The Pleiades Bias

Bias means voltage. Grid bias is by definition the electric potential difference (or voltage difference) of the grid with respect to the cathode. There are many ways to produce bias, here is a much less known way.


What is the Pleiades bias on electron tubes, electronic valves?


It is a way of biasing the vacuum electron tube by using a positive voltage source in series with a high Megohm resistor connected to grid instead of a negative voltage source. Or a positive supply, battery etc connected to the positive side to grid.


Why should a positive supply be used?


When we connect a heater voltage to an electron tube and the cathode heats up, it emits electrons. The cathode immediately becomes positive with respect to control grid by the missing electrons, (positive protons remain). This is an internal negative bias mechanism. By connecting for example 5V to the heaters of the EF183 electron tube and a voltmeter between cathode and grid, the control grid reads when maximum cathode temperature is reached about 700mV negative with respect to the cathode.


The Pleiades bias decreases this negative bias to a much lower negative value, for example -50mV.


So the electron tube can operate with just for example 4V electric potential at the anode. The negative electric field or barrier near the cathode is reduced. So electrons can pass it and arrive at the anode by the presence of a small electric field.


What is the advantage of low voltage operation?


Instead of a typical more or much more than 100V needed for the anode supply the electron tube operation at a few volts has same of the following advantages. Few volt operation means the same battery can supply both heaters and the anode circuit. This gives further reduction in noise and capability of extracting the subtlest information from the source to be amplified. Weight and construction complexity is reduced substancialy. An electron tube circuit can be portable to even easily powered in space by a small photovoltaic cell. The noise is very low as the temperature is very low and no secondary emission effects are present, the transconductance is normal, and pure DC noiseless battery supply is possible. Much less heater voltage is possible when using a low anode voltage, typically less than half of the specified with possible further decrease in noise or equivalent input resistance of the electron tube. Power consumption is of the order of only 1 Watt.


Why using an electron tube in space?


Electron tubes are not destroyed by high frequency radiation. Elcryin tubes have a high bandwidth without the need of feedback making them possibly great low noise candidates for space communication, radio telescopes etc. They are also very useful signal amplification devices as they can fit large amount of information to a finite amplitude space by compressing information in a similar way that our ear brain does; they are very linear at low signals and progressively non linear at large signals. The output signal is peak limited objectively but not subjectively as similar harmonics are produced to what nature produces at high stimuli levels. [Russel O. Hamm]


Example amplifiers are the Pleiades V series low noise microphone head preamplifiers such as the Pleiades V6. Bias is usually by a few Megohm resistor, typically 6MΩ, connected from the anode to the control grid. Suitable electron tubes can be the EF183 triode connected, ECC82, 6SK7, vari μ electron tubes of high transconductance etc.


For conventional high anode voltage a negative bias source should be used as known.


The concept of a positive source of bias was discovered by Hliana who insisted that a positive voltage source will free the electrons.


It all looks very nice. Are there disadvantages?


There is at least one. The input impedance of the electron tube is no longer extremely high but typically 100KΩ. This is not nesesarily a disadvantage as it helps damp the reasonance of an input transformer.


Pleiades concepts or schematics are open source.


References:


Pleiades V6 schematic


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


Operating Features of the Audion - E.H. Armstrong


Tubes vs Transistors (vs ICs) - Russel O. Hamm - Journal of the Audio Engineering Society