Pleades 4.8V super quiet microphone pre pre amplifier booster with EF183 triode connected and 3.9M positive bias from anode to grid

The idea comes from Hliana who insisted on positively biasing grid to free electrons.


Assigned to public domain by George Chakiris.


This little pre preamplifier makes a voice appear times bigger.


YouTube clip of earlier prototype with 6V lead acid battery and 4R7 dropping Resistor.
https://www.youtube.com/watch?v=xPVtXzmfDR8


Please note heaters are under heated to reduce information obscuring electron cloud.


This prototype was tested with Sennheiser MD421 feeding a Canford  low to high impedance converter (419mH transformer primary) feeding pre pre amplifier feeding a Realistic Disco Mixer at its mic input.


Component list:
4x AAA batteries (through fuse they supply heaters directly and anode through Ra) (neg to cathode)
1x fuse holder (very important to protect yourself and the batteries)
1x 0.8A fuse (will be reduced as heater current should be around 260 mA, initial 500mA)
1x EF183 electron tube (EF183 is vari mu, high gm) (in this amp grid 2 and 3 are connected to Anode)
Ra=15K (10K shown)
Rag=3.9MOhms (positive bias resistor from anode to grid to free electrons)
Ci=220nF ( very important input capacitor to keep grid free )
Co=1uF (output coupling capacitor)


Proximity low cut off can be adjusted by the primary inductance of the input transformer. A Pleiades input transformer can be made with 70 - 300 mH so that extremely detailed and clear vocals can be obtained at a distance of 1-2 inch from microphone. The closer the distance needed the less the inductance making construction easier as fewer primary turns are needed. An excellent example of magnetic core is Magnetec's series of nano crystalline extremely high inductance index toroidal cores.


With just 3 AAA (3.6V) it performed even better and perhaps more quietly but gain was lower. Grid bias was close to 0 compared to  -0.4V when 4 AAA were used due to increased negative electron cloud making cathode more positive with respect to grid?)


Very nice performing with electric bass too.
Signal Path:
Yamaha electric bass - Pleiades pre pre - 50K to 500Ohms impedance converter - Realistic Disco mixer mic input
Extremely quiet bass operation and great tone




Schematic of pre preamplifier showing experiments of connecting Grid resistor to various points. Maximum anode current was as expected obtained by having the other side connected to Anode. This does not mean that grid is positive but it is certainly less negative. Being positive or not depends on cathode temperature, lower temperatures reduce electron boiling cloud and this can make grid positive with respect to cathode.