"[Total Harmonic Distortion] is one of the ways that amplifiers are rated, and in the hi-fi world, where people are quite conscious of total harmonic distortion, everybody goes for the lowest possible amount of distortion. In the guitar amp world, people tend to go for the most." --Andy Marshall of THD Electronics, http://en.wikipedia.org/wiki/THD_Electronics
Regarding high-fidelity amplification, many sources describe 1% THD as 'just barely noticeable', and 3% THD as 'just barely objectionable'. Obviously, electric guitar amplification can be driven to much different goals. But, rated for power at some 'high distortion' and not to any recognized standard, many guitar amps can not be sensibly compared on power alone.
Rated to high-fidelity audio standard, the GAGA-50 delivers 50 watts of power at 1% THD. Of course the GAGA-50 provides more power if you accept (or desire) more distortion. Numerous guitarists have said the GAGA-50 actually "sounds more like a 100 watt tube amp", and they inevitably say so when they play the amp into distortion obviously beyond 1% THD.
"Clipping is typically quantified by a THD measurement of 10%." --Texas Instruments, Technical Paper "tpa3007d1"
Wattage, Power, Speaker Efficiency by Howard Davis
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