So I found a couple of nos RCA black plate 12AT7 tubes and was thinking of installing one in my F-1X, replacing the (presumed) stock 12AX7.

Any reason why I shouldn't? It sounds great in my Underground Accelerator pedal (which is 'inspired' by the F-2B).

Give it a rip and report some results... be mindful of the last line here-


https://club.alembicguitars.net/index.php?topic=242.0

Should not be a problem at all.