Yashu,
It can be confusing with Russian vacuum tube manufacturers. SED (Svetlana Eletron Devices)is another Russian company which bought the name, and makes a 6550C. If there is no Winged C, that isn't the one you want. The =C= SED (Winged C) logo is used by the company formerly known as Svetlana. The key is to get a 6550C with the Winged C logo (also says SED).
I don't believe the Winged C 6550C tubes are manufactured anymore, and that is a possible reason why ARC does not provide them with new electronics. Maybe someone from ARC could comment on that.
It is important to get a Winged C 6550C from a reputable tube dealer. Apparently (I cannot verify this) there are some substandard Winged C tubes in circulation.