Without actually looking at the specs for all of those "kinkless tetrode" morphs, I really don't think their electrical properties are all identical even if they're considered "interchangeable". In other words, the operating points for each tube will be more or less subtly different from "optimal", or at least a bit off-axis (so to speak) for any given tube type relative to another. As many folks have pointed out before at ASR, a truly well-designed circuit will allow (correct) for this -- but such circuits are
boring on the audiophile plane and thus seldom implemented in modern commercial designs. So, of course, different tubes impart different flavors, as intended.
One of those
QED things.
I've always inferred that tube rolling is just a way of (possibly) making an amp not work to original spec. But that some people end up enjoying the result.
I'd say that when tubes sound different in my CJ gear, it's departing from the original designed performance. They sound "closest to neutral" with the original recommended tubes, both power and input. However, I've found I enjoy some of the coloration of other tubes sometimes.
Sound changed from somewhat bright and hard (EL34) to laid back (KT120) with each change, and I think it correlates with the color of the light (hard day light to soft evening light).
Interesting, thanks!
The EL34s did sound the hardest to me; I'd attribute it to them having somewhat higher distortion and lower power output, as it was most obvious when the singer got loud. Otherwise, all interchangeable.
My impressions were:
El34
sounded slightly thickened and congested and a bit forward and edgy.
KT88
also a bit thicker/richer though not as edgy and more softer/laid back in the highs. Cymbals and especially drum snares always seemed to become a bit more almost muffled and in the background.
KT120
These produced for me among the most obvious sonic changes, especially right before or after the EL34 and KT88s. The sound cleaned up, a haze was removed from cymbals, drums, especially piano keys, ambience/reverb became more obvious. And the bass seemed a bit more weighty. On my home theater system it was like the sound became a bit more dimensional, more transparent in to the recorded venue.
6550s
Most neutral sounding. Most even, not overly rich, not as soft, more matter of fact, less bass than the KT120s, less clean/clear less ambience retrieval.
I found it fascinating to (at least think I) hear the qualities I hear with the KT120s in my system (though not as obvious as it seems when actually switching tubes in my system), where the KT120s produce deeper, weightier, slightly more in grip bass, a sense of clearer sound, more brilliant highs, larger soundstage/imaging, and more obvious retrieval of reverb and space around images.
Also interesting (to me) are youtube comments echoing this under the video:
- I loved the KT-120 valves. What struck me over and over is the much deeper soundstage and openness of the sound, along with deeper bass and extended highs of course.
- KT120. The most live, the most transparent, the most airy and the most natural sounding. It sounds as if the system doesn't exist and you hear the musicians live in your room!!!!!
- All the KT tubes have a transparent mid / upper, but the 120 also has bass.
- KT120. The bass is deeper and the overall openness of the highs is evident
- KT120 most cohesive, realistic sound stage and timber.
