A Tube Primer for Guitarists: Part Four – Tube Distortion

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So far, in this series, we’ve covered:

I’ve added this 4th part to cover tube distortion for those of you who may have questions about that. Is it good? That is the basic question. First, let’s talk about it in general.

What Is Tube Distortion?

Tube Distortion
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The Fender Twin Reverb from the ’60s is probably the best clean amp ever. There were no “presence” or “overdrive” knobs then so the volume was your only option. This amp just got louder and wouldn’t break up until you entered the dreaded (cue reverb…) “ear-bleed land”. Today’s amps are designed to give you that overdriven sound that eveybody craves.

Remember the picture at left from the first tube primer? I’ve added some rudimentary signal waves at the bottom to illustrate how tubes work. They are called “sine” waves and, in their pure form – the one at the left – they represent an input signal that rises above zero volts, and drops below zero volts an equal amount. The amount it rises and drops is determined by the amount of voltage supplied to the circuit. The length of the wave is measured in time. (We’ll call these 1 millisecond long. (1/1000th of a second)) The red lines indicate where in the circuit you would find this signal.

The middle wave shows a typical signal coming from your guitar, and the third shows what happens when the tube is saturated. You can see the third one is flattened out on the top and bottom, making a square wave. This is where the distortion comes from in the sound. The circuits are actually tuned in modern amps to take advantage of this situation and provide a sweet sound with pleasant harmonics in the signal. In other words, they sound good. This condition happens when the tube reaches the maximum amount of signal it can pass through. Everything over that is “clipped” off and a square wave results, saturated with all the overflow signal in the form of distortion. The tube is saturated, flowing the maximum amount between the cathode and the plate, which produces all kinds of spurious frequencies inside the tube (remember, they’re RF generators), which are the source of the harmonics.

“Distorted amps sound cool”

Now, to keep everything simple (because this isn’t tube electronics 101), tubes were not designed to operate this way. They were designed to operate like the Twin Reverb – clear and bright and loud. Somewhere back before the day, someone got the bright idea that a distorted amp sounded cool. Soon, everybody was turning their amps up, the amp makers added an “overdrive” circuit and, the modern amp was born.

A tube in a well designed circuit will provide wonderful clear and bright clean tones, and complicated crunch or saturated tones when overdriven. The downside to this: the tube is operating beyond its design parameters and can be damaged by continuous operation in this condition. Amp manufacturers know this so modern amps are designed to take advantage of this cool side effect of tube amplification without putting the tube in imminent danger of self destruction. Vintage amps are another story, though. You must use some care to prevent damage to them.

Power tubes can provide some especially sweet tones when they distort – big and meaty with lots of grit. Here again, modern amps will survive this situation and are designed to take advantage of the EL84s tendency to break up before a 6L6 will. Most American vintage amps use 6V6 or 6L6 power tubes. Use care with these to prevent a melt down and a huge repair bill that will ruin your day. (I speak from experience, here!) Don’t operate the vintage amp in power tube overdrive for long periods. Let it cool down and get your overdrive from your pedalboard instead. A happy amp will provide years of trouble free service. An unhappy amp will pass the unhappiness on to you.

The internet is full of some great information on this stuff if you want to know more. Search for “vacuum tube troubleshooting” or “vacuum tube theory” for more info.

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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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