A fixed frequency oscillator built into the tape recorder to supply the bias current.
"Excellent job sir!!!! I know how hard it is to get good sound out of old recordings. You are the best!!! I was reticent to send the tapes because to me they are time capsules and irreplacable momentos of some studio sessions from years past by persons that are dear. You took great care and effort with these obviously and did a SUPER JOB. The ..."