'The waveform contains no 'secret' information as to whether it was produced by a tube amp, a solid-state amp, a digital waveform generator or hundreds of sine wave generators operating in parallel, for that matter," he said.

"The mathematics of Fourier assures us of this. If we can make a solid-state amp produce the same waveform as a tube amp when it clips - including duty-cycle modulation - then we have successfully simulated the tube amp with solid-state components."

• Reproducing the tube amp