Connecting cathode to ground and mic voice coil to grid

This concerns again very low anode voltage operation of an electron tube, typically 3 volts.


It worked worse than expected, no improvement from the brilliant 6MΩ from anode to grid bias. The EF183 electron tube was kept in the Pleiades V6 circuit for this experiment. The voltage supply for the anode of the triode connected electron tube is  aproximately 3.5V. (Note this includes the heater circuit too).


(Note the electron tube when a resistance is connected between grid and cathode still operates with so small anode voltage because the cathode is charged positive by the excess of (positive) protons inside it as some electrons have steam boiled to the nearby electron cloud when the cathode is heated up. So connecting externally the grid to the cathode through a resistance makes the grid less negative wrt cathode. But when connecting the grid through a 6MΩ resistor to a much higher potential such as 3V makes the tube operate with a much higher anode current although the grid bias is still negative (wrt to cathode) but close to 0V allowing brilliant and low noise operation with just 3V at the anode. See other euroelectron posts.)


Signal path, setup:


200Ω BRC Mic - Pleiades V6, Vbattery approx 3.5V - Sony TC-D5 pro - Sennheiser HD 580


Firstly the inexpensive mic was connected through the usual 1:10 input transformer of the Pleiades V6. The sound was loud but peaky and reasonant. Very different compared to connecting the legendary Grampian DP4/L mic.


Then in order to see how low noise V6 is, the input transformer was removed and the 200Ω inexpensive mic was connected to grid through the 22nF PHILIPS MKT coupling capacitor.


There was sound but with more hiss. About half sound, half hiss.


Then it was decided to connect the mic directly to grid and cathode and removing the high Megohm resistor from anode to grid. There still was sound. Not impressive results. The micro amp anode meter showed only very few micro amps.


Then a variable multi turn potentiometer was connected between cathode and ground as a variable cathode resistor. Not very impressive results.


Back to connecting the 6 MΩ anode to grid biasing resistor.


The anode current immediately jumped to 50μA and gain or transconductance increased.


The mic now sounds much louder again.


Then the 1:10 tranformere is inserted in the circuit.


The sound is getting big.


Then the Grampian DP4/L is connected.


The sound is getting very big. Every ambience detail can be heard.


Both Pleiadss V6 (3xAA) and Sony TC-D5 Pro (2xAA batteries) were almost exhausted.


Signal path, setup:


Grampian D.P.4/L. - Pleiades V6, Rag=6MΩ - Sony TC-D5 pro - Sennheiser HD580





This is the schematic of the Pleiades V6 booster microphone head amplifier. Cc is usually 22nF.


Some years ago a similar experiment with the same Pleiades experimental jig took time place. It had been immediately apparent that the anode current and live mic sound at the headphones greatly increased by connecting a high Megohm resistor from anode to grid to free the electrons.


References:


Pleiades V6 schematic


On preserving transconductance of electron tubes at anode potential as low as 3 volts - euroelectron


Operating Features of the Audion - E. H. Armostrong