THE FACE OF HARMONICS


Ever wanted to know how increasing overdrive in your preamp would sound? How about what harmonics are generated in an overdiven preamp versus a clean preamp or in an overdriven single-ended power tube? Want to know what harmonics and what levels are generated by a clipped power amp signal? How about side-by-side audio comparisons? You can find it all here and more; read on.

I have compiled 5 test scenerios of clean stages and overdriven stages all with photos and sound samples. Check them out. I do not draw conclusions, but leave it up to you. The test frequency is 400Hz. Here is an audio sample of all 5 tests. You may be surprised at how much your bias voltages and preamp gain actually affect the sound of your amp.  MP3



The sine wave on top is a clean signal through a triode, and the sine wave on the bottom is an overdriven triode signal. Note how the peaks are rounded off; this produces the predominantly even harmonics for which triodes are known.



This is an overdriven pentode (power tube). Note the clipping of the peaks, which produces predominantly odd order harmonics.



This is a spectrum analyzer that displays the 400Hz signal (the first line) as well as each harmonic frequency generated in the amp and the level of each; the higher the line, the louder the signal, and the further to the right, the higher the frequency. The input frequency is 400Hz, and the harmonic frequencies generated are 800Hz, 1200Hz, 1600Hz, 2000Hz, 2400Hz, and so on. As the frequencies go up, the level goes down, and as you increase the overdrive, the level of the harmonics goes up. Note the harmonics that are generated in a "clean" non-overdriven tube amp in TEST 1 as compared to TEST 5.





TEST 1
A CLEAN PREAMP SIGNAL AND A CLEAN POWER TUBE SIGNAL:   MP3.



TEST 2
A CLEAN PREAMP SIGNAL AND AN OVERDRIVEN POWER TUBE SIGNAL:   MP3.



TEST 3
AN OVERDRIVEN PREAMP SIGNAL AND A CLEAN POWER TUBE SIGNAL:   MP3.



TEST 4
AN OVERDRIVEN PREAMP SIGNAL AND AN OVERDRIVEN POWER TUBE SIGNAL:   MP3.



TEST 5
A SLIGHTLY OVERDRIVEN PREAMP SIGNAL AND A SLIGHTLY OVERDRIVEN POWER TUBE SIGNAL:   MP3.






Here are graphs depicting what is shown on the spectrum analyzer photos. The levels are 0dB to -80dB, and the harmonics shown are 1 through 8: 400Hz, 800Hz, 1200Hz, 1600Hz, 2000Hz, 2400Hz, 2800Hz, and 3200Hz respectively.









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