NOS Quad Groove Tubes JJ KT88sv BLUE GLASS KT88 6550 NR

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Up for auction here is a rare chance to buy a closely matched set of 4 custom made NOS Tesla BLUE GLASS KT88s power pentode tubes. NOTE: these tubes are from the Tesla Czechoslovakian era, originally made under the trademarked transitional name of Teslovak which was a joint venture company that folded when the old Czechoslavakia split up...similar tubes are currently being made under the JJ name. The “s” designated a “custom order” but not just about the unique Blue glass bottle, a rare enough feature all by itself that give the tubes a beautiful appearance. The special “S” and later "SV" productions had a modified plate structure that makes them stay cool when pushed hard and so sound incredible! In fact, the Teslovak (formerly Tesla that moved to Slovakia and now branded as JJ) “s” and "sv" era tubes are regarded by many as the best KT88 ever made anywhere...an opinion I share based on my own personal experience. These early Tesla/JJ made KT88s tubes unique design produced a higher power KT88 as compared to the standard JJ KT88 (also excellent tubes!). IMHO, these are the highest powered, best sounding KT88 ever made, with a more linear (or flat) characteristic. This improved performance is due to a novel idea to borrow a common structural feature found in high power transmitting tubes (like 1,000+ watt tubes!)...the heat sink. These KT88s tubes have added heat sinks, called “plate wings”, that help dissipate heat and keep the plates cool...so they don’t breakdown and start distorting! YOU can see this in the attached cutaway drawing here. These heat wings look like a “W” and are made of laminated Nickel, and are welded to the outer plate structure to add mass and help keep the tube “cooler” and so more stable at higher operating temperatures. This design feature is totally unique, a GT exclusive at the time, and the man reason why these tubes are well known for outperforming even original Genelux KT88!You can also compare the standard clear glass to the special production Blue Glass in one photo, taken from an old GT catalog...it’s a light enough BLue so the filaments glow can be seen nicely combining the Blue glass with the Orange heater making a unique purple glow. There will be no marking on these tubes as they never were released for general distribution but were to be specially labeled for the HI Fi amps line that never materialized, so they are blank now except for my Orange round test labels (NOTE; these numbers correspond to the printed test report as each tube has a number to help in the time consuming testing and matching process).These tubes are listed as NOS because they are old, not new stock. They are at least 10 years old, according to my failing memory... but probably are closer to 20 years old! In any event they not current JJ production (which are still probably the best tubes made today), so we not call them new, so we use the term New Old Stock (NOS). However, they were made in the same the same original Tesla factory during the transitional days of the now 2 Republics...then known as “Teslovak” and probably when this Tesla factory was still located in Czechoslovakia, before the split. They were purchased for a Hi Fi amp line were were going to build many years ago but that line never materialized. So they have been sitting in my personal storage for nearly two decades...just waiting for eBay to be invented!Fortunately I had enough of these tubes left in my vintage tube stash to do some modern computer testing with my Amplitrex 1000. Testing that many tubes gives me a high yield of “matched sets”, so nearly all these tubes matched up very closely into Quads. The 4 test reports shown here are typical of all the Quads I will be selling, they are all very close anyway. But these pages shown here will likely not be the reports for the actual Quad you buy (unless you are the first auction winner!) However in every Quad I sell, each tube will come with it’s individual test report and all tubes will be guaranteed to match within +/- 5% for both Transconductance and Gain! <... read more

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