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1936 Marconi transmitter from Alexandra Palace

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The following contribution to BBCeng by John Liffen is in response to an enquiry posted on the Information Exchange page.

In August 2019 John Rhodes wrote: “I was interested to read the article on the 2LO transmitter which led on to talk about the 1936 Marconi 17kw transmitter at Alexandra Palace. I used to work for the Marconi Company in Building 46 New Street, Chelmsford in the late 1960s. In the entrance lobby to the building was that 17kw amplifier sitting there as a museum piece. I have always wondered what happened to this equipment when the company was closed down by the finance industry. I hope it was moved somewhere safe and not thrown away, but despite my searches I can find no reference to it.”

I can reassure John that in about 1982, when the Marconi Company found they could no longer accommodate the transmitter, they offered it to the Science Museum, London, which was pleased to accept it. Keith Geddes, at that time curator of the Radio Communication collection, was hopeful of displaying it but gallery priorities changed and it had to go into store. More recently, when the Science Museum’s ‘Information Age’ gallery was being planned, we looked again at displaying the transmitter. However, its size sadly ruled it out and it is still in store at the National Collections Centre in Wroughton, Wiltshire. Its object number is 1985-366. I include two photos taken while the transmitter was at Chelmsford and another taken at Wroughton in 2013 which shows part of the interior.

Plaque and simplified circuit diagram of final amplifier unit

Final amplifier unit

Part of the interior

The Science Museum has also preserved some other parts of the 1936 AP transmitter. These are: the final amplifier stage of the vision modulator (object number 1962-273); and the black-level clamp unit, part of the vision signal control apparatus (object number 1962-274). These also remain in store at Wroughton. I include photos of these: in the background of the shot of the final amplifier stage can be seen the surviving part of the STC long-wave telephony transmitter installed at Rugby in 1927 (happily this is now on display in Information Age).

Final amplifier stage of the vision modulator

Black-level clamp

The Science Museum also holds (in the Archives collection) a copy of D C Birkinshaw’s famous ‘black book’ describing the Alexandra Palace equipment (object number 2017-190). It is copy no. 9 but we do not know to whom it was first issued.

John Liffen

Curator Emeritus, Science Museum (formerly Curator of Communications 2003-2017)

D C Birkinshaw’s famous ‘black book’ describes the modulator, but not the final RF amplifier.  So the circuit below the plaque is of additional interest.

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