The Western Electric 1126C automatically reduced amplifier gain when programme amplitude exceeded a predetermined level. Its principal broadcasting purpose was to control peaks that could otherwise cause transmitter overload, allowing average modulation to be increased. In AM installations, this helped prevent the brief adjacent-channel interference associated with overmodulation. In FM systems, it controlled excessive deviation that could encroach on the guard band and cause receiver distortion. The same operating principle also served recording and sound installations requiring automatic regulation of programme level.
The complete system comprised three interconnected units. The 126C Amplifier contained the audio amplification and limiting circuitry, the 298A Control Panel provided operating controls and metering, and the 20B Rectifier supplied regulated power. These assemblies were supplied wired together but could be mounted separately to suit the installation. Their recessed panels were intended for relay racks or equipment cabinets.
At the centre of the Western Electric 1126C was a three-stage push-pull amplifier associated with a DC-biased, programme-operated rectifier. Gain reduction began when the rectified audio voltage overcame the applied bias. A limiter on/off switch disabled the control rectifier when automatic limiting was not required.
The 126C Amplifier incorporated a specific circuit development over its predecessor, the 126B. The earlier amplifier’s 6J5 output valves were replaced with 6SN7 twin triodes. One section of each 6SN7 performed the previous output-stage function, while the other sections formed a push-pull cathode follower. This supplied the limiter rectifier with audio at low impedance to support rapid operation, while also isolating the limiting function from the amplifier’s audio channel.
For a single-frequency signal above the onset of gain reduction, the compression ratio was 10:1: a 10dB increase at the input produced a 1dB increase at the output. The equipment could operate with as much as 15dB of compression, although Western Electric recommended limiting compression to 5dB for programmes requiring preservation of a wide dynamic range. Greater compression reduced programme dynamics more substantially.
Recovery time was selected on the 298A Control Panel. Five settings covered 0.2 to 1 second in 0.2-second increments, with an optional adjustment allowing a shorter range of 0.1 to 0.5 second. The panel also carried a meter used to indicate compression and check valve operating conditions through plate-current measurements.
Input and output levels were established through combinations of fixed attenuators and adjustable panel controls. The adjustable input attenuator provided 15dB of attenuation in 0.5dB steps, while the output control covered 3dB in 0.1dB steps. Fixed input attenuators of 5dB, 10dB and 20dB accommodated different installation levels. The amplifier was supplied with the 10dB input attenuator connected.
On the output side, a 6.5dB fixed attenuator was connected as standard, with additional 1.5dB, 3dB, 6dB and 12dB sections available for setting lower operating levels. The 6.5dB section, or an equivalent external impedance-stabilising pad, provided the required termination between the output transformer and the following circuit. With the standard input and output pads connected, gain was 35.5dB ±2dB. Omitting all fixed attenuators increased maximum gain to 52dB ±2dB.
The Western Electric 1126C operated from a 600Ω source into a 600Ω load, with circuits unbalanced relative to ground. Connection to balanced circuits required an isolating repeating coil. Its frequency response remained within 1dB of the 1kHz reference level from 30Hz to 15kHz. Programme distortion was below 1% under operating conditions involving up to 5dB of compression, while the separate continuous-tone figures distinguished between frequency and compression depth.
The 20B Rectifier provided a nominal 275V DC supply for the amplifier plates and screens, together with 6.3V AC for the valve heaters. Its valve-regulated circuit included provision to prevent the plate supply from rising above its final value during regulator warm-up.
The complete assembly occupied approximately 489mm of vertical rack space and weighed approximately 22.2kg. Front-panel finishes were dark aluminium grey for the 1126C-15 and black japan for the 1126C-3, with dark aluminium grey supplied unless otherwise specified.
Features
- Automatic programme-controlled gain reduction for broadcasting, recording and sound systems.
- Three-part assembly comprising the 126C Amplifier, 298A Control Panel and 20B Rectifier.
- Three-stage push-pull valve amplifier with a programme-operated limiter rectifier.
- 6SN7 twin-triode output valves incorporating a push-pull cathode-follower drive for the limiter circuit.
- 10:1 compression ratio for single-frequency signals above the onset of gain reduction.
- Five selectable recovery-time settings, with an optional shorter recovery-time range.
- Stepped input and output controls supported by configurable fixed attenuators.
- Metering for compression and valve operating-condition checks.
- Limiter on/off switching.
- Regulated power supply with protection against excessive plate voltage during warm-up.
- Separate or combined mounting of the amplifier, control panel and rectifier.