For the sake of clarity (not argument, I swear):
Originally Posted by tomb
You can guess certain flavoring from the topology, but with the tube providing the signal amplification, it's pretty much that - a guess - unless you listen to and test the actual article.
I'd rather say that from the topology, you can determine a basic sound (the harmonics relative weight, a distortion rough ballpark range) and changing tubes (as long as they are suitable for the topology) and pcb will only change the flavor at the margin. Whatever the layout and the tube used, no ssmh will sound harsh nor have vanishing thd (unless you make a mess of things or bring it to oscillation).
But, hey, aren't we all striving for those changes at the margin ? So I certainly wouldn't dare to guess if Fred's version will sound better than Pete's. But I'd certainly be willing to say that both will sound quite different from an O2 (and similar one to another compared to the O2).
Just an FYI, but you might have overstated the significance of the 3rd harmonic as well in Pete's original design - the 2nd harmonic was absolutely dominating. I'm not trying to be argumentative, but it's important when statements are put out there about the quality of an amp's sound or in a comparison. When you stated, "It has rather high distortion but mostly 2nd and 3rd ..." it could imply that the amp is harsh, when it's not. That's because you make no distinction between the levels of the even and odd harmonics. If I'm interpreting correctly, the 3rd harmonic (odd harmonic) is down by 75dB over the signal - that's inaudible with almost every headphone.
More for the information of the public than mine...
My comment about distortion being mostly 2nd and 3rd harmonics is as plain as it can be : distortion in ssmh is mostly 2nd and 3rd. No intention to comment on respective levels. I don't see how I could have implied that the amp was harsh while stating in the same post that ssmh are "tubey", "euphonic", "amps you're never tired to listen to" (judgements actually based on the 2nd strongly dominating, as well as experience, having built such an amp).
If you want, we can rewrite it as "it has rather high distortion, mostly 2nd harmonic and a bit of 3rd, and nothing above".
Btw, 3rd harmonics aren't responsible for amps sounding "harsh". Otherwise almost all amplifiers with a push-pull output would fall in that category. You need higher order odd harmonics for things to go unpleasant (5th, 7th). Iirc, about a person out of three actually prefers amps with dominating 3rd (I could dig a reference from Nelson Pass about that).