quality; guaranteed as advertised or return. $7 shipping CONUS, otherwise actual shipping. Thank you for looking and see my other listings! Info regarding vacuum tubes and testing:
All other things being equal, the tube with the higher gm will NOT have a longer service life than the one with the lower gm. Cathode to plate spacing can also have an affect without any detrimental affect on service life. Mutual conductance for newly manufactured tubes in a given lot will be distributed on a skewed normal (bell shaped) curve. It is skewed to the left because most manufacturers were overly optimistic in stating the bogey value. For American made 12AX7A's for example a corrected, and more realistic bogey is about 0.75 times the specification. You can expect that a batch of, new, good tube to be distributed over the range from .66 time bogey to 1.33 times bogey."
"It is important to note here that a common misconception of tube testing is that every unused tube, new or vintage, should test at 100% of the meter scale of the tester, or at exactly the number listed as the nominal Gm on a digital tester, such as the Amplitrex. In reality, very few tubes test exactly at these readings, some will be above these numbers, most will likely be below. As the RCA manual states that the characteristics may fall in a range above or below the bogey values. For this reason, the original RMA specifications were plus or minus 20% for Gm for an unused tube, and minimum good would be up to 50% lower than the nominal number."
https://www.audiotubes.com/nosnote.htm (Brent Jesse)