New Svetlana 300B (I thought at the time) This is one very classy tube, well constructed and with a solid heft to it.. Smooth and detailed - very similar to the vintage WE 300B I have been exposed to. While not inexpensive, it is a bargain (is it? See below) when compared against others in terms of performance, longevity, and compatibility. I have spent only a limited amount of time listening to this tube in an S.E. amplifier normally equipped with WE 300B, but if anything I thought the performance a little better with the SV 300B..
Update: 3/30/99: Shortly these tubes will really be put to the test as my 300B pushpull amplifiers reach completion..
Update: 6/30/2000: Experiencing high failure rate, all type 1 have failed, several type 2 have failed, now no music. These tubes all failed by developing intermittent shorts from filament to grid during warm up.
Update: 6/23/2001: Purchased gray market SV300B type 3 from Russian Emigre, half dozen so far have not had any problems in a year.
Update: 10/28/01: I have experienced another failure, this time one half of the filament in the right channel amplifier opened up.. In my opinion these clearly these suffer from serious quality control problems and design flaws. I can no longer recommend them

Tesla KT88 I continue to be impressed with the KT88 which is manufactured in what is now the Slovak Republic. This tube has considerably greater plate dissipation than the 6550C, but exhibits a level of performance that is at least the equal of the Svetlana 6550C. This tube is sweet and musical and very detailed, high frequency extension is better than any tube I have ever heard in my amplifier. String basses have a texture and power I have not frequently experienced, and violins no longer sound like a homogenous mass - I can hear some of the individual instruments. This tube resolves well, and does not penalize tonal accuracy to get it. Bass extension is comparable to the 6550C and is tight, well damped, lively, tuneful, and rythmic. If you are a Linnie (or just like quick tuneful bass) then you will probably like this tube.
Update: 9/13/98: I continue to use these in limited quantities and have had no problem with them what-so-ever. I have been criticized for my boosterism of this product, but continue to like it a lot.. The 6550C from Svetlana is also an excellent choice for half the money, but will not dissipate as much power safely..
Update: 3/30/99: The latest batches from JJ are highly consistent in quality, and run at the limits of their screen ratings - and beyond without any evident reliability problems..In addition they seem to be able to dissipate 40W on the plates continuously without failure. Quite rugged - we buy ours from Ruby Tubes, and have nothing but praise for their sound and reliability.. Sound great triode connected too!!
Update: Old hat, DHT is where it's at...

12AX7A I recently purchased a pair of Sovtek 12AX7WXT+ Select, and am happy to report that they sound very similar to the old Telefunken smooth plate 12AX7A in my differential power amplifier. They actually seem to have slightly greater overall resolution, and a tighter, more tuneful bass performance. Overall they seem significantly better, again another sign of progress as I have never cared for the other Sovtek 12AX7 variants due to various audible short comings in my circuit designs.
Update: 9/13/98: I have all but stopped using these due to poor long term matching between the sections, Not much point to spending the extra money frankly. If you need a tube with good section to section match the 5751 is much more consistent over time.. That said the 12AX7WXT+ is a good sounding tube in power amplifier applications, and less so in my pre-amp designs.
Update: 3/30/99: These Sovtek tubes continue to be a source of concern, they perform well in some circuits, and very poorly in others. Life seems to be a problem, and transconductance is generally much lower than a 12AX7A ought to be - rarely exceeding 900 umhos.. Fortunately JJ amongst others makes a very good ECC83, and Ruby Tubes has acquired the tooling for one of the Chinese 12AX7A and has set up shop in Beijing. Good news for guitar players....
Update: 6/23/2001: Please buy the Sovtek 12AX7LP or LPS in lieu of the now discontinued 12AX7WXT.
In the right applications it can sound sublime.