This post is intended as a very basic introduction to preamp tubes. For the sake of simplicity and minimizing confusion, the material here is very top-level. In other words, I will not be getting into in-depth technical discussion here. The purpose is to familiarize the complete noob to the fundamentals of what a preamp tube is and what it does using a 12AX7 as the example.
I'll also give a brief overview of a common gain stage configuration seen in guitar amplifiers and go over identification of schematic symbols in that regard. Again, I'll just be scratching the surface here. A lot of material on this topic starts out by going over very complex analysis. I'll try to keep this as simple as possible by going over the basics in plain English. Obviously, I won't be going over pentode preamp tubes or other odd varieties. Those are less common and the scope of this post is not intended to go over them.
Above is a photo of the most popular variety of preamp tube used in guitar amplifiers: the 12AX7. The 12AX7 is what is known as a noval variety (9-pin) dual triode. What does dual triode mean? In plain English, it means that this tube actually has two distinct, independent amplifier stages inside of its glass envelope.
To further understand this, let's take a look at the elements inside this tube. Pictured below is a common schematic symbol representation of a single triode out of a 12AX7 (one half of the tube).
In the image above, "12AX7" of course refers to the type of tube. "V1a" is the part number as it relates to the schematic. In this example, the part number represents the "a" side (or triode) of valve #1.
Within the schematic symbol, you see representations of three separate elements within the tube. These three elements comprise what we call a triode, hence the name. Starting at the top, this element is known as the plate (or anode in some references). The middle element is known as the grid. And the bottom element is known as the cathode.
Using these three elements, we can create a common preamp gain stage. The gain stage's function is to amplify an AC voltage (like your guitar signal). In many references, this is referred to as a "triode gain stage."
Dual triode tubes like the 12AX7 have two of these triodes within them, hence the name "dual triode." So in practical applications, you can use a single tube to amplify your guitar signal two separate times. Remember how the schematic symbol above only represented one half of the 12AX7 tube? We would call the other half "V1b" to represent the other half, or "b" side of valve #1. It would look identical to V1a. There are of course many different things you can do with these tubes other than gain staging, but the most common use is configuring them as standard gain stages so that is what will be discussed here.
There is one thing about the schematic symbol above that I haven't touched on, and that is the numbering. What do the numbers 1, 2, and 3 represent on the symbol? They represent the corresponding pin numbers on the tube. So pin 1 is V1a's plate (or anode), Pin 2 is V1a's grid, and pin 3 is V1a's cathode.
Let's take a look at the pin configuration of our 12AX7 below:
How do we know which pin is which? Looking at the orientation above, we see that all the pins are evenly spaced from one another except for two of the pins at the bottom. The two bottom-most pins have a larger gap between them. The left of those two pins is pin #1 and the numbering moves sequentially as you go clockwise around the tube. So the bottom-most pin on the right is pin #9.
So wait a minute! If this is a dual triode, meaning the tube contains a total of 6 elements, then how come we have NINE pins in total here??
The other elements I have not addressed yet are the heater filaments. The heater filaments are the ones you see light up nice and pretty when your tubes are powered up. Their job is to heat up the cathode, which I'll get into in a bit. But inside the tube, each triode has it's own heater filament. Triode "a" (represented by pins 1, 2, and 3) claims ownership of the heater connected to pin 4. Triode "b" has the heater connected to pin 5. Moving along, pin 6 is the plate (or anode) of triode "b," pin 7 is the grid, and pin 8 is the cathode. So pins 6, 7, and 8 would comprise "V1b" using our schematic example above.
The final pin, pin #9 is essentially a center tap connection to the two heater filaments inside the tube.
Why does the heater filament heat the cathode?
If you don't care about technical stuff just yet, you can skip this brief explanation. However, if you're curious, keep reading.
Inside the tube, we're dealing with the manipulation of electrons inside a vacuum. The cathode element of our triode has a coating covering it. When the heater filament heats up the cathode, electrons begin to boil off of this coating. There is a specific purpose for doing this, but I will not go into detail here. Just know that this is where your electrons are coming from in a triode amplifier.
So we now know that a 12AX7 is a nine pin tube containing two independent amplifier stages known as "triodes." Each triode is comprised of three separate elements, the plate (or anode), the grid, and the cathode. The other elements within the tube are the heater filaments whose sole purpose is to make the cathodes really hot so that electrons can boil off of them.
Let's take a look at how a standard gain stage is configured within a guitar amplifier. In the picture below, you'll see our V1a triode that we looked at before, but now it's connected to several other components.
For now, we won't get into what everything is doing in this configuration. Let's just identify what we are looking at here.
As you can see, the guitar signal is entering the triode from the left and going into the grid element. R1 is a resistor and is commonly referred to as a "grid leak" resistor. R2 is commonly referred to as a "grid stopper" resistor. R3 is commonly referred to as the "plate resistor" or "anode resistor." R4 is commonly referred to as the "cathode resistor."
C1 is a capacitor that is commonly known as the "coupling capacitor." And C2 is commonly known as the "cathode bypass capacitor."
All of these components contribute to the operation of the gain stage as well as the tone shaping that occurs within the stage. Sometimes you will see additional components in a stage. For simplicity's sake, I've kept the representation basic.
As you can see from the image, the amplified guitar signal exits the tube from the plate and travels through the coupling capacitor C1. From there it can go into another gain stage (like V1b), a tone stack, a gain knob, or anywhere else the amp designer has in mind at this particular point in the amplifier.
In a future post, I will go into more detail about what is happening in a gain stage like this as well as how the guitar signal's gain and tone is shaped and affected by the values of these components. For now, I hope you've gotten a good basic understanding of preamp tubes from this post.