Vacuum Tube Electrodes: thermionic valve electrodes

There are several types of electrode within a vacuum tube or thermionic vale: each has its own function and operates in a very different way.


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Vacuum tubes / thermionic valves have a number of electrodes in them, each performing a different function.

As the different valve / tube types offer different levels of performance as well as having significantly different performance parameters and functions, it is useful to understand the function of each of the electrodes.

By understanding the electrode functions, it is possible to better understand the performance of the different types of valves or tubes, the way they work, and why the different types are chosen for a given position within an electronic circuit design or RF design, etc.

Image of a modern valve / tube amplifier using valves / tubs with different numbers of electrodes.
A modern valve / tube amplifier using valves / tubs with different numbers of electrodes

Valve / tube electrodes

>There are several different types of electrode that can be incorporated within the envelope of a vacuum tube / thermionic valve.

  • Heater:- although not one of the electrodes in the same was as the others, it is nevertheless important
  • Cathode
  • Grid:- there are several types including the control grid, screen grid and the suppressor grid.
  • Anode

Each of the valve / tube electrodes performs an essential function within the overall device.

Cathode

There is a variety of different types of cathode that are used in modern vacuum tubes. They differ in the construction of the cathode and the materials used.

One of the major ways in which cathodes can be categorised is by the way they are heated.

  • Directly heated:   The first type to be used was what is termed directly heated. Here a current is passed through a wire to heat it. In addition to providing the heat it also acts as the cathode itself, emitting the electrons into the vacuum. This type of cathode has the disadvantage that it must be connected to both the heater supply and the supply used for use in the cathode anode circuit itself. This has disadvantages because it limits the way the circuit can be biased unless each heater is supplied separately and isolated from each other. A further disadvantage is that if an alternating current is used to provide the heating, this signal can be superimposed upon the main cathode anode circuit, and there is a resultant hum at the frequency of the heater supply.

  • Indirectly heated:   The second type of cathode is known as an indirectly heated cathode. Here the heater is electrically disconnected from the cathode, and heat is radiated from the heater to heat the cathode. Although it takes longer for these types of tubes to warm up, they are almost universally used because of the flexibility this provides in biasing the circuits, and in isolating the cathode anode circuit from the effects of hum from the heater supply.

Bright emitter cathode

The earliest type of cathode was known as a bright emitter cathode. This type of cathode used a tungsten wire heated to a temperature of between 2500 and 2600 K. Although not widely used these days, this type of cathode was used in high power transmitting tubes such as those used for broadcasting.

The bright emitter cathode suffered a number of drawbacks, one being that it was not particularly efficient in terms of the emission gained for the heat input. The life of the cathode was also limited by the evaporation of the tungsten with failure occurring when about 10% of the tungsten has gone.

Dull emitter cathode

A further type of cathode was known as a dull emitter. These cathodes were directly heated and consist of thoriated tungsten. They provided more emission than a tungsten cathode and required less heat, making the overall efficiency of the tube greater.

Typically dull emitter cathodes ran at a temperature of between 1900 and 2100°K. Although these cathodes normally had a relatively long life, they were fragile and any valves or tubes using them needed to be treated with care and not given any shocks or vibration.

Oxide coated cathode

The type of cathode that was in by far the greatest use is the oxide coated cathode. These may be used with indirectly heated cathodes, unlike the tungsten and dull emitter cathodes that must be directly heated as a result of the temperatures involved.

Oxide coated cathodes were normally in the form of nickel in the form of a ribbon, tube or even a small cup shape. This was coated with a mixture of barium and strontium carbonate, often with a trace of calcium added. During the manufacturing process the coating was heated to reduce it to its metallic form and the products of the chemical reaction were removed when the valve was finally evacuated.

For the oxide coated cathodes it was the barium that acted as the primary electron emitter and it operated at a much lower than the other types being in the region of 950 - 1050°K.

Cold cathodes

Some types of thermionic valve or vacuum tube used what was termed a cold cathode. The valves or tunes that used these cathodes were voltage stabilisers and used a form of activated metal surface.

A number of these voltage stabilisers were used and they were obviously most often found in power supply circuit designs. Typical voltages were around 150 volts, and they could often be used to power the RF oscillator circuits in radio receivers, transmitters and the like.

Anode

The anode is generally formed into a cylinder so that it can surround the cathode and any other electrodes that may be present. In this way the vacuum tube can be constructed in a tubular fashion and the anode can collect the maximum number of electrons.

For the smaller valves or tubes used in many radio receivers, the anodes are generally made of nickel plated steel or simply from nickel. In some instances where larger amounts of heat need to be dissipated it may be carbonised to give it a matt back finish that enables it to radiate more heat out of the valve.

For applications where even higher powers are required, the anode must be capable of dissipating even more heat, and operating at higher temperatures. For these tubes, materials including carbon, molybdenum, or zirconium may be used. Another approach is to build heatsink fins into the anode structure to help radiate the additional heat. This approach is naturally limited by the construction of the valve and the fact that the tube needs to be contained within its glass envelope. However a large heatsink structure will require the glass envelope to be much bigger, thereby increasing the costs.

To overcome this problem the anode may be manufactured so that heat can be transferred outside the valve and removed using a forced air or a water jacket. Using this approach the envelope of the tube can be made relatively small, while still be able to handle significant levels of power.

Grid

The grid is the electrode by which the current flowing in the anode circuit can be controlled by another potential. In the most basic form a vacuum tube may have one grid, but it is possible to use more than one to improve the performance or to enable additional functions to be performed.

Accordingly valves are named by the number of electrodes they contain that are associated with the electron flow. In other words the filaments or heaters and other similar elements are omitted.

Circuit symbol for a pentode valve showing the differnt grids that are used.
Circuit symbol for a pentode valve showing the differnt grids

Different types of valves / tubes have different numbers of grids: a triode only has a control grid, a tetrode has a control and screen gid and the pentode has the control, screen and suppressor grids.

The different grids have different purposes within the overall valve / tube.

  • Control grid:   As the name implies the control gid is used to control the number of electrons passing through it. By changing the potenial on it with respect to the cathode te flow of electrons can be changed.

  • Screen grid:   The screen grid ahs a different purpose. It has a positive potential applied and helps draw electrons though the negatively changed control grid which increases the performance. It also acts as a screen between the anode and control grid. This significantly reduces the levels of feedback and is particularly important for RF designs.

  • Suppressor grid:   The additional energy given to the electrons meant that many of them hit the anode and then bounced off again. This resulted in a significant kink in the characteristic of the tetrode valves with two grids. The solution was to place a third grid closest to the anode to prevent this effect, i.e. suppressing the bounce effect.


Number of grids Total number
of electrodes
Generic name
1 3 Triode
2 4 Tetrode
3 5 Pentode
4 6 Hexode
5 7 Heptode
6 8 Octode

A grid is normally constructed in the form of a gauze mesh or a wire helix. If made of wire, it normally consists of nickel, molybdenum or an alloy and is wound using supporting rods that keep it clear of the cathode. As such they may be wide, possibly oval in shape and they are generally made from copper or nickel.

To achieve a high level of performance that is repeatable, the tolerances within the vacuum tube must be maintained from one device to the next.

In addition to this it is often necessary to mount the grid only fractions of a millimetre away from the cathode or other grids. To be able to maintain these dimensions one approach that is adopted is to use a stiff rectangular frame and then wind the grid wire onto this under tension. This structure then needs to be fixed by the use of glazing or even gold brazing so that it remains firmly in place. Under some circumstances it may even be necessary to grind the cathode surface coating to ensure its flatness. This form of grid is known as a frame grid.

One important aspect of the design of vacuum tubes or thermionic valves it to ensure that the grid does not overheat. This could lead to mechanical distortion and failure of the whole valve. To assist in the removal of heat the grid wire may be carbonised, and often cooling fins may be attached to the grid supporting wires. These supporting wires may also be welded to directly toth e connection pins in the base of the valve so that heat may be conducted away through the external connections.

A wide variety of thermionic valves or vacuum tubes is available even today. Using the techniques that have been developed over many years they are able to offer excellent repeatability, performance and reliability.

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