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you could. the problem is that these are 'dual triode' tubes, so a 12ax7 has two separate gain stages, as does a 12au7. the bassman ri uses one tube to provide one triode as a gain stage and one triode as the cathode follower. if you replaced the 12ax7 in that position with a 12au7 you would get the 12au7 cathode follower, but also a 12au7 gain stage, which doesn't have the amplification of the original triode and you wouldn't be driving the cathode follower very hard. that's why, if you look around, you'll find old tubes that are dual triode with 1/2 as a 12ax7 and 1/2 as a 12au7, where the 12au7 is used as a buffer for the 12ax7 gain stage. not common in current guitar amps, however.
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