Historical:
The origin of tube technology was a carbon filament bulb. After the still air-filled lamps had survived less than five minutes, the air was pumped out. With these first lamps you could already illuminate something. But the inside of the glass bulbs soon turned black, which suggested that some material was emanating from the carbon filament.
Various handicrafts gradually led to success.
A sheet of metal was mounted in the lamp in the hope that the particles would settle there. But this was not so.
The whole thing was like a department store at a sale. The particles spread throughout the "building". The sheet metal had no influence either. It only started to work when the sheet metal was made electrically accessible from the outside with a wire. It was found that the sheet metal was slightly negatively charged and as long as the lamp was burning, this negative charge was "replenished" again and again. And because you knew that there are electrons and that they have a negative charge, you could imagine the filament emitting electrons.
But as long as the tin was not connected to the filament, it was like a warehouse where the entrance is open but there is no exit. At some point, everything was full of electrons and a negative cloud was created, not to say a crowd of electrons, so that no more electrons could escape from the filament.
Now you have connected the metal sheet to one end of the filament. The electrons were thus able to leave the room via this “exit” and thus make room for new electrons. The fact that they ended up back at the point of emission is irrelevant for the electrons.
Unfortunately, the "cleaning" effect on the glass bulb was not yet great, because only the electrons that accidentally landed on the sheet metal (and with them the tiny carbon particles that they carried with them) were kept away from the glass. And a metal-cased incandescent bulb doesn't make much light.
The next attempt was to attract the electrons to the metal sheet. This worked the moment the sheet metal was subjected to a positive voltage (relative to the filament). This positive voltage now (like the rummage table) attracted the electrons. And since you also measured the current that was flowing, you saw that you got a larger or smaller current depending on the voltage.
Now the electron tube was born. And so people started experimenting with it. So, in the “warehouse” a grid with a variable passage was installed after the entrance (cathode) and the exit (anode). This allowed the electron flow between the anode and cathode to be controlled not only by the level of the anode voltage, but also by the negative voltage on the grid.
You have to imagine it like this: The lattice bars are so far apart that most of the electrons can pass through them. A few hit the bars, but for most it's not an obstacle. If you now make the lattice negative (charges of the same name repel each other and the electrons are negative), it looks like making the lattice bars thicker. The flow is significantly slowed down. In extreme cases, the grid closes, even if a positive anode voltage attracts the electrons.
In return, a “backlog” forms at the cathode, i.e. a cloud of electrons. With this first controllable tube, relays were switched without losses in telephony (or at least there was such an idea). So they were a kind of "relay tubes". At that time nobody had thought of an amplifier operation.
At this point, the first two tube types that we now "know": Heated cathode (filament made of tungsten = tungsten cathode) and anode sheet = two electrodes = diode.
Heated cathode, anode and grid = three electrodes = triode.
We have seen that the current through the triode can be regulated by the grid voltage on the one hand, but also by the anode voltage on the other. This circumstance put the "relay tube" limits. If you use a normal relay with a current in the Kitchen sink closes a contact, so it doesn't matter to the now magnetic coil how great the voltage is across the closed contact. It is zero anyway if the contact is perfectly closed. And the driving coil and the closed contact have nothing to do with each other electrically.
But it's different with the tube: if the tube conducts, it only does so if it still has a positive anode voltage. This has the same effect as if the telephone relay, which is to be switched on and off with the tube, is connected via a Resistance connected to the operating voltage. You need a higher voltage because part of the voltage remains (must remain) on this resistor (or on the tube in our case) and this voltage drop is an additional power loss.
The next step was to find an electrode that significantly reduced this reaction of the anode. A second, fairly wide-meshed grid was built in, through which the electrons had to pass. A negative voltage would have stopped the electrons from flying on, so the screen grid was connected to a positive voltage. On the one hand, there were now electrons that were caught by the bars that were on their trajectory. Most of them, however, flew through it and were significantly accelerated in the process. And as long as electrons land on an electrode, a current flows.
So if we use our department store again, the customers are transported on by conveyor belt after they have entered and passed the control grid, regardless of whether they want to go to this destination or not. They are therefore on their way by conveyor belt (screen grid) in the direction of the output (anode). We would call this tube a tetrode thanks to the four electrodes.
Depending on how wide the exit was open (according to the level of the anode voltage), there was bumping at the exit (anode) and it happened that more customers "escaped" from the exit back towards the interior than the number of those who really left the store. In order to avoid a backwater in this region, a troop was deployed, which sent the rioters back to the entrance via a separate route.
Or related to the tube: Under certain voltage conditions, i.e. when the anode voltage is low compared to the screen grid voltage, but the electrons hit the anode with a reasonable force, they eject “secondary electrons” there, which are attracted by the screen grid because this is more positive is as the anode. So the current decreases (in a certain range) as the voltage increases, which corresponds to a negative resistance.
This function can lead to undesired effects and should normally be prevented. The third grid, the brake grid, was used for this. This prevents the electrons from flying back to the screen grid because it is at zero Volt lies and thus has a rather repulsive effect, or the anode is still more positive.
In the field of history it should also be mentioned that the light bulb as a roar-Origin was soon equipped with better filaments that brought more light output because they endured higher temperatures. With the higher temperature, the electron emission also increased. With the use of the tungsten glow wire, a usable emission was achieved. The first radio tubes were still equipped with such heating-emission wires.
First developments:
Soon the emission power of the tungsten wires was no longer sufficient and other materials began to be used. In addition, the heating was separated from the cathode because, with separate elements, the heating could be connected to almost any potential, while the cathode could be connected to ground or to a voltage of more than 100V. It was thus possible to develop devices (the first tube televisions) that could do without a transformer for tube heating.
Depending on the application, tubes with up to seven grids have been developed (EQ80).
In addition, tubes with smaller designs were manufactured. The connection technology has also been further developed. And finally, the heating voltage was also adapted to the special needs.
After virtually every manufacturer named their tubes according to their own code, a designation standard was introduced in Western Europe. This consisted of at least two letters and a number. When the first multiple tubes were introduced since the inception of this code, more letters (up to 4) and more numbers had to be used. The following table provides information about the meaning of the more important letters and numbers.
Let's take the EABC 80 as an example:
First up is the heating. thereby means
A = 4V
C = 0,2A series heater
D = 1,4V
E = 6,3V
G = 3,15V (GY501) or 5V (GZ34)
H = 0,15A series heater
K = 2V
P = 0,3A series heater
U = 0,1A series heater
V = 0,05A series heater X = 0,6A series heater.
Second (and following) is the tube function
A = small signal diode
B = small signal dual diode
C = small signal triode
D = power triode
E = tetrode or secondary emission tube
F = small-signalpentode
H = hexode or heptode (4 or 5 grids)
K = octode
L = power pentode (or beampower tetrode)
M = Magic Eye / Indicator Tube
P = (with suffix) secondary emission tube
Q = Enneode (7 grids)
Y power diode
Z = power double diode
Then the numbers. On the one hand, these indicate the type of connection (socket type), on the other hand they are serial numbers, whereby the odd numbers can sometimes indicate control tubes.
1-9 digits = pin or cup base
10 … = 8-pin key socket (pins look like finger bones)
20… = Loctal, largely corresponds to the octal socket, but has thinner pins. Lorenz built tubes with this socket but with the designation 71... (EM71)
30… = 8-pin octal socket
40… = Rimlock 8 pin
50… pot base 8 poles
500… Magnova socket 9 pin
60… = subminiature tube, soldered
70… = subminiature tube socketed, usually 8 poles
80… = 9-pin Noval socket
90… = miniature socket 7 poles
200… = decal base (like Noval, only 1 pin more) 10 poles
So the EABC80 is a tube with a Noval base, with the sequence number ZERO, with 6,3V heating, includes a small signal diode as well as a small signal double diode and a small signal triode.
After these first explanations some mechanical things:
In the case of a tube, mechanical parameters in particular play an important role. Here again back to the problem of the electrons mentioned earlier, which are knocked out of the anode and no longer fall back on it when the anode voltage is low, but end up on the screen grid, which is more positive than the anode. We have seen that the suppressor grid acts as a "police" to intercept the misguided "rioting" electrons and transport them back to the cathode.
With power pentodes there is another possibility to do without the brake grid. You can build a kind of tunnel after the screen grid through which the electrons have to fly to get to the anode. This sheet metal tunnel is connected to the cathode like a braking grid. Due to the bundling of the electron beam, the (rambling) electrons originating from the anode are fully exposed to the attack of the correctly guided electrons and are pushed in the right direction, so to speak. And if you don't want to, you end up in the tunnel wall.
This construction is called a beampower tetrode. Although it does not have an actual braking grid, the function is no different from that of the pentode. That is why it is not marked with its own letter and is often hardly mentioned in the data sheets.
Only this much can be said about the tube curves: The steepness of the Ia/Ug curve has a lot to do with the amplification. In general, one can say that the steeper, the higher the amplification. Over time, it has been found that bringing the control grid as close to the cathode as possible increases the transconductance. Originally the gratings were gratings, later wires were wrapped around support beams to build a ladder-like thing. The problem with such grids is that they have to be stable so that they can be placed close enough to the cathode. For this reason the clamping grids were developed. Here, spars are not simply wound, but a stable lattice frame is built, which is tightly wound and welded with extremely thin wire. These grid wires can hardly oscillate and do not change their shape even under thermal influences.
Now one could assume that there are only lattice tubes today. That's not the case. Because the grid tube would either result in changed tube data, so that a tube replacement would only be possible after adapting the circuit, or with the same data, the advantages of this technology would not be of any use and only the higher effort for production would be reflected in the price.
And here are a few basic facts:
Today's cathodes are able to deliver high currents. However, they should never be so heavily loaded during operation that all the electrons in the electron cloud (the "store" between the cathode and the control grid) are "burnt away". The maximum cathode current may therefore be exceeded for a maximum of 0,1 seconds if there is sufficient time afterwards to build up the electron cloud again.
The anode plate and the screen grid have to carry a current and are therefore one Performance exposed, which they must radiate as heat. If this power is exceeded, the parts begin to glow, which firstly results in a thermal overload of the entire tube, secondly, uncontrolled expansion of the electrodes occurs and thus possibly short circuits, and thirdly, the mechanical dimensions and thus the tube data can change permanently.
The control grid is incapable of accepting any current (above about 10 microamps) due to its delicate construction. Positive control grid circuits will damage the tubes in a very short time. Exceptions are special pulse tubes that are used in blocking oscillators. But the grid current time is very short and the recovery time is correspondingly long.