I recently received a Mapletree Ultra 4A SE using 12SN7GTA tubes. My assumption is that I could use older 12SN7 tubes is that correct? I think I read (what do you trust on the internet) that the GTA and GTB are related to how the heaters work.
Any tube with an A or B on the tail feathers is not a direct replacement for an earlier tube but a product improvement. Sometimes the new tube can be used and sometimes not and vice-versa.
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