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There are many different ways that a filament (or cathode), a grid and a plate can be constructed and put together, and many of them have been tried over the years. From here on, we will concentrate mainly on indirectly heated tubes, having a distinct heater and cathode, rather than filamentary tubes. These appeared in the early 1930s, primarily as a way to allow the heater to operate from AC rather than DC.
The only disadvantage of indirect heating is that it is less efficient, i.e. more energy input is required for operation at a given temperature. This is why tungsten and thoriated filament tubes (e.g. 211, 845, SV572) are still directly heated, due to their much higher operating temperature, and also why battery tubes were. For oxide cathodes, the best known filamentary tube in common use today is the 300B, together with a few other power types.
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| Figure 5: Types of triode construction |
There are three common tube structures. Figure 5a shows the cylindrical structure, in which the cathode, the grid and the plate are made as three concentric cylinders. This structure has been little used since the 1930s, presumably because it is more expensive to manufacture. (An exception is rectifier diodes, which are almost invariably built like this). It does however have some theoretical properties that may account for the sonic reputation of the classic model 27 and 76 triodes, which are built this way. The grid is wound as a spiral, generally with two vertical support posts. The grid wire is rigid, and holds its shape once it has been formed into shape.
Figure 5b shows the flat structure used in power tubes such as the 6AS7 and (with a filament rather than a cathode) the 2A3 and 300B. Here the grid is also wound around two vertical supports, but in this case it is flat (although often curved outwards slightly around the cathode). Like the cylindrical structure, this lends itself readily to theoretical analysis.
Figure 5c is a compromise between the two. The grid and the plate are flat, but the cathode is a small cylinder. This is a common construction for small tubes such as the 12A*7 family or the 6SN7. Its behavior is very close to the truly flat structure of Figure 5b. There are many variations on this; for example, the plates are often elliptical rather than rectangular as shown. Similarly, the cathode may be elliptical. Since the characteristics depend directly on the geometry, this results in less consistent operation and is therefore undesirable for audio.
The challenge for the tube designer is to get the grid as close as possible to the cathode. As we will see later, this is the principal factor affecting the mutual conductance of the tube, and for most purposes higher mutual conductance means better. However, there are problems. First of all, the slight variation in grid-cathode spacing inevitable due to manufacturing tolerances leads to indistinct characteristics, since different parts of the tube are behaving differently. Indeed, if the spacing is too close then there is a risk that the two may even come into contact, with disastrous results. Secondly, if the spacing is much less than the distance between the grid wires then the tube performance becomes difficult to predict and may even get worse. This in turn led to pressure to wind the grids from ever-finer wire at an ever-tighter pitch. Miniature tubes typically use grid wire that is less than one-thousandth of an inch thick, with a grid-cathode spacing at its closest of little more than this.
Most of the metal inside a tube is nickel, which has a number of desirable properties. First, it has a high melting point. Copper, for example, would be unsuitable because it would soften and distort. Secondly, it does not absorb a lot of gas onto its surface (the technical term for this is adsorption). Such adsorbed gas is gradually released into the vacuum of the tube, and it is important to minimize it. An exception to the use of nickel is the heater (or filament) which is generally made of tungsten because of the high operating temperature.
The plate is generally blackened, because a black surface radiates heat much more effectively than a lighter one. The only way that heat can be removed is by radiation, and it is this that sets the limit on the power dissipation of a tube. Power tubes have plates that include large radiating surfaces having nothing to do with their electrical function, and even small-signal tubes often have wings or corrugations that serve the same purpose.
Because of the very close clearances, and also because of the impact of tube geometry on electrical characteristics, it is vital to maintain the dimensions to very close tolerances. The metal wires that support the various electrodes are generally held apart by spacers made from mica, which is an excellent heat-resistant insulator that is also very vacuum-friendly and easy to work to precise dimensions.
The heater is inside the cathode. Since the cathode itself is small, there is little room for it and even less room for electrical insulation. A thin layer of alumina (aluminium oxide) is normally used, but at the high temperatures involved this is not a perfect insulator, and if the voltage difference is too high then it can break down altogether. This is why all tubes have a rating for the maximum voltage difference between the heater and the cathode, typically 100-200V. Exceeding this may result in insulation breakdown, and hence failure. Even at lower voltages, there is some resistive and especially capacitative coupling between the heater and the cathode.
It is a very instructive exercise to take a tube to pieces and to measure the internal dimensions. Don't do this with a new Western Electric 300B, but take a defunct octal tube such as 6SN7 (the miniature tubes are too fiddly), break the envelope by wrapping it well in a cloth and tapping it with a hammer. (Be careful doing this, and be careful as well with the shards of broken glass afterwards. Remember that some parts are mildly poisonous, and don't even think about doing it with transmitting tubes which can contain highly toxic beryllium). You can then gradually dismantle the electrode structure, seeing how it is put together. You will find, for example, that many tubes have tiny metal plates welded to the tops of the grid supports, whose job is to get rid of the heat generated on the grid due to grid current. (In this case of the 6AS7G, these are comparable in size to the plates themselves).
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