There are three main types of electrode in a vacuum tube or thermionic valve. The cathode, anode and grid all provide different but essential functions.
Their construction is naturally different and there are variations according to the design of the tube and its applications.
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. 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. 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.
The earliest type of cathode is known as a bright emitter cathode. This type of cathode uses 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. It suffers a number of drawbacks, one being that it is not particularly efficient in terms of the emission gained for the heat input. The life of the cathode is also limited by the evaporation of the tungsten with failure occurring when about 10% of the tungsten has gone.
A further type of cathode is known as a dull emitter. These cathodes are directly heated and consist of thoriated tungsten. They provide more emission than a tungsten cathode and require less heat, making the overall efficiency of the tube greater. Typically they run at a temperature of between 1900 and 2100 K. Although these cathode normally have a relatively long life, they are fragile and any valves or tubes using them should be treated with care and they should not be treated to technical shocks or vibration.
The type of cathode that is 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. This type of cathode is normally in the form of nickel in the form of a ribbon, tube or even a small cup shape. This is coated with a mixture of barium and strontium carbonate, often with a trace of calcium added. During the manufacturing process the coating is heated to reduce it to its metallic form and the products of the chemical reaction are removed when the valve is finally evacuated. In this cathode it is the barium that acts as the primary emitter and it operates at a much lower than the other types being in the region of 950 - 1050 K.
Some types of thermionic valve or vacuum tube use what is termed a cold cathode. These are voltage stabilisers and use a form of activated metal surface.
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.
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.
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.
By Ian Poole