Within the American production 6550 tubes, the original Tung Sol, as well as the GE, MPD, and Sylvania versions all produce about the same amount of power output, but the GE and MPD versions of the tube were the most consistent in approaching the lowest levels of distortion achieved by the original Tung Sol product. These tubes still remain the benchmark of performance and durability within this family of tubes.
Within the import tubes, the 6550C from Svetlana was the best of the imports, even employing gold plated control grids for enhanced stability, which was a marked improvement over their previous "B" version of this tube.
All of the imports struggle to produce the power capability of the classic American pieces, with the best ones producing about 90% of their capability. Still, the import versions of this tube have improved significantly over the years, with their distortion performance reaching very acceptable levels with regards to the American pieces.
From a durability standpoint, it is rare to find an import that can go the distance that the classic American pieces can, but within this framework, the import tubes do offer a workable replacement for this wonderful tube.
I hope this helps!
Dave
