On the Pearson paper on noise in electron tubes:
"... Since the noise of thermal agitation is always present, the equation gives the absolute minimum to which fluctuation noise in an amplifying tube can be reduced after all other causes have been eliminated. It shows that for an ideal low noise tube in which thermal noise in the plate circuit is the limiting factor, the noise level may be reduced by a decrease in the cathode temperature, a decrease in the effective frequency band or by an independent decrease in the plate resistance or increase in the amplification factor..."
Does this apply to,the Pleiades V6 head amp? Reduced heater voltage gives in practice a better signal to noise ratio. Also the Pleiades bias (anode bias) immediately decreases plate resistance by increasing anode (plate) current at a very low anode voltage. (A very low anode voltage makes space charge saturation possible at very low cathode temperature so there is no shot noise).
When connecting the Pleiades bias resistor the amplifier immediately operates at a phenomenal signal to noise ratio.
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