Low E 82.4Hz
The following image shows the peak frequency content when the Low E string is played with the gain and volume settings at 10 and the tone at 5. The fundamental frequency is clearly visible at 82.4 Hz and the following peaks can be shown to be certain odd and even harmonics of the fundamental. The second harmonic at 164.8Hz can be seen clearly, however there is also a peak at around 230Hz and 300Hz which does not match to a specific harmonic of the low E.
The image below shows the same audio halfway through the audible period and it can clearly be seen that most of the high end content is dropped, and it is observed throughout the clip that the high end content lowers considerably quickly with the first five or so harmonics remaining consistent throughout the audio. It is also observed that during the audio the harmonics die off in an alternative cyclic manner, with peaks close to each other fluctuating alternatively producing the sustained cyclic period the amplifier produces.
The image below represents the DI guitar played at the low e (82.4 Hz) both peak and half way through the audio. Some of the odd harmonic content found in the previous amplifier output can also be found here, showing that the guitar itself produces harmonic content not related to the fundamental. The main difference in terms of frequency content is that the frequencies produced by the amplifier are much larger in amplitude and more exaggerated than the DI guitar.
The image below shows the two waveforms next to each other (the amplifier output on top the DI below). Just from looking it can clearly be seen that the amplifier adds a great deal of sustain and gain to the input. From the odd shape of the waveform asymmetrical clipping can be observed as would be expected of tube applied overdrive. Reproducing this asymmetrical distortion is key in producing the cyclic sustain and ‘warmth’ that tube overdrive offers.