Electron Tubes operating with much less than 3 volts at the anode, even millivolts

So far a lowest useable limit can be 500mV at the anode.


Why not? If the electrons with Pleiades bias are free to pass trough the grid they will arrive at the anode even with such a small electric field produced by 500mV.


Bravo to Hliana.


For this experiment the Pleiades V6 microphone head amplifier was used in breadboard form.





With Cc=22nF, Pleiades bias resistor 2MΩ


Signal Path, setup:


Birds singing outdoors - Shure Unidyne III indoors with closed window - Pleiades (130Ω,40mH) gentle low cut filter - Pleiades V6 head amp with Altec 4722 input transformer - Sony TC-D5 Pro at XLR mic in (input set to mono) - Sennheiser HD580


The birds could still be heard through the system at 500mV. The lower the anode voltage the less the bandwidth and gain.


How was the anode voltage varied? It was varied exponentially in infinte steps by nature itself.


The Vb=3.9V rail from the 3 AA 1.2V rechargeable batteries was (for the anode circuit only) passed through a 100Ω resistor shunt terminated by a 220μF electrolytic capacitor. The voltage across the capacitor was used for anode supply ie connected to the output transformer primary. As the 100Ω is disconnected from the breadboard, the voltage acroos the capacitor passes through all values of voltage from 2.9V downwards to the limit of 0 volts. A YFE high Z digital voltmeter was connected across the capacitor terminal while sound was monitored through the HD580 headphones. Attention must be paid to ear protection as strong click sounds will appear especially when the Pleiades bias resistor is changed. So the mic in volume control (level control) on the Sony should be really low especially through changes and headphones unworn.


The experiment can be repeated by recharging the capacitor by reconnecting the 100Ω resistor.


As soon as discharging started the anode current quickly dropped from about 50μA but the 7586 Nuvistor electron tube continued to operate.


Better results were obtained with a Pleiades bias resitor of 2MΩ instead of 6MΩ. But the electron tube was operating quite well too with the 6MΩ anode to grid bias resistor.


Some further underheating of the cathode by removing the heater rail was tried with promising results.


The less the anode voltage, the less the gain. It was compensated by turning clockwise the Sony recording level control.


The sound was interesting as it made the microphone sound more focused possibly because of less bandwidth.


At some inbetween value from 500mV to 3V the sound is very natural.


The less the anode voltage the less the bandwidth until it sounds like a good telephone.



The signal to noise ratio seemed to get worse below 1 volt. (Bandwidth reduction must also be taken into account which helps the signal to noise ratio)


If an 2DS4 or 2DV4 Nuvistor electron tube operates similarly to the 7586 with Pleiades bias, a front end microphone amplifier is possible with just one 1.2V battery supplying both heater and anode circuit.


Further reading:


The Pleiades Bias


On preserving transconuctance of electron tubes at an anode potential as low as 3V - euroelectron


Operating Features of the Audion - E. H. Armstrong