White noise
The noise voltage density of a given valve is a function of frequency. At frequencies above 10 kHz or so, it is basically constant ("white noise"). White noise is often expressed by an equivalent noise resistance, which is defined as the resistance which produces the same voltage noise as present at the valve input. For triodes, it is approximately (2-3)/"g"m, where "g"m is the transconductivity. For pentodes, it is higher, about (5-7)/"g"m. Valves with high "g"m thus tend to have lower noise at high frequencies.