[Antipodal]

Hi there,

 I am working on a Philcophone Intercom set . It goes (now!) and uses 2x 6B7s a 6V6 and a 6X5. The "quiet listen talk " lever switch was broken  and I have removed it but in so doing I found I had a spare wire. I traced this back to the output transformer which has three wires leading from it instead of the more usual two. I say it goes and it does, being really only an amplifier with a couple of switches, and using crocodile clip leads as switches I can get it to work as an intercom and I am installing a new lever switch intending to use it as a two station master and slave set  between the kitchen and my radio shed .
 But that third lead on the output transformer intruigues me . I am wondering if it is part of a "call the  master station " facility  so the slave station can call the master to initiate a conversation. Does anyone have the schematic or can anyone tell me where to look to find it?
 Cheers and thanks to all,

 David Fahy