Let’s Add A Grid to That Vacuum Tube Diode

Some of the triode tubes from 1918 to late 1950s

The last post was about the vacuum tube diode. I forgot to mention for you European and other readers when I speak of a vacuum tube it might be what you call a valve. The term valve is a very good one because it really is an electron valve. Diodes, as well as all vacuum tubes, are electron check valves. A check valve for liquid will allow the liquid to flow in one direction but blocks it from flowing back. That is the same thing a diode does with the electron flow in a conductor.

Adding a grid to a diode is like adding a gate with a stem and knob to the check valve. The gate will allow the operator increase or decrease the rate of flow. For many years effective state of the art amateur radio equipment was made possible by the grid or as we will see later multiple grids.

If you don’t know how a diode works then I might suggest you back up to the previous post and read it.

John Ambrose Fleming

John Ambrose Fleming, an English physicist, worked as an engineering consultant for several firms including Edison Telephone and the Marconi Company. In 1904 Fleming conducted some experiments using Edison effect bulbs he had imported from the USA. He found the bulbs could be used as rectifiers which he called “oscillation valves”. Because of their greater efficiency and the fact that a hot spot did not have to be found the vacuum tube diodes detector soon started replacing the crystal detectors in radio.

Just as an interesting side note, though we think of the vacuum tube radio as the predecessor to the solid state radio the crystal detector was actually a solid state diode and thus the solid state radio preceded the vacuum tube radio.

Amplification and oscillation were not possible with the diode vacuum tube.

Lee DeForest

Lee DeForest has been credited adding a third element called a grid to the vacuum tube and making what he called the “Audion Tube” which is the forerunner of the triode.

The grid is a wire mesh placed between the cathode and the plate. Electrons are able to flow past it with very little effect if it is not biased (bias means to apply a voltage). When a positive voltage is applied to the grid (positive bias) the electrons will flow faster and the plate current will increase. The grid will collect some to the electrons as the others pass by it and so there will be a grid current. Most of the time grid current is not desirable.

When a negative potential is added to the grid (negative bias) the electrons are

As the signal goes negative less current will flow through the tube causing the output voltage to go higher (tube resistance increases while R remains constant). As the signal goes positive the tube’s resistance decreases thus more current flows so the output voltage decreases.

repelled by the negative field around the grid wires and thus less will pass through the mesh. By increasing the negative potential the repelling effect will cause fewer electrons to pass. This effect will continue to increase as the negative bias is increased until the current is shut off completely. A very small voltage change on the grid can cause a large change in current flow inside the tube so amplification is possible. As the small signal goes negative it is added to the negative bias voltage and thus decreases the output, the plate voltage will go more positive, then when the signal goes in the positive the grid bias voltage will decrease so the plate current will increase causing the plate voltage do decrease so the small signal will produce a larger signal but with a 180 degree phase shift.

By taking some of the signal output and feeding in back into the input of the amplifier an oscillator can be constructed. This made it possible to create a continues wave (CW) oscillation which is the fundamental foundation of all ham radio, commercial radio, and any other radio communications

A new problem was added with the triode so that another element needed to be added but I will save that for the next post.

 


 

 

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