보통 우리가 듣는 쐐하는 노이즈는 여기서 말하는  Thermal Noise 로 압니다...

Thermal Noise in Tubes

It turns out that there is a very simple way to calculate the thermal noise in a triode. The noise produced by the tube is equivalent to a resistor in series with the grid, at room temperature, whose value is given by:

In other words, the noise produced by a tube is inversely proportional to G_m. This directly explains the modern interest in high G_m tubes such as the WE417A. This tube was intended for the first stages of sensitive VHF and UHF receivers, where minimum noise is a critical feature, but is also of interest for sensitive phono stages.

Flicker Noise

The second source of noise in tubes is flicker noise, also called 1/f noise which clearly describes its nature: it is noise which decreases with frequency. It is of no interest for radio work, but has obvious importance for audio since most of the noise lies in the audio band. It is particularly important for phono stages, since the RIAA correction, by attenuating higher frequencies, boosts the contribution of noise at lower frequencies.

Flicker noise is caused by variations in cathode emission due to movement of atoms within the cathode structure. In oxide-coated cathodes, it occurs primarily at the interface between the oxide layer and the base metal of the cathode, which is generally a nickel alloy. Some alloys are much better than others in this respect, showing a difference of a factor of 20 or more [Smullin59, p65]. A high silicon content increases flicker noise, but unfortunately has advantages in the manufacturing process and so tended to be widely used. The cathode alloy was chosen by each manufacturer, and does not form part of the specification of a particular tube type, which explains the wide variation about tubes from different manufacturers. Smullin [Smullin59] indicates that European manufacturers tended to use alloys which are better in this respect.

Pure tungsten filaments generate flicker noise in a different way, resulting in a noise spectrum which is 1/f ² rather than just 1/f.

Partition Noise

The third source of noise applies only to tetrodes and pentodes, and explains why pentodes are noisier than triodes. The presence of the positive screen grid means that some of the current (typically 10-20%) from the cathode goes to the screen grid rather than the plate. However this division of current fluctuates randomly, just as the current itself does. This very slight random variation in the plate current is called partition noise.

The effect of partition noise is to change the equivalent noise resistance from the simple formula given above to:

where:

Iscreen = screen grid current

In practice, this results in three to five times the noise of the equivalent triode. Connecting a tetrode or pentode as a triode eliminates partition noise, since now the two current flows are recombined.