Let’s be honest: You don’t understand tubes. Of course, I don’t mean “you” as the guitar player and musician you are, but rather everyone else; you know what I’m talking about, those guys who don’t understand how you can get a tube to sound punchy yet warm. However, you should still read this.
What are tubes?
Tubes get their colloquial name because they are glass tubes in a vacuum and some mojo inside, hence “vacuum tube”; However, a more accurate name is “thermionic valve”. There are essentially three concepts in a tube: Temperature, ions, and flow.
Tubes are made of four parts: The filament (Also known as the heater), the cathode, the grid, and the anode (Also known as the Plate).
Suppose you remember anything from your middle-school physics classes. In that case, you probably know that opposite charges attract each other, and electrons (negatively charged) move towards positive charges, thus creating the flow of current we know as electricity (You’ll understand why this is important later). Did you ever wonder why you need to heat your tubes before playing? This is because tubes have cathodes covered in a coat of electron-emitting material, Which, when heated, causes electrons to detach from the cathode and roam freely inside the tube. Or at least that would be the case if the anode (that has a positive charge) wasn’t there to attract the electrons creating electricity inside the vacuum-sealed tube.
This means that there is no flow of electrons without heat, and it also means that current can only flow in one way (which is the principle of the rectifying tube that converts AC to DC).
Congratulations, you’ve now understood the basics of tubes!
Let’s move to the typical tubes you’ll find in your amp’s preamp stage: Triodes. In a rectifying tube, there is no way of controlling how much current runs inside of them, that is why triodes solve this issue by adding a structural part called a grid.
The grid polarizes with positive and/or negative charges, which creates an electrical field altering the electrons running inside a vacuum tube using electricity: If the grid is charged negatively electrons will be repelled from it, reducing conductivity and thus, electrons making it to the cathode (allowing fewer electrons to make their way through). On the other hand, if the grid is positively charged, electrons will be attracted to it, and the plate, increasing its conductivity.
Now, how do we turn a triode into a sound device? Philosophical debates aside about what makes sound: The answer is relatively easy: If tubes have a flow of current inside that can be controlled let’s just use our guitars to control it, by connecting the guitar to the tube’s grid, electrons will polarize in an equal (or analog) way of the output produced by the guitar’s pickups, essentially the guitar is just a remote control that tells the amplifier what sounds to produce inside.
Why did I say earlier that the cathode goes to the ground? Essentially a midpoint for the signal has to be set because if there is no potential difference between the cathode and the grid: There is no flow of current, this is why the cathode and your guitar share a common ground connection, except the cathode has a resistor attached to it; this is called cathode biasing, and the result of it is that the voltage on the cathode dynamically reacts to the signal on the grid adjusting its operational point.
So far, we’ve covered three of the four parts that make a tube and how they are handled, let’s continue on somewhat the simplest part: the heaters.
As previously stated, a heater is essentially a wire that heats up and makes the cathode’s coating emit electrons, now, how much voltage does the heater require? The name is in the tube itself! 12AX7/AU7/AT7 and any other name that starts with 12 requires 12 volts, while 6L6/6V6/6V6GT/6BQ5 tubes require… you guessed it! 6 volts.
So, EL34s must specifically require 2.718281828 volts, right? Well, no. In this case, E has nothing to do with the Euler number, this nomination comes from the Mullard-Phillips tube designation where “E” stands for 6 V in parallel and 12 volts in series. So… tubes are easy, 12AX7s require 12 volts, and ECC83 tubes require 6 volts, but wait, ECC83s and 12AX7s are the same, do they require different voltages according to denomination? Well no, in (almost) all cases, dual triode tubes are used instead of single triodes, what does this mean? Essentially you have two triodes in a single tube package having their filament in series and that’s why dual triodes require 12 volts to function.
Another question you might be asking yourself is: “If amps are essentially tubes wired into each other, how come amps that use the same tubes in the same amount and with the same functions sound different?” Well, the tube is only the part that drives the signal, however it’s the surrounding components that make the circuit, some circuits might be biasing the tubes differently, might have different values of operation, might be stacked in different ways, and the combinations are essentially limitless, what this means is tone in an amplifier doesn’t come from the tubes (as tested), but rather from the combination of both the tube itself and the parts around it that compose the circuit.
