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Every destination, which in 1932 usually meant a transmitter fed by a Post Office line, had its own 'C' amplifier feeding it and the outputs from the control positions were switched to the inputs of the 'C' amplifiers. Various means of performing the switching are possible but in the 1932 control room it was done with relays - around 1,500 of them for the B and C input switching alone.
SOURCE SELECTION
In the following description note that relays were often made to operate by earthing one side of the relay coil, the other side being connected to 24 volts - in other words a negative earth system. Also amplifiers in the control room were powered by simply switching on the heater volts, there was no current drain from the HT or grid-bias supplies if the heater supply was off.
To select a source to a fade unit, the AME first operated a key (the 'first operating key') associated with the channel input that he wished to use on his control position. This did three things; it applied an earth via what was known as a 'private wire' to a relay which turned on the control position 'B' amplifier, it applied an earth to the 'P' relay via the A4 back contacts of the 30 A relays associated with that control position input and it turned on the channel engaged lamp. The P relay operated and provided a path through its contact from the A relay to the 'Punching Key'. There were 30 of these, one for each source, and when one was momentarily pressed, 24 volts was applied via the closed P relay contacts and via the A relay to earth. The A relay operated, contact A3 was arranged to close first and apply 24 volts to hold relay A operated. Contact A4 opened and broke the circuit to the P relay thus preventing any other selection to that channel input.
In the case of a transmission position, contact A5 operated the programme change-over relay which took the source away from the switching associated with the rehearsal positions and switched it to the transmission side of the switching relays. This was done to minimise cross-talk, only sources on transmission were present on the transmission relay wiring. It also meant that sources on transmission could not be selected at the rehearsal positions and sources needed for transmission were automatically switched away from rehearsal control positions. Contact A6 turned on the source's 'A' amplifier via other relays, which also switched on the 300 volts and 8 volts required by the studio microphones. A6 also lit an engaged lamp over all the punching keys for that source. A1 and A2 connected the source to the channel input. Until the 'first operating key' was released no further selections were possible on this channel.
Although the diagram shows one P relay coil and one contact, in fact there were 30 contacts, one for each A relay. As 30 contacts on one relay is not normally possible, the P relay was in fact 4 relays operated in parallel with the 30 contacts shared between them.
The
B amplifier input switching relays. There were two- and four-channel control positions. The picture below shows a two-channel version.
The right hand panel carried the thirty 'punching keys' and two 'first operating keys'. The keys above the faders on the left panel operated studio signalling lights. The transmission positions had keys to operate the studio red lights and the rehearsal positions the blue ones. Both types could also control the green lights. When one of the red or blue light keys were operated, engaged lamps would light on all the control positions, thus indicating to all the operators that the studio was in use. Studios had push-switches which operated a buzzer on the control position, and a further light would indicate which studio had buzzed.
DESTINATION SWITCHING
The destination or C input switching was identical to that for source switching. There were 12 'C' amplifiers, which were fed from 10 sources. The switching was done at the Simultaneous Broadcast position shown on the floor plan as the Trunk Line Switching position. The sources were the 6 transmission control positions and 4 of the 'D' amplifiers. There was an indicator system to show the AME which source was connected to which 'C' amplifier. The 12 destinations included Leeds SB Centre, the Daventry long wave National transmitter and the Brookmans Park transmitter.AFTERTHOUGHT....
The switching system seems very well thought out but assuming the diagram is correct, it would appear to have been possible to hit two punching keys simultaneously and thus select two sources to the same channel which would have paralleled them on the whole switching system. Also it looks from the pictures as if the channel 'first operating keys' were unguarded and thus if accidentally knocked or caught with the sleeve of an operator's jacket, the source would have been instantly disconnected. 
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