If you want to evaluate the selected reel size and speed for the Revox B77 MKI, just like for the MKII, you need this tape tension board. With the Revox B77 MKI up to serial number 53.521, only the winding motor voltage is switched between 43 V and 55 V. The drive control of the first devices doesn't notice anything. There is no Tape Tension or Reel Size input on the deck controller. From serial number 53,522 to serial number 99,999, the reel size is evaluated by the drive control. In fact, our new Tape Tension circuit board is sufficient here to evaluate the speed as well.