So the “ KT” means kinked tetrode , this was a design change to circumvent the original patent. I have NOS Tung Sol 6550 ( 3 hole ) and SED Winged C 6550’s . I have NOS GEC KT-88’s and KT-77’s (smoked glass ). New production GL KT88’s and KT-77’s. Also new production Psvane, JJ , Tung Sol . Over the last 2 years , I’ve had 1 SED 6550 go bang ( bottle split ) with no amp damage . And one almost new current production GL KT-77 go bad . The latter looked fine but the amp produced a huge pop through my speaker and a wisp of smoke from my amp . It’s not a MAC , but it is “ Self Bias “ . I sent it in for repair . I was told by Mr. Had , that when he tested the tube it was bad and had caused a run away plate voltage issue that toasted the self bias circuit . He also told me on a previous conversation that if I wanted to run NOS 6550’s in my amp to get the “ 3 Hole “ version and not the solid plate version. I’ve run most of the tubes in my Dynakits too . FWIW I like the SED Winged C 6550’s in the Dynakits and the NEW Tungsol KT 120’s in the Had. For late night quiet listening I run either of the Nos GEC ‘s and a 5U4G to drop the voltage . If you don’t want to pay crazy prices for NOS , and want a honest seller ( don’t we all ), try Johnny at 3M Audio in Houston . I use him and I live in California. BTW he’s a high volume Mac dealer . Check his web site . He also lists on Agon and EBay . Another source of info is Chip ( RumDog ) on EBay . I bought a box of SED’s from him . He runs Twin MAC 275’s bi amped with SS on vintage JBL’s ( the big stuff) he has a lifetime of colllecting JBL and is into MAC’s . Cheers , Mike B.