This discussion assumes small anode voltage.
It was shown on a previous post that when the cathode of an electron vacuum tube is heated the grid becomes immediately an EMF source of voltage wrt cathode of a 4.7KΩ output impedance. This Zout value is a particular value measured on an EF183 electron tube.
This phenomenon should be possible to be verified on any tube by carefully connecting a heater voltage and then a voltmeter between cathode and grid.
In fact it is the cathode that has become a source of positive voltage as it has lost many electrons to the cloud outside it.
If the secondary of an input transformer is connected directly to grid and cathode all this current (typically 200μA for a EF183) will travel through the secondary back to cathode. The negative voltage on grid that is forced to become zero when the secondary is connected is typically -900mV.
But what if a coin battery is connected in series with the secondary so that it directs grid to the positive (not negative as it is shown in typical schematics working with high anode voltage)? Ie the positive side of the battery towards the grid.
This EMF could be arranged to exactly cancel the EMF inside the tube. And with it the current, that may be more appropriate in this particular case to think of as cathode current rather than grid current. Of course Kirchoff's current law aid us in understanding more. When the tube operates the cathode current should be the grid current plus the anode current. The external return circuits send the electrons back to the cathode.
A value of external EMF may be found that does not affect the DC self assumed condition of the grid whether the transformer is connected or not.
Would this zero external grid cathode current make an even lower noise preamplifier?
The anode voltage may have to be increased from the typical 6V used on previous experiments with Pleiades pre amplifiers.
Would this current null neutralization make an infinite grid input impedance?
Would it make an amazing electrometer?
When grater anode voltages are used is there a grid current in opposite direction by positive ions for example?
Was Georg Neumann trying to balance these two currents in order to get zero current flowing to grid? The variables were, how underheated VF14, the anode voltage, the grid leak resistor etc.
When disconnecting the capsule on the Neumann U47 microphone hiss is so small and beatifully sounding. Almost of velvet like quality without high frequency exaggeration.
There is an excellent paper describing very low noise operation of electron tubes without any grid leak resistor at all.
It appeared on the Electronics magazine shortly before 1947. It is likely that Neumann read it.
A grid leak resistor plays in fact the role of cathode leak? As electrons rush through it to return to cathode from the outside of the tube to cancel the positive potential created by those missing that are making the electron cloud...
Reference:
Open-grid tubes in Low-Level Amplifier - Robert J. Meyer - Electronics - Oct 1944
It was shown on a previous post that when the cathode of an electron vacuum tube is heated the grid becomes immediately an EMF source of voltage wrt cathode of a 4.7KΩ output impedance. This Zout value is a particular value measured on an EF183 electron tube.
This phenomenon should be possible to be verified on any tube by carefully connecting a heater voltage and then a voltmeter between cathode and grid.
In fact it is the cathode that has become a source of positive voltage as it has lost many electrons to the cloud outside it.
If the secondary of an input transformer is connected directly to grid and cathode all this current (typically 200μA for a EF183) will travel through the secondary back to cathode. The negative voltage on grid that is forced to become zero when the secondary is connected is typically -900mV.
But what if a coin battery is connected in series with the secondary so that it directs grid to the positive (not negative as it is shown in typical schematics working with high anode voltage)? Ie the positive side of the battery towards the grid.
This EMF could be arranged to exactly cancel the EMF inside the tube. And with it the current, that may be more appropriate in this particular case to think of as cathode current rather than grid current. Of course Kirchoff's current law aid us in understanding more. When the tube operates the cathode current should be the grid current plus the anode current. The external return circuits send the electrons back to the cathode.
A value of external EMF may be found that does not affect the DC self assumed condition of the grid whether the transformer is connected or not.
Would this zero external grid cathode current make an even lower noise preamplifier?
The anode voltage may have to be increased from the typical 6V used on previous experiments with Pleiades pre amplifiers.
Would this current null neutralization make an infinite grid input impedance?
Would it make an amazing electrometer?
When grater anode voltages are used is there a grid current in opposite direction by positive ions for example?
Was Georg Neumann trying to balance these two currents in order to get zero current flowing to grid? The variables were, how underheated VF14, the anode voltage, the grid leak resistor etc.
When disconnecting the capsule on the Neumann U47 microphone hiss is so small and beatifully sounding. Almost of velvet like quality without high frequency exaggeration.
There is an excellent paper describing very low noise operation of electron tubes without any grid leak resistor at all.
It appeared on the Electronics magazine shortly before 1947. It is likely that Neumann read it.
A grid leak resistor plays in fact the role of cathode leak? As electrons rush through it to return to cathode from the outside of the tube to cancel the positive potential created by those missing that are making the electron cloud...
Reference:
Open-grid tubes in Low-Level Amplifier - Robert J. Meyer - Electronics - Oct 1944
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