The model numbers attached to a tube aren't just
actually encode some data about the tube. The first number in a tube's
model number corresponds to its filament voltage, the letters in the
middle are the actual model of the tube, and the last number is the
number of things in the tube.
For example, a 6DJ8 is a type DJ tube,
with 8 elements and a 6V heater. This can be really handy when you're
looking for a substitute tube, since sometimes a particular tube is
kind of expensive, but is available with a different filament voltage
for a lot less. For example, 6L6, 12L6, 25L6, 50L6 and 117L6 are the
same tube, but
with different heater voltages. European tube numbers are different for
some reason, for example, the European equivalent to the 6DJ8 is ECC88.
No I don't know why they do it like that.
To complicate things, there's also a third
numbering scheme with four digit numbers like 5881 for 6L6, and 6922
for 6DJ8, as well as a fourth, much older system that was used in the
20's and 30's that just consisited of a two-digit number. To cut down
on confusion, I will only refer to tubes by their number/letter/number
designation on this site.