There are some reports that the coin-base 6SN7 are highly microphonic. But there are several stories out there about tubes which don't hold up to close scrutiny. For example, many people hold the red base 5692 up as a premium 6SN7 and outrageous prices are paid for them. However for audio applications, they are a very mediocre tube. Likewise people pay a premium to get the VT-231 "military" version of a 6SN7. There is nothing different about a VT-231, manufacturers simply relabeled them so as to be compatible with military inventory standards.
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