group: letter(s) indicate the serial order of assignment of the
Letters U, V, W, X, Y
and Z are commonly used for rectifiers
S as a first letter indicates
single-ended tubes, related to grid-cap tubes.
S as a second letter
indicates single-ended tubes.
Combinations like AB,
AC, AD and AE were used when all the single letters were used up.
L as a first letter often
indicates a lock-in (Loktal) valve.
P as a second letter
indicates a CRT.
Wherever possible, the
12V equivalent of a 6V tube had the same letters, just 12 instead