Hello MrTennis, and Newbie,
To speak of my own personal findings with my own (Bought new in 1997) Sigma II, and my bought used from A-Gon Alpha DAC, I have noticed quite substantial changes in the sound, and here's my own personal experiences with both Units:
With my Sigma II, when I first got it, I had noted the stock Chinese Tube in it, it sounded very darn good, and then thought I was going to make sonic improvements to the sound, changing out the one 12AX7A, and trying out the then touted Sovteks. In this Unit, every Sovtek I tried, which was every model available 6 years ago including the 12AX7LPS sounded dull, lackluster, and the detail, clarity, and slam that the Sigma II DAC once had was gone slightly. I threw the stock el cheapo Chinese Tube back in, and in fact that tube still sits in that DAC 7 years later with no problems.
Now onto the Alpha I acquired: When I got it, it came with two Phillips-Jan 5751 Tubes, and the sound was extremely brittle, screechy, bright, and essentially unlistenable in this Alpha in my particular system.
A very kind A-Goner by the name of "Zinfan" (Tom) seen my earlier cries for help some months back, came to my aid, and rushed me out both a Copy of the Alpha Owner's Manual, and a pair of virtually New-NOS Mullard CV4004 Tubes to try in the DAC, as he noted a bit of rolled off highs with this particular Tube in his Alpha.
Well, I noted the same, and again, thought of this particular Tube as being lakcluster, and seemed to restrain the Alpha of its true capabilities.
I was right, when I had just gotten these Shuguang, 12AX7, and these are the latest C9 version, which have BlackPlates, Triple Mica Spacers, and a round Halo Getter. Very nicely made Tube IMO, and I'm of the thinking that this Tubes is an improvement over older Shuguang 12AX7's. Just like the three bears, this tube seemed to be "Just Right" (for me anyway, and my particular McIntosh-JBL Based System, 2-2105's MX-130, four L-65's)
I cannot speak for the Shuguang 12AX7Ac9 version longevity, but generally it's been found pretty much that both the Alpha, and Sigma II DAC's are very "easy" on Tubes. In other applications perhaps, this may not be the case for the Shuguang, so I cannot honestly say either way.
It is pretty much a well knows fact the the '50's Sylvania 5751 BlackPlate Triple Mica with D-Getter to be one of the top vintage "12AX7 type" Tubes in these Units as far as sonics are concerned, but I've also heard that this particular Tube has a relatively short life. Perhaps installed in either the Alpha, or Sigma II, this wouldn't be an issue. Biggest problem is, thier rarity, and thier very high desireability, and cost. I seen a pair of these NOS Sylvanias being sold online last week by a dealer for $140 ea!
IMO, this is just too much no matter how nice they are.
Of the two Mullard CV4004 I have which I had noted rolled off the highs-mids a bit, I really cannot sell these, as they were in essence truthfully "borrowed" to me by Tom, and if I didn't care for them, I was to send them back to him. I in fact just wrote him a letter yesterday asking if he would like them back, as they are rightfully his. Mark D.
