A Tube Primer for Guitarists: Part One

Guitar Tube Schematic

Read Time 3 Minutes

How do vacuum tubes really work? How do they provide the sound they do? Do they need maintenance? How long do they last? Why do they cost so much? These are just some of the questions most musicians have. If you own a tube amp, you should be interested in the answers.

Tubes – What Are They and How They Work

Guitar TubeVacuum tubes, so called because there is a vacuum inside the glass ‘tube’, have been around since the late 17th century but practical vacuum tubes didn’t appear on the scene until the 1850s. The modern multi-segmented tube we know today popped up during the 1920s. Made from metal, glass and bakelite (an insulating ceramic-like material very resistant to heat), tubes are used as rectifiers, RF (radio frequency) generators and amplifiers. We are concerned with the first and last use here.

Tube Plate Grid CathodeTo the left is the diagram if 1/2 of a 12AX7, a typical preamp tube. 12AX7’s are dual triodes, so named for the 3 elements to the tube and there are two of them in a single glass enclosure.

The short explanation on how they work: the cathode is made of an electron rich material and connected to ground, or near ground (-). The plate is connected to high, positive (+) voltage and attracts the electrons from the cathode. (Positive attracts negative, basic high school physics 101, guys.) The grid regulates the flow of electrons from the cathode to the plate, sometimes blocking the flow and sometimes helping the plate attract more electrons, depending on the input signal (from your guitar). This process is called amplification because the resultant signal at the plate is the same as the signal at the grid, only bigger.

Guitar Tube Schematic
Click to Enlarge

Now, that’s really simplified and there are a lot of resistors and capacitors and inductors and potentiometers that make up the external circuitry for this to work properly. You really don’t need to know about that stuff and it’s beyond our subject here. To the right is the input for an Original Fender Champ, just to illustrate my point. That’s our 12AX7 in the center.

Why Do Tubes Sound Special?

The truth is… they don’t. They sound retro, because they’re analog and not digital. HUH? Easy. Tubes do not react instantly. Picture a grassy field in the wind. As the wind blows, the grass bends over as a response to the velocity of the wind – the harder the wind blows, the more the grass bends over. If you watch it, it doesn’t react instantly, but moves with a gracefulness that is quite beautiful to watch, much like a flag. As the wind changes direction, the grass ripples and some changes but some doesn’t. It flows, like water.

This is just how a tube reacts to changes in the electrical signal applied to the grid. The reactions take place in milliseconds but still, the tube reacts at its own pace (each one a little differently) so they all seem to have their own personality. Yeah, it’s a stretch… but that’s how we tube guys think of it. Anyway, that’s why they sound the way they do.

Let’s talk a little about that “tube distortion” we all hear so much about. When you have the volume down on your amp, the sound is crystal clear with tubes. So clear it’s disgusting, actually! Why? Because of the paragraph above. The louder you turn up the amp, the more your string attack will affect the tube. You are adding little spikes to the sound wave your guitar is providing to the amp and the first input tube is reacting to those spikes. It sounds like it’s following your playing… and, in reality it is. Some amps will do this better than others because of all the other circuitry around the tubes. Tube amp designers design their amps just to pull the sweetness out of the tubes and give them a “life.”

In Part Two, we talk about tube and amp maintenance, and why they cost so much. See you there.

More reading on Tubes:

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Doug Knight

Our “Man on the Street” reporter, with his “What’s New in Music Stores?” series, resides in Coos Bay, OR. You can find him on Friday nights at The Small Events Center at OrCoast Music in Coos Bay.

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