RETMA (USA) designation system

For example: 6SN7GT
1st number heater
0 no heater
1 0.1 - 2.0 V
2 2.1 - 2.9 V
3 3.0 - 3.9 V
n n.0 - n.9 V


next letter(s) serial code


final number number of elements in the tube


final letter(s) (optional)
A controlled heater warmup time
B improved ratings/performance
C ditto
G glass bulb
GT glass tubular
W ruggedised version
X low loss ceramic base
Y low loss phenolic base

(RETMA is the Radio Electronics Television Manufacturers' Association)