WE417 electron tube with grid at space potential tests

At your risk. Any voltage or current can be very dangerous, all safety precautions should be strictly followed.


When 6.8V heater voltage is connected grid wrt cathode potential becomes more than -700mV. This is possibly due to cathode becoming positive by emmited electrons, positive protons remaining to it.


High tension power supply is 370V.


Cathode is connected to ground or Vb minus.


Grid is unconnected or floating or at space potential.


A 47μF 400V capacitor is connected across cathode.


The positive side of said capacitor is connected to red cable probe of a mA meter. Black cable probe is connected to anode. So anode voltage can be supplied and anode current can be monitored.


A voltmeter is connected across said capacitor.


A few watt 47KΩ series resistor from high tension charges capacitor.


Said resistor can be interminantly connected to capacitor in order to adjust voltage on capacitor.


So the anode is fed by a high output impedance power supply. Almost a constant source. If suddenly a high anode current might flow there would be a great voltage drop at series resistor thereby lowering anode voltage and protecting the electron tube. It can be thought as a 47KΩ load resistor. A current limiting resistor. (Initially a 1MΩ resistor had been tried which obviously created a low frequency oscillation, about 1Hz.


When anode voltage is say 24V there is very low anode current.


At about 70V it might be less than 1ma.


But at 86V anode potential the anode current became about 5mA. Floating grid voltage remains to less than -700mV. Actually potential is increased by the shunt effect of the voltmeter input impedance. In other words grid potential is actually more negative than measured. Voltmeter takes electrons from grid and sends them to electron deficient cathode while making the measurement. This can also be confirmed by anode current increasing when connecting the voltmeter. Bias is reduced, anode current is increasing. The negative lead of the 10MΩ voltmeter is connected to cathode. In other words returning grid circuit to cathode is pull up bias too as cathode is positive with respect to grid.


The WE417A electron tube should power amplify audio at 86V, 5mA conditions.


Changing to 390KΩ. Anode current became 6mA. With 270KΩ, anode voltage is 88V, anode current 8.5mA.


Changing to 10KΩ series PSU resistor . Anode voltage is 105V, anode current is 19mA. This looks great for a class A (electrons flowing all the time) music signal power amplifier. Would it work? How would it sound like?


This post is continued on next post. Output transformer is connected.