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Some very strange notions in this thread. So output clipping is the same as preamp clipping? Every measure it? Unless using a single ended output, the push-pull configuration will cancel even harmonics that each side output generates. If you are not seeing 3rd harmonic stronger than 2nd from the power stage, you simply have a bum tube.
Soft clipping is not voltage limited, it is nonlinearity in the active device at close to saturation and close to cut off and does have a compression effect. Look at the curves of a tube and see the exaggerated "s" where a given change in input signal has a smaller impact on the output. That is the region where the term "touch" comes into play in guitar amps, where slight difference in playing can intentionally move the amp from clean to compressed before the onset of clipping. It is like a compressor with the threshold set high and the ratio set to 10:1. But overplay it and hard clipping takes over.
Preamp stages are usually single ended so a different harmonic content is expected. Stage gain is important in all forms of music production from console, to decks to guitar amps. Most home builders do not pay much attention to that but it separates the great sounding amps, when played well within its design parameters, and a clone that sound uninspired with essentially the same circuit. That natural non-linearity of active devices when near cut off and saturation should be determined for every stage so the signal levels between stages don't unintentionally overdrive the following stage well past that compression zone. Sure sometimes you want to hard clip but much of the time you don't and you can't help it but to push one of the stages into hard clipping if the gain of the preceding stage is too high.
You can look at a spectrum of a power amp with signal being injected into the PI or PA-In jack and you will quickly see the harmonic content at all levels are dominated by odd harmonics. Do the same with the Pre-amp out or effects send and you will notice even in very clean mode, second harmonic is the dominate harmonic just as "nature" would predict. When using opamps, any induced moderate non-linearity has to be created with a nonlinear feedback response. A transistor in common base configuration can certainly act as a nonlinear feedback element for example. You can create soft clipping in solid state, take a look at the output of the classic Fuzz-Face, with just a couple Ge transistors. And it does sound musical as a result.
Soft clipping is not voltage limited, it is nonlinearity in the active device at close to saturation and close to cut off and does have a compression effect. Look at the curves of a tube and see the exaggerated "s" where a given change in input signal has a smaller impact on the output. That is the region where the term "touch" comes into play in guitar amps, where slight difference in playing can intentionally move the amp from clean to compressed before the onset of clipping. It is like a compressor with the threshold set high and the ratio set to 10:1. But overplay it and hard clipping takes over.
Preamp stages are usually single ended so a different harmonic content is expected. Stage gain is important in all forms of music production from console, to decks to guitar amps. Most home builders do not pay much attention to that but it separates the great sounding amps, when played well within its design parameters, and a clone that sound uninspired with essentially the same circuit. That natural non-linearity of active devices when near cut off and saturation should be determined for every stage so the signal levels between stages don't unintentionally overdrive the following stage well past that compression zone. Sure sometimes you want to hard clip but much of the time you don't and you can't help it but to push one of the stages into hard clipping if the gain of the preceding stage is too high.
You can look at a spectrum of a power amp with signal being injected into the PI or PA-In jack and you will quickly see the harmonic content at all levels are dominated by odd harmonics. Do the same with the Pre-amp out or effects send and you will notice even in very clean mode, second harmonic is the dominate harmonic just as "nature" would predict. When using opamps, any induced moderate non-linearity has to be created with a nonlinear feedback response. A transistor in common base configuration can certainly act as a nonlinear feedback element for example. You can create soft clipping in solid state, take a look at the output of the classic Fuzz-Face, with just a couple Ge transistors. And it does sound musical as a result.
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