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ST3500 and ST3600

Catalogue Nos. ST3500 series (Standard) and ST3600 series (Tropical)

A table telephone of modern design, for use with all automatic switching systems.

The corresponding telephone for C.B. manual systems is described in Leaflet STL4.

The moulded Bakelite case, identical in shape with the latest British Post Office model, combines a robust design with a pleasing simplicity of line.

All components except the dial are mounted on a main assembly plate, which can be withdrawn from the case very easily as a complete unit.

The G.E.C. dial is the British Post Office standard, with a stainless steel finger-plate which retains its high polish indefinitely.

The cradle-switch mechanism has been specially designed to eliminate any likelihood of sticking plungers. Twin contacts give maximum reliability.

The handset is the well-known moulded Bakelite Gecophone pattern, with inset type transmitter and receiver.

The flexible handset cord will give exceedingly long service; the conductors are made from tinsel that withstands 200,000 reciprocations without breaking.

The four-way Bakelite terminal strip, connecting the incoming line wires to the telephone cord, has moulded separators between terminal plates to maintain long leakage paths.

There are separate entrances for the line wires and the cord.

The transmitter and receiver are British Post Office standards, and the high-efficiency speech circuit gives maximum volume with uniform frequency response and excellent sidetone suppression.

The unfailing efficiency of the G.E.C. dial mechanism is well known and a special spark-quenching circuit protects the dial impulsing contacts during dialling.

If required, a suppression circuit can be included to prevent interference with nearby radio receivers during dialling.

Tropical Model
G.E.C. telephones for service in tropical climates incorporate various special features, which are the result of long experience (see picture above):-

  • A shunt is connected across the transmitter to minimise the minute arcing that tends to occur at the granules and electrodes as: the cradle switch operates; the accelerated tendency towards frying often shown by transmitters in the tropics is thereby offset. The effect of any increase in transmitter resistance upon line circuit conditions is also minimised by the shunt. The loss in volume efficiency is very small and is partly cancelled by reduction of sidetone, the nett result being a difference of only 1 db. Should the conditions of use render the protection unnecessary and require the very maximum of volume efficiency, then the shunt may be disconnected by the simple transfer of one wire from one terminal to another.
  • Ventilation holes in top and bottom of the case allow air to circulate freely through the case to minimise condensation.
  • All apertures are covered with fine wire gauze to prevent the ingress of insects.
  • Metal parts are specially finished, and all wiring is specially treated, to resist the effects of high humidity and salt-laden atmospheres.
  • Cradle-switch springs have large insulators to ensure long leakage paths.
  • Flexible cord conductors are V.I.R. covered and braided overall. Since textile bindings at the ends are not in contact with the conductor, moisture absorption cannot cause leakage. Rubber fittings give mechanical protection at points of entry to the mouldings.
  • All coils are specially treated to render them impervious to damp, heat and sudden changes in temperature.
  • For very dusty atmospheres, a bakelite cover can be fitted to the back of the dial to give complete enclosure to the mechanism.

Earlier model shown - identified by the flat switch hook plate

The standard colour is Black but enquiries are invited for the telephone in Ivory, Jade Green or Chinese Red, with cords finished to match.

Extension Bell
A magneto extension bell in the G.E.C. Cat. No. BE1100 series (Standard) or BE1200 series (Tropical) is recommended for use with this telephone, to provide calling signals at a distant point, indoors or out, or to augment calling signals in very noisy surroundings.

Length - 7.25" (18.4 cm)
Width - 9" (22.9 cm)
Height - 6" (15.2cm )
Weight - 5.5 lbs. (2.5 kgs)

Enquiries and Orders
Please state on all enquiries and orders:

  • Finish - whether standard or tropical
  • Colour required.
  • If batteries are to be supplied and whether they are to be active or inert.
  • For component parts, please refer to item by name and quote catalogue number of complete instrument ; for example, "Handset cord for Telephone Cat. No. ST3101".

Taken from the GEC Catalogue Leaflet No. STL.3

The ST prefix model numbers were introduced in 1946.
The diagram numbers are all prefixed with KS.


ST 3301 (TEL/1P/CTS) ST 3403 (TEL/2P/CTT) 332 C.B. Manual
ST 3504 (TEL/1P/ATS) - 332 Automatic Non-Director areas
ST 3505 (TEL/1P/ATS/1) - 332 Automatic Director areas
ST 3506 - 332 Mark 1 (GPO) Also made for the Reliance Telephone Company
ST 3509 (TEL/1P/ATS/1) - Automatic Director areas - minor variations
ST 3510 (TEL/1P/ATS/1) - Automatic Director areas - minor variations
- ST 3608 (TEL/2P/ATT) Automatic Dial Non-Director areas
ST 3521 (TEL/9P/ATS) ST 3621 (TEL/10P/ATT) Automatic for D.C. ringing.*
ST 3551 (TEL/3P/ATS) ST 3651 (TEL/4P/ATT) 312 Automatic two-party line, selective ringing, non-director areas
ST 3552 (TEL/3P/ATS/1) ST 3652 (TEL/4P/ATT/1) 312 Automatic, two-party line, selective ringing, director areas

Note : * For use with the 10-line All Relay P.A.X. Cat. No. PX1110 (Standard) and PX1210 (Tropical).

Circuit Diagram for the ST3608 - Diagram KS9645

To Convert the ST3608 to work on the UK plug and socket system

  1. Remove existing line cord.

  2. Connect the red wire of the new line cord to L2.

  3. Replace the link between E and L1 with a 3.3k resistor.

  4. Insulate the green wire of the new line cord.

  5. Connect the white wire of the new line cord to L1.

  6. Disconnect the black wire from the 2uf capacitor.

  7. Connect the blue wire of the new line cord to the black wire and insulate.

  8. Connect a Rectifier No. 205 between R and MR.


Circuit Diagram for the ST3506 - Diagram KS8335

Reliance and GPO Telephone No. 332

To Convert the ST3506 to work on the UK plug and socket system

  1. Remove existing line cord.

  2. Strap terminals 8 and 9 together.

  3. Connect the red wire of the new line cord to terminal 9.

  4. Replace the link between 1 and 2 with a 3.3k resistor.

  5. Connect the green wire of the new line cord to terminal 3.

  6. Connect the white wire of the new line cord to terminal 1.

  7. Remove the link between terminals 10 and 11.

  8. Strap terminals 11 and 12 together.

  9. Connect the blue wire of the new line cord to terminal 11.

  10. Connect a Rectifier No. 205 between terminals 5 and 6.



ST3505 Black - standard automatic telephone with Director dial (letters and numbers)


ST 3506 - CB variant made for the Reliance Telephone Co.


ST3506 - Internal view

ST3604 with chassis removed.  This is a later model with the "non-stick" switch hook plungers

ST3608 Ivory

ST3608 Black


Pictures below are all ST3608 models

Front view

Front view

Rear view

Manufacturers marking on handset

Close up of interior of Terminal Block

Circuit diagram pasted to base plate

Base plate showing model number
Note the two plates, with mesh covered holes, held in place by the feet

Interior view of telephone


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Last revised: August 02, 2022