Mac has changed the bias on the 12AT7 drivers to increase lifespan. I don't know the S/N cutover. This is documented in the service manual.
McIntosh MC2102 Question
Recently doing some reading on the McIntosh MC2102 tube amp. Read on one site that MC2102 flamed 2 or 3 tubes and that early versions of this amp had some problem. Anyone familiar with such a problem or just an isolated incident (urban myth)? Thanks!