A voltmeter will be connected between cathode and grid of the Nuvistor 7586 electron tube.
The reading will be zero.
The heater voltage will then be applied. As soon as the cathode heats up and emits electrons the grid is expected to become much more negative than the cathode by the electron cloud which has left the cathode.
How negative?
2 voltmeters were used. One digital with an input impedance of 10MΩ at the range greater than 333mV and 1000MΩ at the range less than 333mV. The other voltmeter is a passive analog.
The heaters were supplied by 4 freshly charged AA Sanyo Eneloop rechargeable batteries. A fuse must be used for safety. 5.5V is the total open circuit voltage. A variable resistor was connected in series to limit high surge heater current. Initially the heater circuit was not operating. It turned out to be a bad contact problem due to lead aging. It was self cleaned by slightly rocking the Nuvistor tube in its socket.
Even with a series heater resistor the digital voltmeter showed more than 300mV, the grid negative with respect to cathode. Once full 5V were supplied to the heaters and after the warm up time the grid potential reached about -750mV to -800mV maximum.
The plate (anode) potential wrt cathode reached more than -200mV.
Once the heater current was removed and once in the 1GΩ input impedance range the potentials stayed almost constant. This is expected as the electron tube is a charged capacitor. And there is no light falling on the electrodes (sealed tube), so there is no photoelectric effect emission discharge.
When the passive analog needle voltmeter was connected in shunt the max value of grid voltage was -350V. This is due to the voltage drop at the electron tube to drive the load. Or in other words the energy needed to move the needle, (heat dissipation in the needle moving coil).
The input impedance of the analog voltmeter was measured with the digital multimeter to be 50KΩ.
Since the open circuit voltage of -750mV drops to - 350mV when the 50KΩ load is connected shunt and the fact that the closed circuit voltage is roughly half indicates that the output impedance of the heated electron tube as a voltage source from cathode to grid is roughly 50KΩ.
So this Nuvistor tube when heated by 5V becomes a voltage source of -750mV (grid negative wrt cathode) and output impedance of 50ΚΩ.
By looking at the datasheet there is no way the electron tube will operate at a few anode volts with such a large negative (cutoff) grid voltage. So grid bias will be applied from anode to grid with a high Megohm resistor in order to reduce this internal negative bias.
This 7586 Nuvistor electron tube is already with flying leads conneted to its base and it will be temporarily connected to the Pleiades V6 experimental jig in place of the EF183 electron tube.
The EF183 has its grid charged to comparable negative charge when 5V are applied to the heaters. It operates greatly as a microphone head amplifier with just 3.9 volts at both anode and heaters when Pleiades bias is applied by a 6MΩ resistor from anode to grid.
Will the Nuvistor tube operate as well?
(Later same day addition). One of the batteries was removed. The heaters were therefore supplied by 3.9V closed circuit voltage instated of 5V. The grid became -350mV. Connecting the analog voltmeter the reading dropped to -70mV. This indicates a significant increase in output impedance. Expected as less energy is supplied by the batteries to heat the cathode. By the principle of concervatiin of energy the energy supplied by the batteries equals the thermal wire losses, heater expansion, head radiation, analog meter movement, hear dissipation in moving coil,...,etc.
See also part 2:
http://euroelectron.blogspot.com/2018/06/do-electrons-get-into-grid-when-cathode.html
References:
Pleiades V6 schematic - euroelectron
The Pleiades bias - euroelectron
Operating features of the Audion - E. H. Armstrong
On preserving transconductance of electron tubes at anode voltage or potential as low as 3 volts - euroelectron
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