For sale is a pair #3 of NOS Mullard 12AU7/ECC82 long plates manufactured in 1958 at the Blackburn factory, and branded as M.W.T. I purchased perhaps 20 years ago from Mike at KCA NOS Tubes as matched NOS pairs, who had bulk packed stock then. I've never used them.
Particulars: etched date codes are B8A:
B = Mullard's Blackburn factory 8 = 1958 (date codes after 1960 were four digit)
A = January
Tubes have goalpost getters (circular getter with dual support posts). Excellent, virtually identical print on all pairs and packaged in white boxes. I contracted testing to tube dealer TC Tubes in MN, who use an Amplitrex AT-1000. TC Tubes advised a new/NOS tube should measure between 80-150%.
Pair 3 GM/transconductance:
Tube 1 - 81% @ 1800/90% @ 2000
Tube 2 - 81% @ 1800/90% @ 2000
So, known provenance, NOS Mullard tubes from their apex era; guaranteed as advertised or return. $7 shipping CONUS, actual shipping otherwise. Thank you for looking!
Info on new/unused tubes:
"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)
"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." http://leedsradio.com/technical/testslikenew.html