I think the 6550's ARC currently stocks its big tube amps with are the Svetlana 6550C's. They are different from plain 6550's, supposed (of course) to sound better. They certainly sound great in my ARC VT100 Mk III.
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I have the Svetlana 6650C's also. I thought that they sounded excellent in the ARC CL150's which I have. I tried them in the ARC D-250, until I compared them with the PHILLIPS 6550'S. For the sake of experimentation, try the NOS Phillips if you can find them, and let me know your verdict. You will be flabbergasted!