So What Are Vacuum Tubes Anyway?

The extremely simplified explanation is that it's a device that controls electric current, the same way transistors do today. they're big and bulky and power hungry so they've been replaced in most tasks except for some very specialized ones, which we'll get to later.

What Makes a Vacuum Tube?

A Vacuum tube is made of, at a minimum, three things (we'll add more later, don't worry!)

• The Envelope: A Glass, Ceramic, or Metal capsule that's been evacuated of air (or in some cases filled with a rarified gas for special purposes)
• The Anode: The positively charged electrode
• The Cathode: The negatively charged electrode

The Anode and Cathode are placed some distance apart in the tube, in some designs this is done by making them concentric cylinders and sometimes parallel plates. for ease of making diagrams I will be drawing simplified plate style tubes.

this is the general structure, it's missing a few things like structural components and leads. we'll build up to them though

How does this control electricity?

To understand that we're going to have to take a physics detour, don't worry just follow the tour guide's little yellow flag!

The atom is made of a few things protons and neutrons in the center and electrons buzzing around. the electrons are bound to the nucleus by electromagnetism because they have a negative charge and the nucleus has a positive charge.

In order to get the electrons to do something useful we need them to move around, and to do that we need them to be less tied to the atoms their clinging to. there's a few ways to get energy into them but the one we care about is heat. the hotter the material the more the electrons move! With enough heat they will be moving with so much energy that their kinetic energy exceeds the binding force created by their attraction to the nucleus. this breakpoint is called the Work Function of a material, but we don't need to worry about what that is to understand tubes.

So you heat up the metal and electrons are buzzing off in every direction and after they left the cathode is now positive because all the electrons left taking the negative charge with them. Now we have another problem, the electrons are attracted back to the cathode now! that's not enough movement to be useful. progress but not enough. now, what if we pumped more electrons in to replace the ones that left? that would give us a continuous flow!

eventually if we keep throwing electrons around the tube would become negatively charged and flow would stop because the electrons would be pushing back, to fix that we put the anode in there to collect the electrons. the anode is connected to the positive side of the circuit so it attracts and collects all the electrons floating in the vacuum between the plates!

because only the cathode is hot that makes it so only that side of the tube can spit out electrons, making it function like the diodes we all know and love (and also the only semiconductor component to keep the name of it's tube equivalent!) and only allowing flow in one direction

Getting Thinky With It

So we have our one way valve, but that's not enough to build computers or amplifiers or any of the fancy things we use tubes for.

we need some control. this comes in the form of gates. these are little grids of metal put in between the anode and cathode, we can give them a positive or negative charge or leave it neutral. The interesting part comes from when we put a negative charge on the gate.

Like charges repel so suddenly there's a wall there! we've turned off the flow of electrons! If we make it a strong negative charge it will shut it down but if we make it a weak charge we'll just dampen the flow a bit.

this lets us amplify signals!

Adding Some Complications

So we can use them to control signals, this is SUPER helpful! Tetrodes and Pentodes are all built on the same principle, more gates that do various things, like suppressing secondary emission from when electrons bounce off of the anode instead of getting caught.

Those are the kinds most people think of when they think vacuum tubes, but there's other kinds too. You're almost certainly familiar with the venerable CRT, it works on the same principle, except magnetic lenses focus all the electrons into a tight beam instead of an even flow and the anode is covered in phosphors that glow when the electrons hit it. (bonus fun fact: CRTS were originally designed as a way to store data! the glowing CRT was used to store bits at each dot, but that's worth its own post)

But there's a LOT of kinds of tubes out there! so I'm going to talk about my favorite to give an example of what kinds of things can be done with them.

My favorite, the Veertrons...

Lets start once again with the physical structure!

What are they good for? detecting signal strength! depending on the voltage applied to the steering plates the beam moves around, sending electrons to a different detector anode. this can be something as simple as a single plate detector that sends a signal as soon as the voltage reaches a certain level or you can do something like this!

The shutter is arranged so that the flat beam will make binary numbers on the detectors!