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RCA invented the 12A*7 series in around 1947-1948. Production ramped up in 1949, when European makes were still in the caves for small preamp tubes. From 1949-1952 or so, RCA kept the structure of the getter the same--a 'stirrup' D getter with a welded cross-bar across the stirrup (think of the manual labor to do this!), rather than a curved D (single piece bend of the metal).
The point is: the later RCA 12AX7 from '54-58 sound poor to my ears. Too warm and fuzzy. These earlier versions are more extended with better focus in the mids and tighter bass.
This is a matched pair with 1952 date codes, with exactly the same construction, so you know their sound will be matched. Test like new--I used these in my Quicksilver M-60 blocks for a couple of hundred hours. These early RCA 12AX7 are very, very long lived tubes!
If you are using 5751 tubes--give these a try; you might be quite surprised how much better they are than later version of RCA 12AX7--or even 5751!
Hickok Dynamic Conductance Gm data (1000 is min and 2,000-2,200 is typically NOS)
2200/2200 RCA 2-48
2100/2100 RCA 2-04
Eico Dynamic Emission %NOS data (80 is the min and 120 is typically new):
140/140 RCA 2-48
134/140 RCA 2-04
Fantastic match of matched 1952 RCA 12AX7 NOS-spec data. Satisfaction guaranteed.
Thanks for looking. Please see my other ads for the best-of-the-best tubes in 12AX7 (and soon others).