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Inverted Electron Tube High Input Impedance Circuit Amplifier Electrometer Part 2

OddMix.com - Technical Note - 1203A - by Karl Nagy

Figure 2. Electrometer Circuit [4 KB]
Figure 2. Electrometer Circuit

For the electrometer circuit on Figure 2, the preferred vacuum tube VT is a professional, official, electrometer tube Picture 2. Connect all additional grids to ground or to the control grid if the tube selected is not a triode. If obtaining a real electrometer tube is not possible, the next best selection is a tube with a top grid or anode terminal. Notice the professional tubes also have an electrode top terminal with on the side shielding if possible. With this circuit 50,000 Ohms/Volt is the least obtainable using ordinary vacuum tubes.

Filament and grid voltage has to be very stable. Either laboratory grade power supplies or rechargeable batteries have to be used. R1 rheostat can vary in accordance of the tube type and the filament voltage the tube uses. Smaller filament and grid voltages, increases the possible input impedance. Tube can be operated with half filament voltage or as small as possible to get useable reading. The meter movement M1 is in a bridge circuit. Potentiometers R2 and R3 is used for setting zero and full scale.

Picture 2. Beckman 931 Electrometer Tube [4 KB]
Picture 2 . Beckman 931 Electrometer Tube

Vacuum tube manufacturers specifically developed electron tubes for electrometer applications. Many of those special tubes have their anode or control grid terminal coming out of the glass envelope as far away as possible from the cathode, usually on the opposite end of the tube's socket and electrodes. This arrangement provides additional, inherent, larger resistance to the tube.

Instruments measuring pH, titrators and many additional electrochemical measuring circuits were not possible without inverted tubes. Before the FET [Field Effect Transistor] devices were widely available, only vacuum tubes were available. Tubes are still superior, because a relatively small static field kills an unprotected FET device that does not affect the tubes at all. And any protection component destroys the desirable high input resistance in a solid state circuit.

Among many other tubes designed and developed specifically for electrometer applications was the 6BS7 vacuum tube. It appeared around 1966. Since electrometer tubes were made in limited quantities, and the extreme requirements for the tube itself and its operating conditions, electrometer tubes were always expensive. Chemical laboratories had applications for most of the electrometers.

The 6BS7 - a 6BR7 or 12BR7 with a top grid G1 contact - was a "modern" pentode in it's time. It was an all glass noval vacuum tube that has nine connecting pins. The unusual property of this special device is that it has an additional terminal on top for the control grid. The tube has an indirect heater cathode arrangement and it uses the standard 6.3 Vac for its filament voltage.

The specified maximum input resistance was 10,000 Megohms. Grid ion current was less than 5x10-12 [Pico Amperes], sensitivity is better then 10 uV. Additional professional tube numbers; Beckman 931, Beckman 932, Westinghouse RH507, CK5886, 5889 or regular types; 3BE6, 6BE6, 12BE6.

Electrometers are not to be confused with the well know static electricity detecting instrument the electroscope. The electroscope was invented more than a hundred years ago. It works by electrostatic repulsion. It uses a pair of super thin metal strips made of gold. The two strips open more or less accordingly to the voltage connected to the instrument.

            Parts list for Electrometer Circuit:

            M1  - Current Meter, 100 uA, 1 - 2.5 Kohm
            R1  - Filament Rheostat, 100, 5 Watt, WW - Function of tube selected & voltage
            R2  - Potentiometer, 20k, 1 Watt, CC
            R3  - Potentiometer, 50k, 1 Watt, CC
            R4  - Resistor, 91k, 1/4 Watt, CC
            VA  - Filament Battery - Voltage is function of tube used
            VB  - Anode Battery -  - 9 V - Function of tube used
            VT1 - Electrometer Tube - See text 

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