That’s right, we’re finally getting to tubes. If you’re starting with this article, you’re a sneaky effer. Good job. But go back and read some of the basic concept pages sometime because I put a lot of work into them (not really) and they were very boring to write (except the parts about explosions or bananas).
Tubes are basically magic. They take electricity (that wants to kill you) out of the wall and turn it into sweet music from your speakers. This transformation is both complicated and steeped in legend. The most basic tube used for amplification is the triode. Triode is from ancient greek. Tri is ‘three’ and ode refers to the greek god Odin, who threw electricity like Joe Montana. There are three tiny electron-slinging demigods in your tubes and they get super pissed every time you heat them up. This is where the magic comes from. Rad.
Ok, none of that was factually accurate, per se. But you don’t need to know exactly how the tube does what it does, do you? You just have to know enough to harness its magic (and/or science). So let’s talk the basics.
The three (aka four) parts of the triode tube:
Anode – This is also called the plate by old farts. The anode is positively charged and so it attracts electrons. It’s usually at the top of the tube symbol on a schematic.
Cathode – This guy pukes out electrons all day long. The cathode is typically depicted at the bottom of the tube diagram. It needs to be warmed up for the electrons to start flowing. Just like your mom.
Grid – The grid is the bouncer. The grid decides how many of the cathode’s electrons are going to the anode’s party. In the tube symbol, it’s the dotted line in the middle. Some tubes have more than one grid, but we’ll get to that later.
Heater- The heater and its connections aren’t always shown in schematics, but it is always there. Some tube’s cathodes are the heaters, too. The heater does what it sounds like. It gets hot.
So when a tube is operating, its heater is getting hot and warming up the cathode. When the cathode is hot, it starts blasting out electrons. These negatively charged electrons are attracted to the positively charged anode, but the grid sits in the middle and determines how many electrons get through.
How does the grid determine how many of the cathode’s electrons get to the anode? With its own voltage charge. Like with magnets, same charges repel one another, and the stronger the charge, the stronger they repel. If the grid has a strong negative charge, the negatively charged electrons have a harder time getting past it to the anode. When the charge on the grid is less negative, electrons can flow to the anode without a problem. If the grid becomes positive in relation to the cathode’s electrons, it starts attracting them. That’s usually a bad thing, so we will almost always ensure that the grid is negative (relative to the cathode) throughout its operational range. That sounds a little complicated, but it isn’t. More on it in later sections.
The grid, then, controls the conduction (flow of current) within the tube. We use this to harness the little gods of the tube to make music. Think about recorded sound for a second. You’ve seen it represented as a wave, right?
Sounds are made up of waves (ok not exactly, but don’t be a dick). Waves go up and down. The audio recording process turns up and down sound waves into up and down voltage waves. These small up and down voltage changes are applied to the grid of the tube and thus the grid’s relative charge goes up and down. The tube uses this up and down voltage change at the grid to modulate the much larger flow of electrons from the cathode to the anode. So a little voltage signal at the grid can become a bigger and stronger voltage signal at the anode.