ARC started with 6550C so that's why it is in the manual. But the 6550C is no longer available at the quality level that it had with Svetlana. ARC buys in large quantities and the 6550WE is available. In the ARC power supplies either tube (Svetlana or WE) will work fine. The WE is a tad more analytical. The current 6550C is not suitable and a reputable dealer should sell them for use in ARC.
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