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Thermal Noise
All electronic, and indeed electrical, devices produce noise. The most fundamental, and unavoidable, form of noise is called thermal noise or sometimes shot noise. It occurs because the flow of electricity is actually due to individual electrons. Even though the number of electrons is huge, there are still statistically predictable fluctuations, just like traffic on the highway. Because it is random, this noise occurs equally at all frequencies. The noise voltage developed by any conductor or resistor is given by:
| $e_{noise} = 2\sqrt{kTR\Delta _{f}}$ | where: | $e_{noise}$ = noise voltage $k$ = Boltzmann's constant, 1.38·10-23 $T$ = temperature (°K) $R$ = resistance $\Delta_{f}$ = bandwidth |
For example a 1kΩ resistor at room temperature (approx. 300°K), across the audio bandwidth from 0-20kHz, produces a noise voltage of about 0.6µV. When dealing with very small signals, such as a received radio signal or a phono pickup, this is a significant amount of noise
Thermal Noise in Tubes
There is a 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:
| $R_{noise} = \dfrac{2.5}{G_{m}}$ | where: | $R_{noise}$ = equivalent noise resistance |
In other words, the noise produced by a tube is inversely proportional to $G_{m}$. This directly explains the later 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. It has also been considered for sensitive phono stages, though its other noise sources (see below) may be an issue.
In addition to thermal noise, tubes generate noise in two other ways, called flicker noise and separation noise.
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 increasing 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 in 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 $1/f$ - in other words, the audio component is even more predominant.
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:
$$R_{noise} = \frac{2.5}{G_{m}}\left( 1+8\frac{I_{screen}}{G_{m}} \right)$$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.
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