Your Guitar Amp “Tone Formula”

Let’s jump right in and continue with part 2 of our discussion: “Your Signature Distortion”

My last article covered the DNA of a speaker and how the materials used factors into your tone.

This month let’s dive deeper and breakdown how a speaker distorts; and is the speaker producing what the amp can deliver?

Distortion is a broad term these days and there is a number of ways to achieve the distorted tone you’re looking for. If you have a tube amplifier you can get distortion from clipping your pre-amp and power amp tubes, not to mention that you can clip your input stage adding a bit of gain and compression right from the input of your amplifier.

Of course you have a plethora of distortion pedals on the market that give you an infinite amount of distortion produced by “fets (field-effect transistor)”, “op amps (operational amplifier IC)”, “leds (light emitting diode)” etc…

And all of these distortion makers fall to the mercy of the speakers; it’s how the speakers interpret this signal will determine your ultimate tone.

What is distortion?

Distortion is the coloration caused by physical resonance in a speaker creating additional sounds resulting from unwanted tones caused by the speaker’s mechanical or electrical operation.

How does a speaker distort?

When a speaker distorts it produces two types of frequencies, the first one is Harmonic distortion: This is heard as additional tones which are multiples of the original note played.

Example: If the original sound produces 100Hz, you would also get 200Hz, 400Hz and so on, even though these tones are not part of the original sound.

(The “Hz” symbol is the electrical abbreviation of Hertz; a SI unit of frequency that equals to one cycle per second)

The second is Non-Harmonic distortion, also known as “Odd Harmonic Distortion” and is often referred to as a buzz or a rattle in the sound.

Example: If the original sound produces 100Hz, the odd harmonic distortion would produce frequencies of 300Hz, 500 Hz, and 700Hz etc…

When setting your tone there are a series of specs you need to consider, starting with your guitar and amplifier.

The signal from a guitar pickup is mostly all midrange and is not rich in harmonics, with practically nothing coming through above 4000Hz. The sixth string “E” tuned to pitch comes through at about 80Hz, two full octaves above the 20Hz low frequency our ears can pick-up. The standard 4-string bass has a range one octave below the guitar with the low E at about 40Hz.

How a typical Fender/ Marshall amplifier is designed also plays into affect on how the speakers will respond. The most obvious difference is that the Marshall circuit lets more signal pass through, and the tone controls offer less frequency range. The higher signal means that the preamp tube stage can overdrive the output tube stage more.

The Marshall circuit has a slight dip in the mid range section almost an octave higher than Fender amps metering in around 700Hz, the Fender’s mid range dip is around 400Hz the bass response on both amplifiers meter in around 10Hz.

Fender's tone controls allow for a higher midrange frequency to pass with the treble response, meaning more dynamic range for that sparkling/ tight sound they're famous for.

To save time I’ll spec out the three most popular speakers associated with Fenders and Marshalls starting with a 25 watt, 12” speaker with a sensitivity rating of 98dB, 1.75” copper voice coil, Ceramic magnet, and a resonance frequency of 75Hz.

Another popular choice is a 30 watt, 12” speaker with a sensitivity rating of 100dB, 1.75” copper voice coil, Ceramic magnet, and a resonance frequency of 85Hz.

The speakers in Fenders are designed to stay clean so they spec out at 100 watts with a sensitivity rating of 99dB and a voice coil inductance of 1 kHz, Ceramic magnet with a resonance frequency of 104Hz.

How does this translate to your guitar tone?

It’s these specs that are directly responsible for your tone, and it’s here where the “disconnect” usually happens for most musicians. We are not trained on how these features translate into the sound we hear, and it’s this knowledge that can serve as a guideline for you to build your own “Tone Formula”.

Here is an example of a classic “Tone Formula”:

Example: 50 watt tube Marshall running through two 25 watt speakers. (100 watts with four 25watt speakers will produce similar results) The Marshall circuit let’s more signal to pass through meaning that the guitar’s input section, the pre-amp, and power section are going to distort. The lower wattage speakers with the smaller magnet and voice coil is going to break-up faster at a lower volume. The voice coil will clip and compress giving you that “Sizzle” in the upper frequency range.

This is by definition the classic “Marshall” tone, but here’s the rub. The lower wattage speaker distorts so fast that the speaker will not be able to produce the lower frequencies (Bass response) that the amp can put out; thus giving you the illusion that the amp has no bottom end. By adding a 30 watt speaker will increase the bass response but cut the level of distortion.

Now if you went to the other end of the spectrum and had a Fender amp that was to clean and brittle sounding, you could add a lower wattage speaker with a smaller magnet and voice coil, and achieve some nice mild distorted tones, changing the magnet to Alnico will add a bump to the mid-range section.

This is of course a very basic analogy of how we achieve our tones, but it’s these basic facts that are the building blocks to dialing in the sound you are looking for, and knowing how these specs affect one another will help you define your signature sound.

Stay tuned for part three in this series, where I’ll explain how cabinet construction affects the overall tone of the speaker, and how the enclosure could be your secret weapon in your “Tone Formula”.

Cheers,

Tony J. Pasko

©2009 - 2010 Fred Russell Publishing, All Rights Reserved. This article can not be used without permission from the Author. To Contact the Author email curt@RockHouseMethod.com