A source of confusion in electron tubes

(Use all safety precautions at any potential)


is the very useful fact that all electrodes are referenced to cathode.


So cathode is arbitrarily assumed at 0V.


Another fact is that electrons emmited from the cathode make it positive as there are remaining positive protons.


A heated cathode is by default more positive that the control grid. Or the grid is by default more negative than the cathode. Ie it is by default negatively biased. Even at relatively low cathode temperature the grid bias or grid potential or grid voltage make be less than -600mV.



To repeat, if the positively charged cathode is assumed at 0V then the grid is at negative potential.


So why do we need to negatively bias the grid?


In fact we may not always need to.


We need to make the grid bias more negative when we wish to operate the electron tube at a potential of hundreds of volts at the anode. We may need this in order to get more power output. Or when we need to have a grid of very high input impedance.


By if we wish to make a front end amplifier just to boost the signal of a mic and then drive another preamplifier then we do not care much about output power. All we care of is a great loud sound, sensitive response, a dynamic sound and of the lowest possible noise.


An example front end mic head amplifier is the Pleiades V6 which can operate even with 1.5V at the plate.





By using pull up or Pleiades bias the grid is still negative but much less so allowing arrival of electrons to the anode and therefore music signal amplification.


To summarize, the grid is negative as soon as heater voltage is applied to an electron tube since cathode temperature rises and electrons are emmited with various velocity values from the warm cathode.


In general there are the following options:


The grid is pulled up or Pleiades biased for operation between 0V (the plate might even be slightly negative as electrons have momentum?) and say 9V plate voltage.


The grid is left open or at space potential (negative bias by nature itself) for operation at potentials between say 9V and 40V.


The grid further negative biased (for example by a cathode resistor making the cathode even more positive) for plate operation between 40V and say some hundred of volts.


When we connect a pull up resistor such as shown in the Pleiadss V6 schematic there is a few μA grid current following from grid to to positive supply in this case anode. This does not not seem to produce noise. The Pleiades V6 is exceptionally low noise. See Grampian DP4/L microphone experiments.


If a high Megohm resistor is connected from grid to cathode (the cathode is positive) there is grid current from grid to cathode. This is the electrons which are attracted by the positive cathode (protons and electrons are attracted) so electrons come back to externally to their beloved protons through the grid cathode resistor. A 12AU7, ECC82 can be operated at 12V or less plate volts by a grid pull up resitor (say a few Megohms) from cathode but it can possibly operate more easily with further pull up from grid to anode or Vb and using a higher resitor. Or a lower resitor and much less anode potential etc. See Pleiades V3 etc.


We can bias the grid even more negative than it is by pulling up the cathode further up trough a cathode resistor. If we would then connect a high Megohm resistor from grid to cathode more grid current would flow as we are effectively trying to further pull up the grid (how would this sound for low plate potentials?). But what we usually do is connect the grid through a resistor to ground. So the grid remains heavily negatively biased.



So there is no grid current as the electron tube is now like an electrometer tube with a very very high input impedance. But we now need a fairly high plate potential for operation. Electrons move at great accelerated velocities, secondary emission takes place, positive ions might be attracted to the too negative grid and grid current now might reverse direction. This process may be noisy.


At the moment some of the Pleiades experiments concentrate in the case of floating grid or open grid or grid at space potential where grid current is 0. Such experiments concentrate at keeping using low plate supply. A low plate supply seems still possible as the positive anode field penetrates the field around the grid and cathode [Deketh]. Promising results with sensitivity and possibly low noise are so far obtained with the 1H4 operated by 1.3V heater voltage and 9V plate voltage on the Pleiades V6 schematic. For further reading please see the recent euroelectron posts.


Reference:


Fundamentals of radio-valve technique - Deketh - Philips Technical Library


Some further reading:


The Pleiades bias - euroelectron


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


Low Plate Potential Tubes - C. E. Atkins - Radio and Television News - January 1957