Re: What kind of 6L6
5881 is a ruggedized military version of the 6L6...
KT66 ('kinkless tetrode') is a heavy duty British equivalent 6L6...
7027 is a different critter, more like a 6550, different pinout, not compatable without rewiring...do NOT plug one into a Fender or 6L6 dedicated amp.
The Winged C 6L6GC are a good bang for the buck...solid tone, strong, dependable, affordable, good QC. Didn't like the JJ's...midrangey, honky, high failure/microphonics rate. I don't have much use for the Sino/Sovtek stuff either (except that Shanguang KT66; nice tube).