[turretslug]

The EL34 was apparently designed to withstand unusually high HT voltages for its class, there's even circumstantial talk of its having been a spin-off from line output valve design- perhaps it's better able to withstand the initial over-voltage from solid-state rectifiers before significant HT current is drawn. Part of the problem with the whole 6L6/5881/KT66 family of valves is that they were so popular that different variants appeared over the years with increasing anode dissipation and HT voltage ratings. 6L6 variants in particular gave range to lots of suffixes that need to be followed carefully, limiting 5881 specs might also have been a moveable feast between US industrial spec. originals and inexpensive Russian substitutes with similar caveats applicable between GEC and Far Eastern KT66s- Marshall might have been safeguarding against lower-rated versions being substituted, given that musicians' amps worked their bottles hard anyway and were bought to be thrashed!