Even and Odd Harmonics??

M

MesaHead

New member
i've heard the subject in passing.........

through experimentation with a friend we tried our two amps side by side.

his amp was a YB-1 Traynor Tube amp from '67.
mine was a 2003 Mesa Boogie F-100

we put a distortion pedal through the traynor and played a chord. we let it ring out and waited for the feedback to come through. it was a harsh note that came out and it sounded dissonant.

we did the same thing through the boogie and the feedback that came out was resonant. not only that, it was in key with the chord, either the third, fifth or unison. but the amazing thing was that no matter what chord you played, even semitones up, the feedback would always hone in on the right key and match the chord.

is that "even and odd harmonics"?

if not what is it? and then what's the explanation of even and odd?

the F-100 ran 4 6L6's and 6 12ax7's
the traynor had 3 12ax7's and two larger tubes (larger than 6L6)

i don't think the F-100 had a rectifier in it either.


so many questions, so little time.

the next question will be about....... speaker phase?
 
Even and odd harmonics refers to the harmonic overtone series. I don't remember all of the intervals (I am sure you could find it if you did a Google search), but every note is made up of a series of frequencies, and they are always at the same intervals. The tonal quality of the instrument is made up of the relative loudness of each frequency. The order, as far up as I can remember it, is an octave, a fifth, a fourth, a major third, and then it is either another major third, or a minor third, then a minor third, and then it starts getting into seconds, and even smaller intervals. At any rate it continues up like this as far as x-rays (although I am guessing the average musical instrument does not put out many x-rays).

The basic formula, from a physics point of view, is to keep adding the frequency of the fundamental (also the first harmonic, though not the first overtone). So for instance, the open A string on a guitar (which I am using for the simplicity of the math) has a fundamental (or first harmonic) of 110 hertz (Hz). The second harmonic (or first overtone) is 220 Hz. The third harmonic will be 330 Hz, and the fourth is 440 Hz, and it continues on from there.

As you go up the series, the odd harmonics tend to be more dissonant, the even more consonant. For instance, the flute is made up of primarily even harmonics (though it is in fact as close to an instrument with a pure fundamental as you will find in acoustic instruments), where as the oboe has very loud odd harmonics.

When you get into amplifiers, Tubes tend to amplify even harmonics, and transistor amps tend to amplify odd harmonics (this is of course a gross over-simplification, but it is more or less accurate). The feedback is caused by one or more frequencies in the guitar being so loud that it over-excites one or more of the strings, and they ring in sympathy to that frequency. With a tube amp, they will be even harmonics, and with a transistor they will be odd (typically, but not always). They are also almost always lower order harmonics (within the first four or five overtones).

Amazing the shit you can learn in a Physics of Acoustics class.


Light

"Cowards can never be moral."
M.K. Gandhi
 
The feedback test you did only proved that the amps sound different. The reason you got happy feedback from the boogie and crappy feedback from the tranor is more or less because the boogie sounds good on a guitar and the tranor probably doesn't. If you mess with the tone controls on the boogie to more or less match the sound of the tranor, you will get the same feedback from it. BTW all tube amps have rectifiers. Some tube, some transistor. Boogie just gives you the choice.
 
Farview said:
The feedback test you did only proved that the amps sound different. The reason you got happy feedback from the boogie and crappy feedback from the tranor is more or less because the boogie sounds good on a guitar and the tranor probably doesn't. If you mess with the tone controls on the boogie to more or less match the sound of the tranor, you will get the same feedback from it.
Click to expand...

Sometimes you can't though, the tubes themselves generate overtones, and different tubes sound different because of this. So it might not be the amp, it might be the specific tubes in the amp. You just gotta swap things around & try 'em out.

In case anybody is really bored, the overtone series is:

1: tonic
2: octave
3: fifth
4: 2nd octave
5: third
6: fifth
7: minor seventh
8: 3rd octave
9: second
10: third
11: tritone
12: fifth
13: minor sixth
14: minor seventh
15: seventh
16: 4th octave

Of course these intervals are in the natural scale, not the equal tempered scale used in modern music. So if you play one note, odd order distortion might sound OK, but when you play an equal tempered chord with odd order distortion, it will sound very bad.
 
I know the harmonics are "odd" when I'm playing a Mozart adagio for my wife, and I hit a wrong note, and she gets up and goes to the kitchen to pour another glass of wine.
 
Last edited:
mshilarious said:
In case anybody is really bored, the overtone series is:

1: tonic
2: octave
3: fifth
4: 2nd octave
5: third
6: fifth
7: minor seventh
8: 3rd octave
9: second
10: third
11: tritone
12: fifth
13: minor sixth
14: minor seventh
15: seventh
16: 4th octave
Click to expand...

Actually, that is the harmonic series. The overtone series starts at the 1st octave.

And yes, that is an incredibly anal semantic correction.


Light

"Cowards can never be moral."
M.K. Gandhi
 
I used some FX rack about 12 years ago that had a 'harmonic exciter.' This was supposed to act as a partial emulation of a tube amp, rather than a solid state amp, no? That is what a sound tech/engineer told me about it, anyways.

:cool:
 
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