I have yet to find a decent chart which explains the reason for the
numbers/letters used on tubes. (For USA tubes).

There seems to be more online for european tubes.

For example:

6L6 tube. the first "6" is the heater voltage. The "L" means Beam Power
Pentode. And the last "6" is supposed to mean the number of elements in
the tube.

Yet, I have found no chart which says what "U" means, and 5U4 is very
common.

Oddly enough, some tubes dont match the filament voltage. For example,
the 813 tube requires 10 volts on the filament. (Not 8), yet the 807
tube needs 6.3 volts on the filament.

But this gets even more complicated.
What is a 6AU6?
6 volt filament, (6 elements *maybe*) but what's the AU for? (remember,
"U" is not listed). "A' means "Diode".
(A 6AU6 is a sharp cutoff pentode).

Or 12AX7.
12 volt heater, 7 elements (that is correct), but what does the AX mean?
["X" is supposed to mean "Gas-filled full-wave rectifier". *12AX7 or
it's near identical 12AU7 is NOT used as a rectifier, it's a dual
triode.*

And then there were those old 1B3 tubes in TV sets. Where did the 1 volt
heater voltage come from? (and why didn't they just stick with 6.3 volts
like other tubes?)

One other thing. Is there any rhyme or reason that transistors are given
the numbers they get, or are they just random numbers given for no real
reason? I wont even get into the IC chips and what leads to the
number/letters used on them....

* If anyone knows of a COMPLETE listing that can be downloaded, such as
a PDF file, please post the URL. (or even a text file). I am not finding
anything that's complete......