GE purchased Ken-Rad (Kentucky Radio Corporation) in 1945, although tubes under Ken-Rad brand have been produced until 1951, so a lot of references are for GE not Ken-Rad
” The GE Black plate 12AX7 (and 5751) have an enchanting and beautiful sound. Articulation is excellent, though not as detailed as the TungSol. Sound stage is expansive and inclusive.” Read
Very interesting tube. It has a very warm very deep, black coffee like sound and great soundstage. Very similar to RCA 12ax7 with carbon plates. The tubes were produced from 1948 to about 1952, so they are really hard to find. All later production tubes by GE had gray plates, so this is the only black plate GE tube. On the negative, this tube has very early roll off of HF, so despite it’s great bass and fantastic mids, this tube looses a lot of details due to the roll of. Might work great in a bright system, but in McIntosh 275 it sounded OK.