On adjusting for pull up, pull down or space potential bias on electron tubes

Once the cathode is heated and emits electrons it becomes more positive than the grid. Typically tyengrid becomes -700mV for an EF183 even if it is underheated by 5V applied to the heaters.


The electron tube can operate at this grid space potential. See for example the 1H4 operating as a mic booster amplifier with 9V at the plate using the Pleiades V6 schematic.


If cathode is connected to ground and a high (order of Megohms) resistor is connected to ground (ie cathode) then we have pull up bias and the electron tube can operate with a low plate potential. The above example would have more anode current.


If we return the resitor to plate or other source of positive potential (typically 6V?) then we have further pull up bias (the grid is still negative wrt cathode typicaly -70mV). The electron tube can operate with 4V or even 1.5V at the plate.


If we connect a cathode resistor to ground, depending on the current flowing and the value of resistor, if the grid return resistor is returned to ground we may have pull up or pull down bias. If the potential drop by Ohm's law on the Rk is high enough it would be pull down bias. This is usually called negative bias. The electron tube can operate even with hundred of volts at the anode without the anode current be extreme.


There must be a value of Rk where the potential across Rk is the same as the negative open grid bias mentioned in the first sentence. Connecting a grid return then from the same external potential to the same grid space potential should produce no current flow ie no grid to cathode current.


Perhaps more experiments can be made by a variable bias source spanning all the values from pull down to free grid to pull up bias.


(Always use a fuse in series with batteries for safety).


A way could be 2 AA batteries in series and the centre point connected to ground. Then a potentiometer across 3V. Then a high Megohm resitor connected to the pot wiper. The other side of the high Megohm resistor connected to grid. The signal to grid of course in most of the cases described in this post should be supplied with series capacitor for DC isolation.


Another way could be even a potentiometer connected from cathode to ground. The wiper connected to grid through a high Megohm resistor.


Of course useful experiments can also be made by keeping the grid at space potential and starting with a very low anode potential, then increasing it until best sound and s/n is obtained. With an 1H4 even at 9V plate potential and only 10μA anode current the sound was impressive and big. See relevant post:


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


(Later addition): the grid space potential increases when anode voltage is applied and can be positive:


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


Further reading:


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