In short, there is no "standard" numbering scheme for American
vacuum tubes.

In the "beginning" numbers 01 though 99 were assigned pretty
much on a "first come first serve" basis.
More confusing were, for example, adding a number to signify
manufacturer. For example, the type 27. 127, 227, 327, 427 etc
were the same tube but by different manufacturers.

Starting in the '30s, they got together and agreed that the
first number would be the filament voltage.
6A7 for example, 6.3 volt filament. 5Y3, 5 volts etc.

An original "idea" of assigning 1 or 2 letters in the middle
on a per function basis went to Hell surprisingly fast.

During WWII, the military also came up with VT-nnn numbers
that referenced "normal" tubes, such a 27, 80, 6SK7 etc.

Transmitting tubes were different, usually "high numbers"
such as the 807, 811, 813, 1625 and so forth.

Then there's the entire gambit of 4-digit "industrial" types.
Nominally in the 4000-9000 range.

The only other thing they agreed on was keeping the type the
same across different filament voltages. 6BE6 and 12BE6 as an
example.

Then of course, a few manufactures came up with their own
******* numbers. Like Taylor TZ-40. "Taylor, Zero bias, 40
watts."








--
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com