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5XX Transmitter Valves Return to Daventry

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By Rod Viveash, Voluntary Curator, Daventry Town Council Museum.

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In 1925 the BBC opened their first long wave station on Borough Hill next to the town of Daventry. Using the newly developed water cooled valves and producing an output of 25kW, it was then the most powerful transmitter in the world.

Droitwich took over the famous 5XX call sign and frequency in 1934 but the Daventry transmitter was retained, first as a reserve for Droitwich, and later for transmitting coded messages into Europe during the war.

In 1949 the “Old Gentleman” as it was known was scrapped and the building cleared for installation of the new medium wave Third Programme transmitter. Three of the water cooled valves, one of each of the types that were used, were kept on display in the entrance hall of the building.

In 1992, when H.F. transmissions were ceased at Daventry, the 3 valves were put on display in the main transmitter hall for the closedown day, then they disappeared, I was working at Daventry until my retirement from the BBC in 1995.

Quite by chance in February this year I was shown an advert in a museum magazine stating that the BBC was disposing of some broadcasting items from its Heritage Collection including transmitter valves. On enquiring I was amazed to discover they were the missing 5XX valves. They were found at Television Centre in London while it was being cleared for the BBC move, no one knew what they were or how they got there, as they had gone from Daventry to the Heritage Collection in Bradford. The valves were transported by taxi to the BBC Archive Centre at Perivale and are now at the Town Council Museum in Daventry.

The valves as they arrived at the museum. They still have the chrome display stands attached that were used to display them when they were on the Daventry transmitter site at Borough Hill. Click the picture to see a larger version.

The three Marconi valves are a C.A.T. 1, one of 5 in use, one as the oscillator and 4 in parallel as the R.F. output valves, a C.A.M. 1, one of 8 in parallel as the modulator, and a C.A.R. 2 one of 8 used as HT rectifiers for the other valves. The C.A.T. 1 could accept 10kW of input power as an RF amplifier and the C.A.M. 1 could dissipate 5kW dead loss as a modulator (class A high power mod), and the C.A.R. 2 could deliver 1 amp of rectified HT. All needed a minimum of 2 gallons of water per minute flowing over the anode with an HT of 10,000 Volts.

They must be the earliest examples of Marconi water cooled valves to have survived. Water cooled valves were developed in the USA by the General Electric Company in the early 1920s. As most of the loss in a valve occurs at the anode, the temperature of the anode limits the possible output. Putting the anode on the outside of the structure enabled it to be liquid cooled usually by water. One of the manufacturing difficulties was “welding” the metal anode tube to the glass envelope, the metal had to wet the glass and solidify at the same rate and temperature. Also, as the metal and the glass had different coefficients of expansion, the metal was tapered so that only a thin layer was in contact with the glass allowing it to expand and contract with the glass as the valve heated and cooled during manufacture and when the transmitter was turned on and off.

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