White noise
The noise voltage density of a given tube 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 tube input. For triodes, it is approximately (2-4)/"g"m, where "g"m is the transconductivity. For pentodes, it is higher, about (5-7)/"g"m. Tubes with high "g"m thus tend to have lower noise at high frequencies. For example, it is 300 Ω for one half of the ECC88, 250 Ω for an E188CC (both have "g"m = 12.5 mA/V) and as low as 65 Ω for a tride-connected D3a ("g"m = 40 mA/V).