The WE417A preliminary tests at open grid

At your risk, take all safety precautions.


The amplifier is currently on the balcony.


The input impedance of the voltmeter used is very high to high (it depends on automatic range scale). From 1GΩ to 10MΩ.


Firstly measuring cathode to grid voltage with grid open and no power supplied. It is just 2-3mV. But today it is very cloudy. If there was direct sunlight the photoelectric effect may had produced hundreds of millivolts.


When supplying 6.8V at heaters the potential of grid with respect to cathode decreased to -700mV. Actual value is expected to be less than -1V. Systematic error due to the lowering of input impedance of the voltmeter used as range scale automatically increases and impedance lowers down. Using an electrometer or pH meter should give the correct potential of grid. The space potential?


Why is the grid negative anyway when heater voltage is applied?


As soon as cathode temperature increases electrons are thermo emmited (boiled off) from cathode.


But number of positive protons inside cathode remains the same. So cathode becomes positive. Or in other words grid becomes negative with respect to cathode.


Another way to look at it may be the (negative ) electron cloud near the grid.


A negative bias without even touching the grid may be good news for starting supplying an anode potential, carefully increasing from 12V.


(In fact reducing this already negative bias by an external high Z anode to grid resistor is what makes very low voltage operation possible on the Pleiades V6 low noise microphone booster or front end amplifier.)


Increasing the plate supply more than needed can have a very dramatic effect on an open grid amplifier as at some point the anode filed will start to penetrate the grid. Anode current may inceease too suddenly.


Anyway, how would this amplifier sound like if increasing the anode potential to such a value that anode current (at open grid) becomes say 20mA? Cathode connected to ground.


Grid at space potential.


The music signal coupled to grid through a capacitor.