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Put your hand in front of an empty electric socket, and you won’t get a shock— because electrons just don’t fly through space, right? Well … they will under the right conditions—like inside a vacuum tube.
Here we’re going to take a look at the inner workings of standard amplifier circuits—the tubes, transformers, resistors, and capacitors that work together to create the amazing tones that have powered countless songs for the past 60+ years. While this stuff may be daunting to some of you, take heart—this is century-old technology. The basic concepts really are not too difficult to grasp.
We’ll discuss amplifier circuits by looking at my absolute favorite small amp, a 1960s Vox AC4. While it’s small and simple, an AC4 actually is not the simplest guitar amp out there. Unlike Fender’s earliest tweed Champs, the AC4 has a tone control and tremolo, which gives us a bit more to talk about.
But before we get started, let’s make it clear that this article is not in any way encouraging or equipping you to open up the back of your amp and start poking around. Make no mistake: Amplifier circuits, even when unplugged, contain voltages that can kill you. And if you’re an amp tech, please excuse any oversimplification in the discussion—this is a primer for general consumption, not a compendium of possible exceptions and anomalous phenomena.
The Vacuum Tube
A Vox AC4, like many amps, is designed to make the power tube’s cathode slightly positive—a state that is commonly referred to in the guitar universe as cathode biased. Other amps, instead, put a negative charge on the power tube’s grid. That’s called fixed bias, and it has a similar effect. Either method causes electrons to stay put on the cathode until needed.
Learn how to control that magnetic field accurately and, as RCA did, you can display an image of Felix the Cat on a phosphorescent surface at the far end of the tube. The tube used in that case was the cathode ray tube (aka CRT)—better known today as an old, pre-LCD/LED/plasma television.
In guitar amps, we’re not that interested in displaying images with our tubes, but we’re still very interested in controlling those electrons—and we can use a guitar to do it. Picture this: In the center of a tube’s glass envelope is a cathode. It carries just a slight positive charge, and it’s ready to release a gazillion electrons. It’s especially ready if it’s been heated. Surrounding the cathode is the anode—although in the guitar universe we typically call it the plate. The plate carries a high positive charge that’s ready to pull those negative electrons toward it. To the highly positive plate, the cathode’s slight positive charge still makes the cathode seem negative (we’ll talk more about this slight positive charge later). If you place these two elements in a vacuum and power them up, electrons will fly relentlessly toward the plate. When you add a third element—the grid—between the two, you can control the flow of electrons. And when you position the grid close to the cathode and connect the grid to the relatively tiny voltages coming from your guitar pickups, something interesting happens: The tiny signal unleashes a flood of electrons, allowing them to fly freely to the plate. That rush of electrons from the cathode to the plate mirrors the signal from the guitar, amplifying its signal many times over.
Okay, so let’s get back to that earlier mention of the slight positive charge. The reason we want the cathode to carry a slight positive charge is that it makes the grid, with no charge yet applied, seem negative. Voltages are relative. And while opposites attract, like charges repel. The apparently negative grid close to the cathode will keep those negatively charged electrons in place until the guitar signal is ready to swing the grid positive to release them.
One other useful electron-related fact to know is the difference between voltage and current. Think of current as the amount of water flowing through a pipe. More current means more water being delivered. Voltage, on the other hand, is like water pressure— it’s the force behind that water. Increase the voltage (pressure) and you’ll increase the current (amount of flow). A resistor acts like a constriction in the pipe, with more resistance being analogous to a tighter constriction. So it follows that placing a different resistor in a circuit will affect both the voltage and the current.
What actually goes on inside a guitar amplifier is obviously a bit more complex than just the flow of electrons in tubes, though. Next we’ll do a quick overview of the additional parts involved, followed by more detailed, part-by-part descriptions.