[RadioSvit]

MrFixr55
Ten years ago and until recently, I was actively involved in the design and creation of tube amplifiers and HiFi speaker systems, so I can share my experience. Here are some results of observations that I got thanks to measurements on a spectrum analyzer and in the process of listening. By the way, they are confirmed by the theory of high-quality sound reproduction.
The advantages of triode inclusion of 6F6 radio lamps over triode is that the spectrum of triode distortion is large at the 2nd harmonic, quite significant at the 3rd, but completely absent at the 4th, 5th and higher. In the pentode, the spectrum of harmonics is more uniform and much lower on the 2nd harmonic, but larger on the 3rd and extends up to the 10th, and therefore is generally much more noticeable and not more pleasant to the ear, because it causes discomfort. The higher the harmonic number, the more noticeable it is. That is, - the 5th harmonic with a value of 0.5% is much more noticeable to the ear than the 2nd harmonic with a value of 5%. The spectrum of harmonics of the amplifier in class A mode in triode switching is limited in number to the second and third harmonics, and does not extend to the tenth or more. At the same time, the second harmonic is hardly noticeable to our ears even up to a value of 10%. The third harmonic is on the contrary - its value is quite low and rarely exceeds 3%, but it is very noticeable and very uncomfortable for the human ear. And the presence in the signal spectrum of the fifth or seventh harmonic with a level of even 0.5% is a disaster for the ear and causes a sharp rejection. Therefore, even low-power single-cycle triode amplifiers in Class A are much sweeter and have a more correct sound, because their distortion spectrum ends with the 3rd harmonic, although at the same time they are very inefficient in terms of power compared to pentode amplifiers.
But pushpool has one feature - even harmonics of the 2nd order are mutually compensated, and odd (that is, 3rd, 5th, 7th) harmonics are added. So, - if we build Pushpool on triodes, we will get a very low value of distortion on the 2nd harmonic and a little higher on the 3rd and almost complete absence of distortion on all subsequent harmonics. We have such a phenomenal amplifier in Philco 37-116 deLux - it is difficult to find a console with a more pleasant, soft and accurate sound. And this is my favorite radio.
A completely different spectrum of distortions in push-pulls on 6F6 pentodes in pentode switching is a low level of even harmonics, but a much higher level of odd 3rd, 5th, 7th and up to the 11th. Because of this, such amplifiers sound more dynamic and sharp, very often unpleasant and quickly tire our ears. Everything described applies to amplifiers of class A, which were used in radio in the 30s...50s. When the radio and audio industry, in order to save money and increase power after the war, abandoned output triodes and began to use amplifiers of class AB or even B, to reduce distortions, they had to use Negative Feedback, which destroyed the naturalness of the sound, although the overall level of harmonics became lower than 2 %, but the range of harmonics extended up to the 12th...15th number. Therefore, their visibility grew.

I hope that this information will be useful to someone.