HOW THE HECK DO TUBES WORK ANYWAY?
So you run to the bookstore and buy a book on basic electronics. Hmmm, no tube chapter. Even some ‘good’ tube amp books don’t do a good job of explaining how these work. There are many different tube types out there, but guitar amps use three types of tubes: triodes, pentodes, and rectifiers.
Let’s start with the rectifier, because it is the simplest type of tube. Inside the glass bottle, there are a few metal parts: filament, cathode,and plate. The filament is sometimes called the heater, because that is exactly what it does – it heats the cathode. Like the filament in a light bulb, the tube filament is a thin length of wire that gets quite hot when electricity flows through it.
When the filament heats the cathode, electrons ‘boil’ off the cathode and flow toward the plate. Electrons flow from negative to positive, so the cathode is negatively charged and the plate has a positive charge. This flow is a sort of one way valve. Remember that electron flow is the opposite of ‘conventional’ current.
(The ancients did not know which way the electrons really flowed. They knew that there were invisible bits of charge that flowed through wire in a circuit, but had no way of determining the direction the bits traveled. They took their best guess. Bzzzzt! Wrong! By the time the not-soancients realized the mistake, it was too late. To this day, when we talk of electric current, we really mean ‘the negative flow of electron current’.)
So, current flows from the positive charged plate to the negative charged cathode, just like current flows through a solid-state diode, from its anode to cathode. But the rectifier tube used in the AX84 has two plates. Just think of it as two diodes with the cathodes connected. In our circuit, it is used as a full-wave rectifier.
The grid in a triode is a mesh of thin wire or wires positioned between the plate and cathode – very close to the cathode. If you were to connect the grid to the cathode, the tube would behave somewhat like a rectifier. Most of the electrons flow right past the grid on their way to the plate. Please do not try this. Most triodes are not designed to be operated this way.
When charge on the grid is made negative with respect to the cathode, the electron flow starts to get ‘pinched off’. The electrons are repelled by the negative charge on the grid. Now, make the grid voltage negative enough, and the electron flow will stop.
This DC voltage applied to the grid is called bias. The voltage that cuts plate current to zero is called cut-off bias. Increase the bias voltage and more plate current flows, up to a limit. The point where plate current no longer increases, regardless of increases in bias voltage, is called the saturation point. Between cut-off and saturation, a triode behaves as a linear device.
How do we make sure the tube operates in this linear range? By setting the grid bias about halfway between cut-off and saturation.