Lalitk, You wrote,
"The tubes in output stage in theory has no voltage gain as @lewm pointed out but in real world, they behave quite differently. A cathode follower has no voltage gain, it’s there to lower output impedance, provide current and buffer stages. So on paper, it shouldn’t influence the sound much, right…lol!
But in real world, it still does. Tubes differ in transconductance and behavior under load, which affects microdynamics and low-level detail. They also shape the distortion profile, influencing tone, texture, and decay. Since a cathode follower is about current delivery not gain, how a tube handles dynamic current impacts attack, ease, and overall musical flow."
What you say here is quite true, but you are conflating differences among different tube types with differences among different brands of 6922 and its congeners. The performance of a CF is very dependent upon its plate resistance, mu, and most of all transconductance. The output impedance of a CF is roughly equal to the inverse of its transconductance. So to achieve the lowest output Z and the max current delivery at the output, you want tubes of highest transconductance. For this purpose, the 6922 and other high Gm/low Rp triodes are best. I like to use the 5687 as a CF but it does require lots of current, which is among its virtues as a CF but also puts demands on the PS. This is also why the 12AX7 makes a terrible CF. (Very low current handling and very low Gm.) Among tube types, yes, performance as a CF will differ, but among 6922s that are performing within RCA manual spec, there should not be much difference.
That leads me to my next point. I have found that any sonic differences among decent CFs can be ameliorated by adding a constant current source (CCS) between the cathode of the tube and ground. The CF per se is a nonlinear device, and therefrom derives its character if it has any. The CCS stiffens it up, makes it much more linear and transparent. I have done this at least 3 or 4 times in different gear and always with a noticeable uptick in SQ. The caveat is you need enough cathode voltage to support the CCS itself. I realize that not many are going to try this, but K&K used to sell very compact solid state CCS kits, small enough to fit in most tube gear. I've installed many of their kits. You won’t hear the dreaded sound of solid state, because a CCS by definition has a super high impedance, meaning the audio signal does not pass it.
