Why tube bias?

Attention to any voltage. All safety precautions should be strictly followed.


We should not be biased.


Perhaps it might be the same for electron tubes.


Why should they be biased?


What does bias mean? Does it mean a voltage?


By definition the grid potential is the grid potential with respect to cathode [Gray]. So to avoid confusion it is not the potential difference between grid and ground or 0V. It is the potential difference between grid and cathode. This is what matters in electron tube operation. This is why we usually isolate the grid from DC by a coupling capacitor. This is why grid to cathode current flows when we return a resistance to grid. There was already a voltage at grid with respect to cathode. It is generated by the missing electrons from the cathode moving towards the grid. Missing boiling electrons from the cathode leave positive protons behind. So cathode is positive wrt grid or grid is negative. When we close the circuit by a grid return resistive path, negative electrons take this path to return to their beloved positive protons, A current flows.


When the cathode is heated up electrons are emmited and the grid becomes negative. This is its natural electric potential under such conditions. Why not leting it be at that potential?


So the question is:


Do we pull down the grid by what is called negative bias?


Do we pull up the grid potential (still negative but less negative or closer to 0V) by what is called in euroelectron Pleiades bias?


Or do we leave it as is, let it be at its space potential?


1st option allows operation of an electron tube at high to very high plate or anode potentials. More than a hundred volts even for small electron tubes.


2nd option allows operation at very low plate potentials, less than 6V. See Pleiades V series front end amplifiers.


3rd option, free grid, or floating grid or open as it is called grid or grid at space potential allows operation at typically between 10? and 70? volts. Is this what Georg Neumann had in mind when he supplied the VF14 anode of the U47 microphone amolifier by approx 40V. Is the cathode resistor designed to neither pull up, or pull down the grid letting it keep its space potential? Would anode current stay the same if the 60MΩ return resistor was disconnected? Why did Georg Neumann suply the heaters of the VF14 with just 35V instead of 60V? Did he want to reduce the negative space potential at the grid [Atkins] by the boiling electrons? Did he just disconnect heaters and heard an increase in signal to noise ratio?



See also,that the grid space potential can increase when anode voltage is applied:



http://euroelectron.blogspot.com/2018/08/the-grid-space-potential-increases-when.html


References:


Applied Electronics - T. S. Gray - MIT


Low plate potential tubes - C. E. Atkins - Radio & Television News - Jan 1957



Patent US2850674 - Carl E. Atkins


The Pleiades bias - euroelectron


Open-grid tubes in low-level amplifiers - Robert J. Meyer - Electronics Oct 1944


See also some preliminary mic head amp tests:


http://euroelectron.blogspot.com/2018/07/1h4-with-grid-at-space-potential-mic.html