In the 1950s a 5-element system (GOST 5461-59, later 13393-76) was adopted in the (then) Soviet Union for designating receiver vacuum tubes.
The first element - number, means a filament voltage in volt (approximately).
The second element - letter, designating type of the device:
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
R - double tetrode and double pentode;
G - diode-triode;
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
L - tubes with focused beam;
E - electron-beam indicators of adjustment.
For electronic mehatrons the second element of a designation is made of three letters: the first M - mehatron; the second letter corresponds to the
basic purpose of the device (in some designations mehatrons, developed earlier, this letter is absent); the third letter designates type of the device according to the list resulted above.
The third element - number, corresponding a serial number of the given type of a tubes.
The fourth element - letter, describing 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
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 tubes, for example:
V - increased reliability and mechanical ruggedness (such as low susceptibility to noise and microphonics);
R - even better than V, extra high reliability (10000 hours and more);
E - extended service life (5000 hours and more);
D - exceptionally long service life;
I - optimised for "pulsed" (i.e. switching) mode of operation.
Example for 6P14P-ER tube:
6 - filament voltage, 6,3V;
P - output pentode;
14 - serial number of type of a tube;
P - envelope: miniature, glass;
ER - raised durability, operation time > 10000h
There is another designation system for high-power tubes such as transmitter ones.
The first element (from left to right) is always G.
The second element (with some notable exceptions such as the G-807) is:
K – shortwave (≤ 25 MHz) tube;
U – ultra-shortwave (25–600 MHz) tube;
S – centimetric-wavelength (> 600 MHz) tube;
M – modulator tube;
I – impulse tube.
The third element consists of a dash ("-") followed by the design serial number. Letter can be used here in some special cases (GMI-6 – impulse modulator); if the tube has to be force-cooled, there might follow a letter A for water-cooled or B for air-cooled.
Popular transmitter tubes include the GU-29, GU-50, GM-70 and G-807 (the Russian 807 analogue).