Tubes - What do the letters mean

Jim and Dekay, thanks for the advice; I will take both of you suggestions and see if I can make sense of this business. So far, it's a great learning experience!

Dekay, thanks again for another tip. I was not aware of the search feature at Audio Asylum. I'm heading to "Out Of The Closet" tomorrow, but was able to pick up 20 virgin jewel boxes from a friend of mine in Culver City. His shop is next door to a fullfillment house and they happened to have a bunch of corporate instructional CD's. Free, and he gave me something that fixed the small leak in the radiator of my Alfa. It was a good day.

You seem to have a good handle on tubes. I just ordered Telefunken 12AX7's and RCA 6L6GC's -- both 60's vintage -- for my Mesa Baron.

Thanks again for the info...

Using the 12AX7 as an example, the 12 is the approximate filament voltage.
All AX7 tubes share similar characteristics, of which there are about 20: pinout, base size, gain, plate volts, etc.
The letters on the end indicate different versions of the tube. A thru F are modifications such as an change in maximum plate voltage, usually you can substitute a higher letter for a lower, but not the other way.
The other letters like the W signify some special characteristic such as a special base material or the shape of the tube.