Would electron tubes surrounded by no light make even lower anode voltage operation possible?

It has just been observed:


http://euroelectron.blogspot.gr/2018/05/why-are-ther-potential-differences.html


that if an electron tube is surrounded by light and the heater voltage is applied then an electrode away from the cathode becomes negatively charged relative to the cathode. So far a 1GΩ voltmeter has been connected between cathode and anode of an EF183 in a dark room.


When 5 volt at heaters is applied and a torch shines on the electron tube a -40mV potential difference is observed. The anode is 40mV negative with respect to the cathode. (Under daylight conditions a many hundred millivolts negative voltage is observed on the control grid. And this is why a high Megohm resistor is used between anode and control to make operation possible of the Pleiades V6 at only 4 volts.)


But not (the -40mV disappear) if a torch is not shinning on the electron tube.


When in dark the potential is near zero.


Next the cathode to grid potential must be measured with parameters the cathode been heated and light shinning on the tube.


The grid of the EF183 was always observed to become negative when the cathode was heated up. But this was so far done at daylight. It never occurred to try the experiment after sunset.


Would a tube enclosed in a box operate better or with even lower anode voltage?


Perhaps even less than 3V or with no anode to grid high Megohm resistor needed to neutralize the negative self induced bias by the heated electron emitting cathode?


Is this another reason some tubes like 6SK7, metal enclosed, operate relatively easily with a few anode volts?
Same for some German tubes including VF14?


Electron tube microphones are rare cases of amplifiers were the electron tube is in light sealed container.


Next comes the question of the red glowing cathode light, is it affecting the other electrodes?


Is this another reason why Georg Neumann reduced the heater voltage on the Neumann U47 microphone?


The Pleiades V6 is planned to have the EF183 inside a Hammond aluminum die cast orange octagon box. Would this affect the performance? Should a different anode to grid biasing resistor be used?


(Later addition. Cathode to grid potential was now measured by connecting the 10MΩ voltmeter. Note only a voltmeter and a heater circuit is connected to the electron tube. When light shined on the electron tube by the torch about 20mV appeared on the grid. When then a 5V heater voltage was connected to the heaters the grid voltage with respect to cathode started to decrease up to - 700mV. Shinning light or not did not make almost any difference. Perhaps slightly decreasing the negative bias by 20 mV.


It can then again be seen how negative the grid becomes by the cathode heated or rather the cathode becomes positive by the missing electrons.


Hence then need of the external high Megohm bias resistor from anode to grid to make operation possible of the Pleiades V6 etc preamplifiers with just 4 volts at the anode.