You are here to learn more about a technology that has been obsolete for a quarter-century. Why? Because tubes give a warm sound that solid state amplifiers can only try to emulate. Tubes are less reliable and much more fragile, but to a tube enthusiast, they are the only choice. So how does a tube work? That's what this section is all about.
Here we go. We will start with the picture on the far left and work toward the right.
Inside a basic tube there are four parts: the heater, the cathode, the grid and the anode. The heater is a filament, like a standard light bulb that gets very hot. Electrons are "boiled" off of the heater and are free inside the tube. From basic physics we learn that electrons are negative and opposites attract. In the first image we see that the boiled off electrons are attracted to the positively charged plate.
The second image shows what happens when we insert a grid with a charge that is 0 relative to the cathode. The electrons still are still attracted toward the anode.
In the third image we see that is the grid is negative with respect to the cathode, the electrons are repelled from the grid, and consequently repelled from the anode. If we made the grid positive the electrons would be sucked through quickly and accelerated to the anode. This "speed" is the current or amperage going through the tube.
The idea is to "Set" the grid so that when no note is being played the grid is negative, and no (or very little) current goes through the tube. When a note is struck, it raises the grid voltage to positive and the electrons accelerate to the anode. We can now see that the grid controls the tube and must be set to some base level. This level is the bias of the tube and is often referred to as an "idle."
Bias is one of those things that is very misunderstood in the tube amp world. If you can understand that there is this perfect base voltage that allows you to properly control the valve, you are way ahead of the game.