Yes - the sequence went: 6L6G - 18watts plate dissipation, 6L6GB-23 watts, 6L6GC-30(35)watts. You can swapp a higher wattage variant for a lower one but don't swap a lower wattage one for a higher (6L6G for a 6L6GC for example). Now the "real" 5881 is a 6L6GWB - the "W" being a ruggedized military designation. The "5881/6L6GC" being offered by Sovwrek is an almost 6L6ish Soviet era military tube that has it's following and it's good point but isn't a real 6L6 - lotsa tubes will substitute for other tubes without being an exact substitute.
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