Telefunken is a german company that produces audio and visual electrical equipment. The U 47 was a large-diaphragm condenser microphone made in the 1950’s by another german company (Neumann GmbH) but which used Telefunken VF 14 vacuum tubes. So maybe Zappa is singining about the microphone he’s using to record the song on?
Edit and more googling sugests that this is infact the case