From fto–(at)–etcom.com Sat Aug 26 22:23:18 CDT 1995
Article: 3121 of alt.guitar.amps
Newsgroups: alt.guitar.amps
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From: fto–(at)–etcom.com (Tom May)
Subject: Re: 6v6gt, gta, gty
In-Reply-To: Dr.Distortio–(at)–bs.mhv.net’s message of 25 Aug 1995 18:02:03 GMT
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Date: Fri, 25 Aug 1995 22:58:24 GMT
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In article <41l36r$cr--(at)--ver.mhv.net> Dr.Distortio–(at)–bs.mhv.net (Dr Distortion) writes:
>That makes sense. If the operating points of the two tubes are somewhat
>mismatched, the clipping points should be skewed, which supposedly
>generates more even-order harmonics. I don’t know the actual
>mechanics of why that is, since it involves shit like Fourier analysis
>that I really don’t understand too well.
Here is an intuitive (I hope) explanation of this odd/even harmonic
stuff with no Fourier analysis. It may not be rigorous, but it helps
me to remember what’s what with harmonics and distortion:
Take one sine wave (this is gonna be tough):
***
** **
* *
* *
-*———–*———–*
* *
* *
** **
***
That is the first harmonic, also known as the fundamental. The second
harmonic is a sine wave of twice the frequency. Here’s a plot of the
first(*) and second(+) harmonics:
***
** **
* *
* +++++ * +++++
-*–+—–+–*–+—–+–*
++++ +++*+ +*+
* *
** **
***
If they are added together, the resulting wave is asymmetric. The
second harmonic increases the peak of the first half, but flattens the
peak of the second half (I cheated a little on the addition here to
make it look better):
–(at)—(at)–BR> –(at)–*–(at)–BR> –(at)– –(at)–BR> * *
–(at)–+++++–(at)– +++++
—(at)–+—–+–(at)—-+—–+–*
–(at)–+ +–(at)–+ +–(at)–BR> –(at)– –(at)–BR> –(at)—(at)—(at)—(at)—(at)–BR> ***
That is what your asymmetrically biased push-pull output tubes are
doing to the wave — giving it lots of second-harmonic distortion
(same for a SE output). Admittedly, I adjusted the phase of the
second harmonic to make the result look like this (i.e., I moved it
left or right until its peaks coincided with the peaks of the
fundamental), but that is not important. What is important is that
whatever it is doing to one side, it is doing the opposite thing to
the other side, regardless of the phase.
Here is the picture with the third harmonic:
***
** **
* *
+++ +++ +++
-+—+—+—+—+—+—+
+++ +++ +++
* *
** **
***
Adding together:
–(at)–***–(at)–BR> –(at)—(at)—(at)—(at)—(at)–BR> –(at)– –(at)–BR> +++ +++ +++
–(at)—-+—+—-(at)—-+—+—-(at)–BR> +++ +++ +++
–(at)– –(at)–BR> –(at)—(at)—(at)—(at)—(at)–BR> –(at)–***–(at)–BR>
Here the wave is clipped symmetrically. Even if you adjust the phase
of the third harmonic, the resultant wave will always be symmetric —
what happens to one side happens to the other. This same symmetry
holds for all odd harmonics.
So the moral of this whole thing is, if you have an asymmetric wave
(clipped harder on one side, say, as in a single-ended circuit or
mismatched push-pull), you got even harmonics (mostly second), and
possible odd harmonics as well in smaller amounts. If your wave is
distorted symmetrically (as happens in push-pull with matched tubes),
you got odd harmonics only (because even harmonics would cause
asymmetry).
I should be clear what I mean by symmetry. What I mean is, with a
symmetric wave, you can take the top half and flip it over the
horizontal axis, then slide it to the right and it will match up
exactly with the bottom half of the wave. E.g., square waves are
symmetric, sawtooth waves (/|/|) are not.
—
Tom.
From tremolu–(at)–ol.com Sat Aug 26 22:23:46 CDT 1995
Article: 3134 of alt.guitar.amps
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From: tremolu–(at)–ol.com (Tremolux)
Newsgroups: alt.guitar.amps
Subject: Re: 6v6gt, gta, gty
Date: 25 Aug 1995 22:49:08 -0400
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This unequal clipping amplitude is EXACTLY what I did in Mike Fuller’s
FullDrive overdrive pedal specifically to increase the even-order content.
It works!!
Regards.
