Not to throw a wet blanket on the tube rolling going on here but... If anyone doesn't know, this is a push pull buffer stage with the tube on one side and the transistor on the other. My thinking is that any audible differences are departures from the Saga's astoundingly low distortion and are caused by the chosen tube not being as well matched to the transistor it's paired with as the original ones Jason chose to use. It's fine if you like it, but it's a strange circuit and it bothers me mentally to be messing with it too much. That said, I do have a vintage RCA in mine that I put in before I took a close look at the board and realized what the circuit was. I must no be a golden ear because I don't think I really hear anything different. I may just stock up on the original Russian tubes as they are currently available for around $4.
I agree with you for the most part (based on what you've posted regarding the Saga's hybrid tube buffer circuit), and my observations of the various 6SN7 tubes I have on hand in the now as heard in the Saga aren't necessarily at odds with your findings. All audio circuits employing vacuum tubes introduce distortion, for better or worse. Unless that distortion is objectively unpleasant it effects on the source audio will be perceived differently from lister to listener.
I'm certainly no golden ear but I am able to make observations and comparisons, even subtle differences, when compared against the same signal chain with the Saga's buffer bypassed (I don't use the Saga's level control BTW - I'm using the Saga for source selection, and for its buffer as an optional sound).
Generally speaking, what I'm finding is that imaging, staging, dynamics, tonality, and detail
may be slightly affected by the tube used - again, for better or worse. The only significant changes are those that detract from the passive mode (same signal chain otherwise). If perceived detail is decreased, if tonality is negatively impacted anywhere, if dynamic range seems to flatten out to any degree, etc. with any given tube then it'll never see use. If a tube yields no discernable difference from passive mode then I've no interest in using that tube. If a tube affects the source music in a way that yields an enjoyable listening experience then It'll see further use if or until that perception changes in any way that detracts from my enjoyment of the source.
I trust my ears, even in their aged state (now in my early 50's). What my ears are telling me is that the differences from tube to tube in the Saga are quite subtle (one new production Russian made reissue Tungsol 6SN7GTB, one new production Russian made Electro-Harmonix 6SN7, one GE 6SN7GTB, several RCA 6SN7GTB's, and one RCA 7N7 local base - electrical equivalent of a RCA VT231 / 6SN7GT).
Generally speaking I'm finding the vintage 6SN7GTB tubes I have on hand to be less desirable than the Tungsol reissue 6SN7GTB and the RCA 7N7, as treble seems a bit smoothed in a way that blurs detail (this is, once again, a very subtle difference - one that has taken a good bit of listening to discern). The Tungsol doesn't have enough hours on it to draw any final conclusions. I haven't put any hours on the Electro-Harmonix yet so no no observations.
The 7N7 is a standout so far in that nothing is perceived as negatively affected, imaging and soundstage are perceived as improved slightly, treble is 'sweeter' sounding without sounding overly smooth or any perceived loss of detail, and bass seems to have slightly improved texture on many tracks. The overall difference from passive mode with the 7N7 is pleasant with no perceived negative impact on the source music. More listening time is needed to verify my findings so far but this one is the most pleasing listen so far.
These differences may be attributed to distortion, and others may or may not hear the same things if given an opportunity to listen to this signal chain with this tube installed. At the end of the day all of this is subjective.
Cheers!
