Why rock musicians prefer valve amps decoded

Scientists have looked at the science of valve amps to explain why their rich, warm sound is more pleasant to the ears of so many rock musicians than cheaper transistors.
For many guitarists, the sound of an overdriven valve amp - think AC/DC's crunchy Marshall rhythm tones or Carlos Santana's singing Mesa Boogie-fuelled leads - can not be beaten, researchers said.
David Keeports, a physics professor from Mills College, California in the US, looked at the science of valve amps to explain why their sound is 'better' to the ears of so many guitarists.
"Although solid state diodes and transistors are cheaper, more practical, and technologically more advanced than glass valves, valves survive because so many guitarists are exacting about their tone, and prefer the sound a valve amp gives them," Keeports said.

"At its most fundamental level, this is because a moderately overdriven valve amp produces strong even harmonics, which add a sweetening complexity to a sound. An overdriven transistor amp, on the other hand creates strong odd harmonics, which can cause dissonance," he said.
Keeports explored the physics of why even harmonics enrich a sound, and why the timbre of the sound from a valve amp changes when a guitar is played more loudly.

He ran a 200 hertz (Hz) sine wave - a pure wave with a single frequency - through a small Bugera hybrid amplifier, featuring a valve preamp and a solid state power amp.

He then tested both 'sides' of the amp; first turning up the gain knob, which controls the valve preamp while the master volume knob (controlling the solid state power amp) was set low. He repeated the process with the preamp set low and the master turned up.
Using Logic Pro X music production software, he examined the resulting sound waves in both frequency and time domains.
"The output from the amp showed that a moderately overdriven valve preamp produced prominent 2nd and 4th harmonics at 400 and 800 Hz, and only a very weak 3rd harmonic at 600 Hz," Keeports said.

"For the solid state power amp, this pattern was reversed. All of this behaviour is consistent with the common claim about the harmonics that valve and solid state amplifiers produce. But the story is not quite so simple. Overdriving the valve preamp harder produces strong odd harmonics," he said.
"The shift towards odd harmonics at increasing gain is a characteristic of valve amplifiers that further explains their appeal.
"An electric guitar player can overdrive an amp two ways: by turning up the amp's gain control, and by attacking guitar strings more strongly," Keeports added.
The research was published in the journal Physics Education.