The only connection to the EF183 being the heater voltage.
Heaters supplied by Vh=5.9V.
Then a high impedance digital multimeter was connected between grid and cathode.
When the cathode is heated at max temperature:
Vg=-855mV !!
Ig=-185μA !!
In fact when measuring current from cathode to grid in this way we effectively short circuit from cathode to grid.
So by Thevenin's theorem the grid to cathode terminals behave like a voltage source of output impedance Vg/Ig.
This is aproximatelly 4.7KΩ !!
So in fact when we measure grid current in this way we effectively measure cathode emission current intercepted by grid. A nice simulation to see emission is by keeping a glass under hot water for a few seconds and then holding the glass and observing the steam coming out violently in a cold room.
When the cathode temperature is gradually reduced by disconnecting Vh, Ig decreases but Vg initially is reduced and then remains constant at about -300μV (systematic error?, why does it not stay at -800mV?).
The value stays constant as the cathode to grid structure is a charged capacitor.
So at cold heater temperature Vg/Ig approaches infinity ie the grid to cathode output impedance approaches infinity.
Of course when Vg is short circuited to become zero and then the electron tube cathode is heated we observe the grid potential gradually decreasing from 0V until it stabilizes at -855mV with the Vh voltage used.
These observations explain why a grid bias resistor from a positive source is needed to neutralize the control grid and make possible the electron tube operationing at small transistor like voltages. Very low noise performance can be obtained too as observed on the Pleiades V series preamplifiers. (Reduced cost, low power consumption and small weight is an advantage too dispensing with complicated and noisy power supply in favor of just a noisiless single battery for both heater and snode).
These observations also explain why when reducing the heater voltage on the Pleiades V6, V5, V4 series preamplifiers the gain increases and noise goes further down. As is done on the Neumann U47 microphone with its underheated VF14.
It is a great playing with electrons.
Heaters supplied by Vh=5.9V.
Then a high impedance digital multimeter was connected between grid and cathode.
When the cathode is heated at max temperature:
Vg=-855mV !!
Ig=-185μA !!
In fact when measuring current from cathode to grid in this way we effectively short circuit from cathode to grid.
So by Thevenin's theorem the grid to cathode terminals behave like a voltage source of output impedance Vg/Ig.
This is aproximatelly 4.7KΩ !!
So in fact when we measure grid current in this way we effectively measure cathode emission current intercepted by grid. A nice simulation to see emission is by keeping a glass under hot water for a few seconds and then holding the glass and observing the steam coming out violently in a cold room.
When the cathode temperature is gradually reduced by disconnecting Vh, Ig decreases but Vg initially is reduced and then remains constant at about -300μV (systematic error?, why does it not stay at -800mV?).
The value stays constant as the cathode to grid structure is a charged capacitor.
So at cold heater temperature Vg/Ig approaches infinity ie the grid to cathode output impedance approaches infinity.
Of course when Vg is short circuited to become zero and then the electron tube cathode is heated we observe the grid potential gradually decreasing from 0V until it stabilizes at -855mV with the Vh voltage used.
These observations explain why a grid bias resistor from a positive source is needed to neutralize the control grid and make possible the electron tube operationing at small transistor like voltages. Very low noise performance can be obtained too as observed on the Pleiades V series preamplifiers. (Reduced cost, low power consumption and small weight is an advantage too dispensing with complicated and noisy power supply in favor of just a noisiless single battery for both heater and snode).
These observations also explain why when reducing the heater voltage on the Pleiades V6, V5, V4 series preamplifiers the gain increases and noise goes further down. As is done on the Neumann U47 microphone with its underheated VF14.
It is a great playing with electrons.
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