Svetlana KT-88

KT-88's are relatively difficult to manufacture and there are a lot of mediocre to bad ones being sold - people with amps that use them must be very careful. E.A.T. makes high quality KT-88's, but they are expensive (the list price for an octet was $1,400, the last time I looked).

Ghosthouse is correct: current Winged C production comes out of St. Petersburg.

I would contact Andy Bowman at Vintage Tube Services in Michigan - he is an excellent tube vendor.

the right equipment and it is worth noting that very positive things are being said about the Gold Lion reissues, in particular (esp. their KT88s)

^^^^
Yes the GL 88/6550  are popular and have accolades.
I personally found the Svet's a  better sonic balance and voicing in my Defy.(I returned the GL witha  restock fee) Also read another audiokarma comment pretty much in agreement with my listening to the GL's.
The ebay seller posts on his site, that the NOS 6550  Svet are identical to the new prod 6550
I am not a believer in this idea of winged C , SED thing.

Also note on TAD's web site, their top of line 6550 @ $100 are in fact from the Svet lab = New Sensor.
So a  NOS Svet <wing C/SED > = New Prod/New Sensor Svet 6550
Both with  have idential performance, as it is the very same exact tube.
Best site for exactly matched Svet 6550's is Amplified Parts, pay a bit more, but  receive exact match,, which for this highly sens Defy 7, <<only 1 bias UNDER CHASSIS/= Need Tech to bias>>>  perfect matched 4 sets of trios is critical.
These 6550's will perform 2500 hours, 
So lets say you use your amp 4 hours a day (I listen at least 6 hrs) X 350 days = 1400,
 so every 18 months you need to bring in new power tubes. 
I'm going to stick with Svet 6550's, as they have a  nice tonal balance and are clear, articulate, bass rich, vocal reproduction  <,stage like>.
+ they image Allan Pettersson's 8th sym BIS with emotional involvement = testing reference passes with high ratings. 

Just some general info: There are no SED, Winged =C=, Svetlana tubes, currently in production.     I really liked them (bought from Upscale), but- the St Petersburg plant burned down, years ago (along with it’s equipment) and SED closed the doors.     New Sensor bought the name (ONLY) and is manufacturing their version in the Sarartov, Russia plant.      Some info from a reputable dealer:  https://www.thetubestore.com/winged-c-sed-6550c                Try contacting known to be honest tube mongers, like the above, Brent Jessee Recording, TC Tubes or Upscale Audio, for knowledgeable info, as to whether they are, "the very same exact tube."      An opinion/observation regarding the new version:  https://www.tubesandmore.com/comment/2577/report                                   Compare the plate and getter construction, to the above (original Svetlana 6550) pic, here:                   https://www.ebay.com/itm/PLATINUM-Matched-Quad-4-Svetlana-6550C-tubes-Russia/184570896793?hash=item2...

well I am sure the NOS <<most liklely are superior>> to the new stock,, but  considering out options, the new stock is perhaps best sound 6550 in production. 
Thats all that matters to me. One guy made a  comment on his ebay purchse, <<These new svet 6550s, are just as good as my GE NOS>> so there you go.
I'd rather upgrade all caps/resistors  and put the cash there, than chasing after NOS over priced tubes.
The shootout in the 6550/88's 
are 
Tungsol
EH
GL
Svet
I've only heard GL and Svet's. 
The Svets beat out the GL's. 
Tried to locate a  comment on audiokarama concerning the GL 6550, not standing up to its repitation, which right off, i found the GL losing to the Svet 6550 in the ref test cd shootout. 
But I can not loacte that particular comment, ONly comments  in high praise of the GL 6550's.
Oh well each will have to pick his own. 
I  KNOW what what i am hearing in the Svet 6550, and have no interest  to  consider other contenders. 
This tube presents music as high fidelity. . 
That is  to say, with a  sublime  realism. 

No argument!    When it comes to what will sound best, with what gear, in which room and to whose ears/tastes; the variables are incalculable.                  When it came to the audio business; I was all about, "The customer always thinks he's right"   and catered to (whatever) their tastes.