Tube Theory and Function
What is a tube?
A tube is a valve contained in a glass vacuum for the purpose of sound amplification. Considered obsolete by many designers since the seventies, the tube is still the hands down favorite for use in guitar amplifiers. They have an organic quality to their sound which most describe as "warm" sounding. Tubes also have idiosyncrasies which endear them to performers of music. They reciprocate subtle changes in technique which guitar players feel as a lovemaking experience.
In recent years they have become popular to process computer recordings to warm them up and give back some analog characteristics.
Bottom line; tubes are here to stay due to the demands of the audio industry, and their consistent results.
Tubes are thermionic valves. That is they get excited when they are hot. This is done by the filament which directly or indirectly heats the cathode. Lots of current but low voltage (about six volts). When the cathode gets excited it emits electrons (as men sometimes do). The electrons flow at a controlled rate; from the cathode to the plate...
What happens on the trip to the plate? They get bigger! A tube can be a diode; two components. A triode; three components. A tetrode; four components or a pentode: five components. We know that we need at least a cathode and a plate, so that would make a diode (two components).
A man named Forest introduced a third part called the grid, so that made a triode. A simple amplifier was born (a 12AX7 is a triode). To make a tetrode a screen grid was added. The screen grid works like a screen door. Only instead of mosquitoes, electrons are kept out. As a screen door keeps out the big bugs but lets in the gnats, so a screen grid lets the little electrons get through while blocking the bigger ones. This regulates or smooths out the flow of electrons.
At this point the electrons are screaming up the highway. They might be going so fast that they bounce off the plate instead of sticking. To fix this a suppressor grid was added. This part was charged negative as a reminder so the electrons would go back to the plate.
So a tube is basically; a grid, where the signal goes in. A plate where the signal comes out louder (high voltage, low current). How loud? That depends mainly on the cathode. The cathode can work like a brake on the tube. If the cathode is grounded, the tube will draw all the current it can. If a resistor is placed between the cathode and ground, that resistor's value will control how hard that tube works. This makes for a practical amplifier called a cathode bias amp.
Output tubes can be either cathode bias or grid bias. A grid bias amp has its cathode connected to ground. But it does not draw too much current because a negative voltage is applied to the grid. It rides piggy back on your guitar signal and limits the current draw on the tube by a fixed negative voltage applied to the grid.
Needless to say these two amp circuits sound and respond differently. So bias can be like a cop who says "Cool out tubes, you are having too much fun!" Less bias, they run hot, more bias they run cool.