are you asking if you can use the carnhill VTB9046 line INPUT transformer as an output transformer or are you asking if you can use an ungapped output transformer on a 312 circuit? sorry, i'm not understanding the question. ???
the 312 preamp circuit does not require a gapped output as it is capacitor coupled so no DC should be reaching the output transformer. if you have an ungapped output transformers use them without fear.
now if you are asking if you can use the VTB9046 line input transformer as an output transformer i could not definitely say it would work well but you could try.
i would think that the limitation would be the input level limitations and depending on how you wire it to satisfy the reflected impedance of the output you may be losing some dB. someone who is more knowledgeable than i may correct me here though. danger would be saturating the core with too much input level?
