I have heard sometime ago that the reason older tube amps sound so tubey is due to higher levels of distortion when compared to newer designs.This is partially true. Its the 2nd and 3rd harmonics that are in question- together with a lack of higher ordered harmonics. Older amps tend to have a significant amount of the 5th thrown in; many newer amps do too. But some amps have less 2nd harmonic these days- if the amp is fully balanced and differential from input to output, the primary distortion component will be the 3rd due to a cubic non-linearity, whereas the 2nd results from a quadratic non-linearity. Amps based on the 3rd harmonic tend to have less distortion overall with higher orders falling off at a faster rate as the order of the harmonic increases. These types of amps simply were not in use back in the 50s and 60s.
Why don’t tube amps sound like tubes anymore?
When I hear the latest tube amps I’m more reminded of what a solid state amp sounds like than what I remember a tube amp once sounded like. I say that, with most tube amps I hear today, but not all. Gone seems to be the lush tones, warm glow and natural harmonics I used to hear. What I hear is more of a thoroughbred, faster, sharper sound when I listen to a modern tube design today. Then why use tubes?