Was visiting a guitarist friend who was talking all these different tubes he tries in his amp- like using a KT-66 in place of an EL34 for more or less what he calls "sag" or maybe it was more "chime" I admit I lose track after a while-
Wait, here it is, the actual text-
"So the KT66 is "louder" but breaks up later with less "gritty" crunch. It sounds fatter/warmer a little darker pushing the mids. What an original JTM45 would sound like. It needs to be pushed and EQ up front. Definitely more dynamic and responsive sounding. Kinda old school."
Some of his tubes-
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Dude I don't have a clue- I just plug in and play-