How to operate electron tubes (vacuum tubes or electronic valves) at small anode potential (low voltage)

At your risk. Please take all safety precautions (for example left hand always in the back pocket, a fuse in series with a battery etc). When the cathode is heated it emmits electrons, so it becomes positive wrt grid or the grid negative wrt cathode. This intrinsic (negative) bias usually more than -1Volt (measured accurately with an extremely high impedance voltmeter) can cut off anode current at low anode potential. If we use a lower impedance voltmeter to measure this contact potential in fact we increase anode current by pulling up the negative grid by the positive cathode. A cathode resistor on the other hand will decrease this pull up bias. How can we make an electron tube operate at a low anode potential say 6V to 20V? Cathode is connected to ground. We can leave the grid floating (at space potential) and couple the input signal by a high quality capacitor. Depending on the mu (amplification factor) of the electron tube, if mu is say 10 then the penetration of the anode electric field above say 12V will make the grid less negative and the electron tube will operate with a healthy anode current. Advantages are extreme low noise by battery (in series with a protective fuse) operation, low temperature of electrodes leading to low secondary emission, low heater current, low radiation photoemmision from one electrode to another, low electron velocity therfore low secondary emmision by bombardment of an electrode by high velocity electrons. Less ionisation of residual gas etc. (see older euroelectron posts for references). For rediculusly low anode potential a high megohm pullup or Pleiades bias resistor can be used from anode or Vb to grid. Grid is still negative but less so. Input impedance of the electronic valve is decreased to say 100K. A great advantage of open grid is no grid current. Electron tube is so sensitive at such condition that it may become photosensistive too (gm of tube changind or modulated by light)! Further reading: Operating Features of the Audion _ E. h. Armstrong, Open-grid tubes in Low Level Amplifiers - Robert J. Meyer - Electronics, The Pleiades Bias - Euroelectron, History of low plate voltage electron tubes - euroelectron