Privacy set 6AA (GPTN) Diagram SA 5061, c.1937-1949.

The Secraphone was initially based on the so-called Privacy Set No. 6, also known as Frequency Changer No. 6, a fairly simple voice scrambler based on the principle of frequency inversion. It contained a valve-based balanced ring modulator with a reference frequency of 2500 Hz and was used in combination with a black GPO telephone set No. 394 with green handset. All the party at the other end of the line needed, was an identical Privacy Set with a similar No. 394 telephone.

This system was in use between 1937 and 1949 and was rather unsafe as it was based on a simple principle and had no key settings. By mirroring the frequency around 2500 Hz once again, an eavesdropper could recover the original voice data.