The London Twin-Wave Broadcasting Station
The Transmitter Hall
Running along each side of the hall, facing each other and at right angles to the switchboard, are the two transmitters, each of which consists of five separate units.
These units are completely enclosed and are constructed of polished aluminium framework with enamelled steel panels. This type of construction provides the necessary amount of screening to prevent interaction between the various units.
The two transmitters are identical in construction, and operate on the principle known as 'choke control at low power'. That is to say, the high-frequency oscillations are first of all generated at low power, then modulated, and afterwards magnified to the full power of the transmitter.
The functions of the five units mentioned above are as follows:
Beginning at the opposite end from the switchboard, the first unit contains the 'master oscillator', the separator stage, the sub- and main modulator stages, and the modulated amplifier. All the valves in this unit are air cooled.
The second unit contains two water-cooled valves connected in 'push-pull', water-cooled valves being used because this is the first stage in which any considerable amount of power is used.
The third unit contains one-half of the total number of valves forming the final power stage, the other half being contained in the fifth unit, which is really a duplicate of the third. These two groups of valves are connected in 'push-pull'.
The fourth unit contains the high-frequency tuning circuits, to which are connected the high-frequency feeder lines, which run out at right angles from the building to the aerial transformer house.
The doors of all these units, with the exception of the one handling high-frequency currents only, are provided with an interlocking system of switches, which makes it impossible to open any door without automatically switching off the power.
This is unnecessary in the case of the fourth unit, containing the high-frequency apparatus, because no fatal voltages exist. In each stage a spare valve is mounted in position and ready for use, and it is only necessary to close a switch to bring it into operation.
In the middle of the hall there are two control tables, each facing the transmitter which it controls. An engineer sits at each of these control tables and from it he can operate the transmitter and adjust the various input voltages without leaving his seat.
Circulating water for the water-cooled valves used in the power stages of the transmitters flows by gravity from two tanks (one for each transmitter), mounted on the roof of the building, and after passing through the jackets of the valves it flows into two tanks situated in the vault.
It is then pumped through two banks of tubes mounted over a concrete pond outside the building. After passing through this cooler the water is returned to the tanks on the roof. Water is pumped over the outside of the banks of tubes in order to cool the hot water flowing through them.
Thus it will be seen that the water which actually comes in contact with the valves is entirely enclosed, which allows the use of soft water, without the necessity of constant replenishment.
A History of Brookmans Park Transmitting Station by Lilian Caras 1982 (revised January 2002)
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