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UK
Phone
Voice
Scrambler
GPO
  
No. 6AC →
No. 6AA →
  
Privacy Equipment No. 1
Complete Frequency Changer telephone scrambler setup

Privacy Equipment No. 1 is a generic name for a complete telephone speech scrambling system, used by the British Army Signals Corps and the Signals Research and Development Establishment (SRDE). It consists of Frequency Changer 6AA or 6AC, SA5063 desk set, junction box, indicator, crank-operated inductor, batteries, cableing and an optional auxiliary unit (telephone selector).

There are two known configurations: (1) a single telephone installation, and (2) a sharing installation with up to three telephones. Both setups are illustrated below. In the block diagrams below, a model 6AA Frequency Changer is used. This model was later replaced by Frequency Changer 6AC, which has more power supply options and has an improved receiver circuit [D].





Versions
The following versions of Privacy Equipment No. 1 were available:

Parts
Frequency Changer No. 6AC/3
Voice terminal (converted telephone set)
Frequency Changer 6AC
The actual voice scrambler was a Frequency Changer 6AA/0, or 6AA/1 or 6AC. It is housed in a metal enclosure in black wrinkle paint finish that measures 585 x 360 x 185 m and weighs no less than 41 kg.

The unit would normally be installed in a cup­board, under a desk, or in an adjacent room. Once installed, there is no need to have quick access to it, as it has no controls. It is powered from the AC mains or from a 12V battery. At the top is a hinged lid that provides access to the serviceable parts (valves, vibrator, fuses, etc.).

➤ More information

  

Advanced terminal   SA 50xx
The Frequency Changer 6AA or 6AC was used in combination with a special (modified) telephone set, such as the SA 5063, or the SA 5063/1 shown in the image on the right.

These telephone sets were modified, and allowed the user to switch between clear and scrambled voice calls, using push-buttons on top of the telephone set. These phones had a green handset to show that it was a 'secure' line.

➤ More information

  


Documentation
  1. Fig. 1 - Block diagram, Single telephone installation
    Source and date unknown.

  2. Fig. 2 - Block diagram, Sharing installation
    Source and date unknown.

  3. Fig. 3 - Block diagram, Frequency Changer Unit, Privacy Equipment No. 1
    Source and date unknown.

  4. Pamphlet No. 371A
    SRDE, undated. 1

  5. SA-6023 DC/AC Converter
    Origin and date unknown.

  6. EMER T 710/1, Privacy Equipment No. 1, Mks. I, I* and II
    Telecommunications. Issue 1, 10 February 1945.
  1. Document kindly provided by Louis Meulstee [3].
References
  1. Louis Meulstee, Personal correspondence
    18 November 2023.
Further information
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© Crypto Museum. Created: Saturday 18 November 2023. Last changed: Wednesday, 05 November 2025 - 11:55 CET.
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