I don't know much about vacuum tubes, but often I have read statements like "the cathode heats up, causing electrons to boil off and fly to the anode".. which makes me scratch my head..
How does an electron boil? Can anyone explain this?
Are there degrees of boiling, like a pot of water, or is there an on/off threshold?
Then there's the grid mask, whatever that is... how come the electrons don't smash into that? How do they find the holes? Is this like synchronized diving, they're trained to hit the water at specified spots?
Richardson equation; work function; partial field emission; LaB6; tungsten; pure field emission (hot [schottky] or cold [very tiny tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.
I worked for a short time (a year +) with _the_ electron microscope company in a small technical capacity. Some things managed to rub off. The emitters I worked with used a combination of thermionic emission and field emission (partial) and the Wehnelt is pretty much impossible to miss, as well. LaB6 was one of those materials sputtered on for enhanced effect, as I recall, on some emitters (other materials were also used.) Work function is obvious -- can't avoid the concept for even a few seconds, as it permeates everything interesting in the world. Richardson, Fowler, and Nordheim get name dropped from time to time, as do others.
Not quite a correct description, because once they "boil off" it is the electric fields in the tube that cause them to be accelerated and "fly" to the anode.
Pretty much like water does. In water you have a "sea" of water molecules bouncing around off each other. The amount of action if determined by the temperature of the water. And the distribution of water molecule velocities has a statistical distribution with some having high velocities and some low. When you start to have lots of high velocity molecules some of them start to leave the liquid and are vapor (or a cloud of individual molecules).
In a metal, one pretty much has a "sea" of free electrons. These also have a statistical distribution of velocities that depends on temperature. Again, if you get the temperature high enough there are enough "fast" electrons that they are leaving the metal.
There is not a distinct 'boiling point" like with water, but the number of electrons escaping does depend on temperature. Usually this temperature has to be pretty high. Edison discovered the tube by noting the current that flowed when he put a metal plate inside one of his light bulbs. However there are distinct lifetime advantages if you can get eletron emission at lower temperatures. 20th century tube "technology" was very much involved in the black magic of producing long-lived low temperature high current cathodes. Today, some of this old technology appears to be lost, like the technology of how to make a decent mummy.
A grid is just a bunch of wires that is use to create an electric field to accelerate the electrons away from the cathode. (or repel them back to it). Electrons DO smash into it. But like a garden hose at chicken wire, most of them go through the holes. The grid is MOSTLY "holes"! There is no need to have them all go through the holes. [Well usually, if we were to discuss the theory of beam power tubes, we'd talk about the problems of grid heating due to the electrons that smash in, but this is no problem in smaller tubes.] PLUS, it depends on how the tube is being operated. If the grid is negative, then it is creating a repelling field. The field gets stronger as you get near any given grid wire so the electrons tend to be deflected away from the grid wires. OK?
Oh look Muffy! It's a guy who knows "everything" but is willing to explain nothing! (much of which has little to do with the subject in question except tangentially)
Thorium gives off electrons rather nicely when heated, although radium (Ra226) gives off electrons when stone cold or even when cryogenic cold. Radium is a cold cathode electron emitter, and otherwise extremely nifty beyond most imaginations. Radium is also one of the most secretly horded elements on Earth, with a 1600+ year half life to boot.
Not exactly. Electrons will flow from cathode to anode with zero field, or even a bit of reverse field. The electrons ate kicked out of the cathode with a couple ev of energy. There have been thermionic generators that work that way.
Electrons in a metal or similar are bound by bonding forces. As you heat things up, some electrons acquire more energy and, if near the surface, may break away. The energy is called the "work potential", measured in volts (or electron-volts.) Once free of the surface, any extra energy past the escape potential gives the electron some velocity.
Below some critical temperature, there's insufficient energy to escape. Above that, electron emission goes up fast as temp rises.
It's a very sparse grid of wires, so most electrons miss, just drift through the big holes between the tiny wires. If you apply a high negative voltage, you can force the electrons further from the wires, crowd them into the gaps betweem, and eventually shut down the gaps completely.
Is this like synchronized diving,
They are random and unsynchronized, although they do repel one another, which adds a certain sort of order.
In fact, one of the methods of biasing small-signal tubes is to drive the grid through a high (10M-ohm in some cases) grid-leak resistor. This lets the grid bias itself negative.
It can't be done on just any tube (and no, I don't know enough to know which ones it can or can't be done on), and if the grid ever gets hot enough to start emitting electrons on its own it'll go positive and the tube will run away and (probably) toast itself, so it can't be done _at all_ on power tubes.
As pointed out, the electrons "boiling" is an analogy of what's really happening. Do a search on "thermionic emission" and "work function" to get more of the scoop.
Or go find a tube physics textbook from the late 40's or early 50's, when they had found out a _lot_ about tubes but hadn't started using transistors yet. You'll also find older (and, oddly, brand new) ARRL handbooks helpful -- their explanations are aimed at giving a technician enough understanding to work with circuits, not giving an engineer the best understanding possible, so they simplify things to the point of leaving erroneous impressions, but they're very good for getting you up and going.
Like this one:
formatting link
The electrons don't "find" the holes -- the grid repels the electrons by being negative, or it attracts the electrons by being positive. If it attracts electrons, then grid current flows.
Once again, an old tube electronics textbook from "the days", or an old ARRL handbook will make this clear.
AFAIK, class C transistor amplifiers aren't all that much more efficient than class C tube stages, because the switching isn't at all crisp. But tubes have much too much capacitance to make a class E stage, and with clever design you can make a (transistor) class E stage that has efficiencies approaching that of a class D (base band switching) stage.
(I haven't actually tried a class E stage, but I know it's been done. One day...)
This is clearly an application of the original question about do electrons hit the grid (mask). Note that the field that the electrons see has not only to do with the metal structures around (grids, cathodes, supports etc.) but also with the other electrons that are close to the given electron (the so-called electron cloud). Not that we want to go into this in any detail, but since electrons are "boiled off" with some energy, if the electron "cloud" is minimal, those electrons striking the metal of the grid will charge it. If that grid is connected to ground by a VERY high resistance, that charge will build up on the grid eventually repelling the new electrons coming off the cathode and biasing the tube. If the tube is built with such dimensions that an electron cloud can exist in front of the grid, that negative "cloud" will repel all electrons from the cathode and none will reach the grid. Hence it will never get negatively charged and hence will not work with a "grid-leak" bias. OK?
By heating up the atoms you give the electrons energy(the nucleus is tightly bound). If they get enough energy they are able to escape the nucleus and eventually the material(this isn't completely true though).
The energy needed to accomplish this is called the work function.
Depends on a lot of factors. Temperature is the main thing but it also depends on what surrounds the material. What generally happens is that when an electron does escape it gets immediately attracted to the material(remember the material ends up becoming positive). So what you end up with is a "cloud" of electrons around the material and eventually no electrons can escape the material. (this is why you need a electric field to get them to move and a current to replace those lost.)
No, They do hit the mask. The mask exists to attract those electrons from the cathode. e.g., you put a voltage across it and electrons will be attracted to it. Since the grid has holes in it the electrons usually will go through it. Also if they do hit it then that grid will build up a charge... those electrons must be removed. Hence the grid usually is grounded by a large resistor so electrons can flow off the grid.
Also, when they hit the grid they usually can just bounce off because they have so much momentum. This is called secondary emmissions and usually occurs when there is another grid such as a tetrode(which causes a pretty big problem and why the pentode was designed).
It's quite simple:
You heat up the cathode. This causes electrons to gain enough energy to break away from the material. If any do they are replaced by new electrons from the power supply. (you can heat up the cathode with the same current but generally this is a bad idea because you what a steady temperature)
Then the grid supplies an attraction so that any electrons that do escape will move away from the material and be attacted to it... they are able to go through the grid(since it is a mesh) and they finally end up on the anode.
Note that not much current is actually flowing(usually uA's to a few mA) but high voltages are present.
If the external electric field strength is high enough, you can rip electrons out of metals even at temperatures near absolute zero. The required fields are huge, 5e10 v/m or some such. With a very sharp metal tip, you can reach fields like that with mere kilovolts of potential, and interesting things can happen, like imaging individual atoms.
This is actually a field-ion image, but it works with electrons, too.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.