Tesla had many different tube factories all around the former Czechoslovakia, but the largest receiving tube factory was located near the old border of today's Czech Republik and Slovak Republik. I was fortunate to partner with four other equal partners in the rebuilding and restarting of the Tesla factory about 10 years ago. This was after the Berlin Wall had been torn down and Czechoslovakia had become a free country and so began to 'Privatise' the government factories. After a hard couple of years, our privatisation plan for Tesla was interrupted by the dividing of the country into the now Czech Republik and Slovak Republik, and so our 'company' disappeared overnight. I lost touch with the factory and several of my former partners during this time, only to find one of the partners had 'rescued' the Tesla factory machines and tools out of a liquidation sale and transferred them to a new location inside the newly formed Slovak Republik.
Jan Jurco, who was one of my/our five original Tesla partners, is now the owner and operator of the Tesla legacy, now renamed after his initials. I am happy to report that two of the tubes we had developed while still at Tesla are still made exclusively for Groove Tubes by JJ, the GTE34LS and GTKT88SV.
Mr. Jurco has also developed many NEW TYPES, and greatly improved all the former tubes to a new level of performance. His tubes are extremely high on output ratings, usually meeting or exceeding original specifications in the areas of gain and output. This makes them great for souping up vintage amps, but also less suitable to high-gain amp designs, where the increased gain and output can make them literally too hot and so they can have hum, noise and microphonic issues in some designs originally made with the Russian type tubes. That said, however, this is perhaps one of my favourite tube factories, and Jan Jurco is a great old friend and provider of some of Groove Tubes' most popular tubes.
Valves made by JJ in Slovakia carry the suffix 'S'.