The LTA preamp is supplied stock with a pair of hand-picked NOS 12sn7 dual triode tubes, and a pair of current-production Russian Gold Lion 12at7. The 12sn7 tubes in this case are United Electron grey glass. I will have to remember to get the letter codes to try to determine the actual origin of them. United Electron did make tubes during WWII, and very good ones, but shortly after the war became a supplier of glass and tube parts to other makers, and a remarketer of tubes made by RCA, Sylvania, GE and others. Since grey glass envelopes were pretty much abandoned by the first couple of years of the 1950s, it's possible that Keith's tubes originated from the WWII era United Electron factory. If a few years newer then I expect they were made by either RCA or Sylvania. The grey glass obscures the tube internals from inspection.
The 12sn7 is a 6sn7 with a 12v heater. Xsn7 tubes from that era are some of the best sounding dual triodes you can find for preamp-level gain. I brought some alternates, since the preamp can use 6sn7 types with a jumper change, so we compared the stock United Electric 12sn7s with some known high grade 6sn7 types in my inventory: 1 pair of RCA red base 5692 (usually provides a dose of snap and dynamics to an oversmooth 6sn7 component), 1 pair of Sylvania 1945 production 6sn7w (tone-dense and articulate), and a pair of Sylvania 1950s chrome domes (full-bodied with fast bass).
The United Electron 12sn7s supplied are excellent and LTA is doing a good job with their NOS tube selection. No alternative was better holistically. None were worse either. Just different by small degrees. I think the Berning preamp circuit has a sonic thumbprint that narrows tube differences, and it's an objective thumbprint, which I recall from all of his electronics over the years. We were trying to find a little more snap and even the RCA 5692 wasn't delivering much more.
However, I immediately suspected the Russian 12at7 as being responsible for glare in the upper strings and female vocal range, as well as sanding down articulation, especially notable during arpeggios of any kind. I know those 12A*7 Russian production tubes, as they show up in guitar amps and current production preamps, and I don't really like them in anything -- or I should say any gear shipped with them stock can be quickly improved through substitution. We found the missing snap in its replacement.
I didn't have as wide a variety of 12at7s as I have of 12au and a12ax types but I have a narrower range of very good choices. I brought along a pair of Amperex Bugle Boy Holland 12at7, a pair of Amperex Germany 12at7, and a pair of RTC (France) 12at7. The 12at7 position clearly reveals differences in tubes, even very good ones. The RTC 12at7 had the best balance of objectivity, articulation, tone and transparency in that circuit and with the Ampzilla involved. While all the NOS choices I had with me were a large improvement over the stock Russian Gold Lions, the German Amperex overmoothed arpeggio articulation and thickened bass. The Bugle Boy delivered a slightly larger soundstage but did not mitigate the glare as much as I thought possible. The RTC brought everything into balance. Any of them would have been subjectively fine replacing the current production tubes, so I am referencing relatively small differences but they are differences that once heard cannot be unheard. The longer we listened the bigger the advantage offered by the RTCs.
Phil