Lighthouse Electric Tutorials

Vacuum Tube Theory

Part 1.
Invention of the Electron Tube

Thomas Edison discovered the principle of Thermionic Emmision as he was experimenting
with his new invented light bulb. Edison placed a metal plate inside his bulb along with the filament.
A Battery was connected in series with the plate and filament, with the positive pol connected to the
plate and the negative to the filament. When the filament was heated, an ameter connected in series
indicated current flow.
The heated filament caused electrons to leave its surface what is known as Thermionic Emmision.
The application of the thermionic emission making electrons to flow accross the space between
the filament and the plate has later become known as the Edison Effect.

The Fleming Valve The english scientist J. A. Fleming was trying to improve on Marconi's wireless receiver when he went back to Edison's earlier work. At first he duplicated Edison's experiment. With the plate POSITIVE relative to the filament (or heater), the heater hot and the circuit completed as shown in Fig.1-1, the ammeter in the plate circuit will indicate a current flow. Because current amount is the same in all parts of a series circuit, the same current must flow across the space between the heater and plate.


Electrons emited from the heater ( or cathode) are NEGATIVE and are attracted to the POSITIVE plate
because unlike charges attract. Flemings next experiment was to reverse the plate battery as shown
in Fig.1-2. With the plate NEGATIVE relative to the heater, the ammeter indicated no current flow.
Fleming found that the NEGATIVE charge on the plate, cut off the current flow. Since electrons are
negative and the plate was negative, electrons were repelled back to the heater. No current flow
was present.

Even though the heater was now positive in respect to the plate, there was no current flow.
The cold plate does not emit electrons. Fleming concluded that plate current flows only in one
direction - from cathode to plate. Fleming's next step was to apply alternating
voltage to the plate as shown in Fig.1-3.

The plate current follows the positive half of the plate voltage.
During the second half-cycle period, the plate's polarity is negative and current flow is cut-off.
Fleming concluded the following:
        1. Plate current flows only during the positive half-cycle.
        2. Plate current flows in PULSES because it is cut-off half the time.
        3. Plate current flows in one direction, therefore it is a (pulsating) D.C.
In effect, the first rectifing tube was invented - the DIODE.

The Diode.

Many changes have been made from the original Fleming Valve. Modern diodes or any other tubes for that matter, are more efficient and reliable. Cathodes last longer, emit more electrons and many operate at lower temperature than in the early days. Two types of heaters are used, directly heated or indirectly heated. A.C. voltage is now used to power the heaters. Plates are cylindrical shapes and have larger surface areas. Diodes can be used as rectifiers, switches and many other applications.

The Triode.
As with all inventions, attempts were made to improve Fleming's Diode Tube. In 1906 Dr. Lee De Forest, an American, added another active element to the diode, the Control Grid. After much of experimenting with different elements between the cathode and plate, he placed a wire mesh close to the cathode. In this way electrons could flow from the cathode, trough the wire mesh, to the plate. Applying different voltages to that third element, De Forest could change the plate current. In the first experiment, Fig.1-4a no grid voltage was applied. Plate current was 5 mA and did not change. Tube was acting like a diode.

In Fig.1-4b  +3 volts were applied to the grid. The plate current increased too 10mA, it doubled.
The reason for that is that the grid, being very close to the cathode acted as a tiny "plate", it
assisted the plate in attracting the electrons. Since the grid has a very small surface area most
electrons passed trough and were propelled to the plate.

In Fig.1-4c -3 volts were applied to the grid. The plate current dropped to 2mA, more than half
the amount of the first experiment. A variation of +/- 3 volts caused a variation in plate current of 8 mA.

That was not what De Forest had in mind. He wanted amplification not current variation. That problem was
quickly resolved with the help of Ohm's Law:        E=I*R.
Adding a resistor in series with the plate would cause varying voltage drop accross the resistor.
This is shown in Fig.1-5  If we enter currents from previous (Fig.1-4) experiment into the Ohm's
formula, we get the following results:
                                 E=0.005*10000    E=50 volts
                                 E=0.010*10000    E=100 volts
                                 E=0.002*10000    E=20 volts

With a grid voltage variation of only +/- 6 volts De Forest caused a plate voltage variation of +/- 80 Volts.
That is a tenfold amplification. The first amplifying tube, theTriode was invented.
Further improvements of the Triode resulted in developing the Tetrode Tube, Pentode Tube and others
which we will discuss in part 3.
Next: Tube Biasing

Home | Circuits| Projects

Copyright 1997-2010        Lighthouse Electric        All rights Reserved.