Testing if a preamplifier is low noise

If we connect across its input a 200Ω resistor in parallel with a switch...


When the switch is closed there should be extremely little noise at the output of the preamplifier as the input is short circuited and there is no thermal noise at the input.


When the switch is opened, ie 200Ω across the input,  the thermal noise hiss should increase.


This apparently means the preamplifier amplifies the Johnson's thermal noise of R=200Ω.


The lower the value of R that can make a difference (between short circuit and R) in noise the better the noise figure of the preamplifier?


An ideal preamplifier should produce 0 noise when a short circuit is at the input?


But there is always some noise and this apparently brings in the concept of equivalent noise input resistance.


Is it correct in saying that a real world preamplifier with R equivalent input resistance noise behaves as an ideal preamplifier with R at its input as a noise source?


The Pleiades V4 pre preamplifier has extremely low hiss when a short circuit link is connected between ground and the capacitor that feeds the control grid of the EF183. Anode voltage is a bit more than 3 volts. A high Megohm resistor from anode to grid frees electrons and makes fantastic and smooth operation at such low plate voltage. Heaters are underheated supplied by around 3V instead of 6.3V. This apparently helps freeing electrons as less cloud induce less negative charge on the control grid.


The very low anode and heater voltage minimize secondary emission effects.


The cathode does not glow much so there is least photo emission from grid.


Electrons strike the anode at low speeds so there are no soft x rays produced, therefore no grid emission from x Ray photons.


Electrons travel at low speeds so there is least chance of secondary emission if they collide with a gas molecule.


So is the only source of noise in such an amplifier any irregularity on the about 1μA current from the electrons drawn from grid to anode by the external (about 4ΜΩ) resistor?


Or is the noise coming from the 10KΩ anode load resistor?


The Pleiades V4 as well as the other Pleiades V series preamplifier are very quiet and it is a joy to hear such a low and creamy smooth noise with a very dynamic and strong signal sound.



Reference: The Use of Multigtid Tubes as Electrometers - J. R. Prescott The Review of Scientific Instruments Volume 20 Number 8 August 1949