A misconception of automatic cathode bias by a cathode resistor

Referring to the usual amplifier circuit with a cathode resistor connected between ground and electron tube cathode.


It is not that the Rk increases the potential of the cathode.


It is that the already positive potential of the cathode by the electrons left to the electron cloud is decreased at the other side of the resistor by voltage drop (ohms law).


This other side is connected to ground and the grid is connected there too.


The misconception may have arisen from the fact that tubes were usually operated at high anode potentials. We were always interested in further increasing the negative bias and this effect may had been overlooked. That the bias was already negative!


It is very easy to verify that the cathode is already positive or the grid negative by just connecting only the heater voltage to an electron vacuum tube. Any voltmeter even of low input impedance will show a significant of few hundred millivolts negative potential at the control grid with respect to the cathode.


It is this internaly produced negative bias that blocks operation of an electronic valve or electron tube at low anode voltage.


To circumvent this situation and make an electron tube operate at low battery anode voltage at least 2 ways are possible.


1. Connecting the secondary for example of the input transformer to grid. Then adjust exoerimentaly the bias by a variable cathode resistor. At very low values of Rk the tube will operate normally as the positive as explained above cathode will negate through Rk the negative bias of the grid. See Pleiades V-1 booster microphone preamplifier. More investigation is needed as there may thermionic current flowing in the grid circuit through the secondary winding. Another way of looking at this example is that the cathode resistor with its lower side being connected to grid acts as an electron accerating resistor as effectively we connect a positive potential (the cathode) to grid through a resistor. This is why when Rk becomes 0 the anode current becomes so excessive even using just 6 volts anode potential.



2. Another way to make an electronic valve operate at low Va is connecting the secondary of the input transformer for example, through a coupling capacitor to the control grid. As explained above the grid is already negative because the heater voltage is applied and there are electrons boiling off the cathode towards the grid forming the electron cloud or space charge. An anode to grid resistor of a few Megohms can be used to negate this negative bias and make the electron tube to operate at a very low anode potential, typically 4 volts on a triode connected EF183. Using the above technique the grid bias may still be negative but less so.


Note: Whenever we say grid bias we mean the voltage of the grid with respect to cathode.


Under heating the cathode has a beneficial effect on the above described techniques as the electron cloud is reduced.



References:


Operating Features of the Audion - E. H. Armstrong


Pleiades V-1 - euroelectron


Pleiades V6 schematic - euroelectron


On preserving electron tube transconducatnce at anode potential as low as 3V - euroelectron