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| The amplifiers found in the Control Room. |
Amplifier switching
In order to lengthen valve life, the amplifiers were switched on only when required. The diagram shows the general arrangement. By operating the key (which was on the jackfield) an engaged lamp was lit on the jackfield. Relay LT operates and switches on the LT supply to the amplifier and, in the case of the 'A' amplifiers, the 8 volt supply for the valve filaments in condenser mic amps and for polarising the carbon mics. Also for the 'A' amp's, another relay (HC) switches on the 300 volts for the condenser mic amps HT supply.
The manual operation key was provided mainly for use when testing amplifiers and probably when setting up sources for the Dramatic Control Rooms. The switching was usually done by the source selection and routing system. 'A' and 'D' amps were automatically switched on when the associated input switching relays were operated from the CR control positions. 'B' amps were switched on by the control positions' first operating key and 'C' amps by a similar key on the simultaneous broadcast position.
There was an alarm system to warn the engineers of an amplifier failure. The H relay would release if the amp's HT feed fell below a certain level, perhaps because of a fuse blowing or a valve failing. This lit an alarm lamp on the bay and operated the P relay. The P relay, in turn, lit another alarm lamp and a bell. The engineers could then find the bay with a lit lamp, and check the relevant amplifier by looking through the valve windows in the amp casing.
'A' Amplifiers
Each 'A' amplifier bay was equipped with five amplifiers, a jack field,
relays, alarm lamps and termination strips for connecting the cables from
the studios, etc. to the amplifiers. Of the 31 'A' amps, 21 were for use
with the studios; 5 for the echo rooms; 2 for the Dramatic Control Rooms
and 3 were spares. The jackfield provided apparatus, line and listen jacks
for the inputs and outputs of the amplifiers.Another bay in the same rack as the 'A' amps had jacks accessing a second output of the amps. This was used for echo; the inputs to the echo room loudspeakers also appeared on this bay. The echo room mics were wired to further 'A' amps. the outputs of these were then plugged as required to the source selection system. The same bay contained the jacks for connecting the inputs of the dramatic control panel.
Above is a view of the front of an 'A' amplifier with the cover removed.
Note the stud potentiometer, which gave gain control in 4dB steps. Maximum
gain was about 50dB. Below is the back of the amplifier.
'B' and Programme-Meter Amplifiers
Right - The back of a 'B' amplifier and, above it, a programme-meter amplifier. |
The 'B' amplifiers were mounted on bays together with their associated programme-meter
amplifiers. In the photo the 'B' amps are the ones with four inspection windows. The relay
switching arrangement included an extra relay to operate the meter amp which was considered
as part of the 'B' amplifier circuit. A stud potentiometer controlled gain in 4dB steps,
the maximum available being about 40dB.  |
'C' Amplifiers
The outputs of the 'B' amplifiers were wired to 'C' amp input switching
relays, which were operated from the simultaneous broadcasting position.
There were twelve 'C' amps and ten sources - the outputs of the six transmission
control positions and four of the 'D' amplifiers. An indicating device informed
the SB engineer which source was connected to any of the 'C' amps.Each 'C' amplifier was associated with a specific outgoing line to a transmitter or SB centre. They had a high input impedance, allowing several amplifiers to be connected to the same source. The stud potentiometers allowed level to be adjusted in 2dB steps.
'D' Amplifiers
The six 'D' amplifiers were used for incoming SB lines (from Leeds and Daventry) and for long-distance outside broadcasts.Trap-Valve Amplifiers
These were buffer amplifiers which were connected to the outputs of each of the transmission 'B' amplifiers. They had three separate outputs, isolated from one another, feeding the check-phones, house-phones and loudspeakers in the building. 
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