When the cathode of an electron tube starts emitting electrons the grid starts immediately becoming negatively charged with respect to cathode.
In fact the cathode is charged positive as it has lost electrons to the cloud outside of it.
This effect limits anode current at low voltage. Therefore it limits transconductance.
This of course means that the ability of the grid to control current is restricted.
But what if an external source of DC voltage is carefully connected to the grid through a very high Gigaohm resistor.
The negative potential of the grid will be neutralised and anode current will increase together with transconductance ie amplification ability.
The external DC source may have to be quite high so all necessary precautions need to be taken. And the higher it is, the higher the grid resistor that can be used to get the grid from negative to as close as zero volts?
Would by experiment values be found that make the electron tube behave as an infinite impedance to DC? Can the grid leak be omitted as described on the next post?
Is there an opposing grid current source? Could they cancel each other by appropriate choice of external applied voltages or components values?
A variable is also the heater voltage as it beautifully controls or rather creates sensitivity on a Neumann U47 microphone.
These concepts are also used in the Pleiades V series low level amplifiers or microphone preamplifiers using a few only, 6V for example at the anode. On the Pleiades V5 4.7MΩ is connected from anode to grid to free electrons. Pleiades schematics and concepts are open source.
More on the next post...
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