Why is a coupling capacitor needed from the input transformer to the control grid?

An electron tube with the cathode normally heated behaves like a source of voltage (EMF) between grid and cathode. And the source output resistance is quite low.


This is very easy to verify. We take a tube and carefully (using fuse etc) connect just the heaters and nothing else.


Immediately we can measure a few hundred! millivolts between negative grid or positive cathode.


This potential between cathode and grid should be able to be measured even with a needle analog voltmeter of quite low input impedance.


Similarly by connecting an micro Ampere meter between cathode and grid we can measure the current of boiling electrons traveling from being captured from grid back to cathode by the external measuring circuit. This is the short circuit current.


So if the secondary of an input transformer is connected directly to the grid it forces it to assume a different potential and current will flow through the transformer secondary.


On the Pleiades preamplifiers the triode connected EF183 is capacitor coupled from the secondary of the microphone input transformer to its control grid.


The cathode is grounded. It was found that the tube operates excellently with just 3 volts on the anode when a 4Megohm resistor is connected between anode and grid.


And this low noise battery powered amplifier operates even better when the heater voltage is reduced from 6.3V to somewhere near 3 volts!


Note: there is also the common cathode resistor bias. Is it possible to find a suitable cathode resistor so that the external bias negates the self assumed internal so that they cancel and the grid to cathode external current becomes 0? Would this give a very low noise operation too? There is also the technique of high Megohm grid leak bias by a grid resistor to cathode. It needs higher anode voltages, for example about 47V for the Neumann U47?  There is also the technique of no grid resistor, ie almost infinite. This is described as a low noise technique.


References:


Operating features of the Audion - Edwin Armstrong


Other Euroelectron posts on Pleiades V4, V5, V6 booster preamplifiers and the article On preserving electron tube transconductance at very small anode potential


Open-Grid Tubes in Low-Level Amplifiers - Robert J. Meyer