USSR/Russian tubes designations

This is really simple categorization.

1st symbol - аilament voltage in volt (apx).

2nd symbol - tube type

D - diode, including dempfers;
H - double diode;
C - low-power kenotron;
S - triode;
N - double triode;
E - tetrode;
P - output pentode and beam tetrode;
J - high-frequency pentode with short characteristic, including with double management;
K - high-frequency pentode with long characteristic;
R - double tetrode and double pentode;
G - diode-triode;
B - diode-pentode;
F - triode-pentode;
I - triode-geksode, triode-geptode, triode-oktode;
A - frequency-converting tubes and tubes with two operating grids (except for pentodes with double management);
V - tubes with secondary issue;
L - tubes with focused beam;
E - electron-beam indicators of adjustment.

3rd symbol - serial number of the given type of a tubes.

4th symbol - describe constructive desing of a tubes:

P - in a glass environment, tiny (finger-type), diameter 19 and 22.5 mm;
A - in a glass environment, supertiny, diameter from 5 up to 8 mm;
B - in a glass environment, supertiny, diameter from above 8 up to 10.2 mm;
G - in a glass environment, supertiny, diameter over 10.2 mm;
S - in a glass environment, with a socle or without a socle, diameter more than 22.5 mm;
N - in a ceramic-metal environment, tiny and supertiny;
K - in a ceramic environment;
D - in a metal-glass environment, with disks.
Tubes in metal to an environment of the fourth element of a designation no have.

Additional element. To a standard designation of a tube are sometimes added (after a hyphen) the letters describing special properties of lamps, for example:

B - tubes of raised reliability and mechanical durability;
E - tubes of the raised durability (5 thousand hour and more);
D - tubes especially durable;
I - tubes, intended for work in a pulse mode.

For example -
6N6P - filament voltage, 6,3V / double triode / in a glass environment, tiny (finger-type), diameter 19 and 22.5 mm;