The General Electric BA-5 Limiting Amplifier’s remarkable performance is the result of an innovative circuit design, originally developed by the Columbia Broadcasting System’s technical staff. This design incorporates control-bias generating circuits that enable an exceptionally fast development of the automatic-control voltage, essential for responsive performance. Additionally, a time-delay network in the signal path temporarily holds the signal, allowing the bias generator circuits to adjust the amplifier gain appropriately. This configuration effectively minimises the audible “thump” by providing a high signal-to-control voltage ratio, especially critical during the initial dB of gain reduction.
One common issue with limiting amplifiers, “motor-boating,” is eliminated in the General Electric BA-5 because the automatic-control voltage operates independently of the amplifier’s output voltage. Another advantage of this design is its ability to sustain a higher average modulation level without causing over-modulation of the main transmitter. The amplifier’s high compression ratio maintains a steady output, keeping the level constant within 0.5 dB, even if the input level increases by up to 12 dB above the threshold.
To address potential “pumping” effects, an “anti-pump” circuit is included to automatically adjust gain recovery times. Single peaks, such as a gunshot, result in a quick recovery time of approximately half a second. However, when multiple peaks occur in rapid succession, like during an announcer’s speech over applause, the recovery time lengthens, effectively minimising the undesirable pumping effect typically associated with limiting amplifiers.
The General Electric BA-5 Limiting Amplifier is composed of two separate units: an amplifier unit and a power supply. The primary controls and meters, which are necessary for regular operation, are conveniently mounted on hinged panels, eliminating the need for removal or disturbance when making adjustments or performing maintenance on the rack. Additional controls, required only for occasional adjustments, are mounted on a sub-panel within the amplifier unit, protected behind a separate hinged cover panel, ensuring they remain secure and accessible only when necessary.
A gain reduction meter, mounted on the front panel of the General Electric BA-5 will:
- Indicate automatic-gain reduction directly in dB
- Indicate balance of various tubes
- Measure tube-plate currents
- Measure plate voltage
A panel-mounted rotary switch allows the user to select the desired meter function of the General Electric BA-5, with a standard VU meter connected to a selector switch to measure both input and output levels. The power supply consists of two independent, regulated power supplies providing 250 volts positive and 250 volts negative, ensuring stable d-c output voltages across a wide range of a-c line voltage fluctuations.
On the front-cover panel of the Power Supply Unit, a power switch and a red-jewelled pilot lamp are prominently displayed. The d-c voltage can be adjusted via two controls located on a sub-panel within the power supply chassis, which also houses a 5-ampere power fuse. These voltage controls and fuse are easily accessible by opening the hinged front-cover panel, simplifying maintenance and adjustment.
The amplifier is designed as two physically separate units that can be positioned apart as needed. The amplifier unit occupies 12¼ inches of rack space, while the power supply unit requires 10½ inches, making the setup flexible for various installation configurations.
Tube complement of the General Electric BA-5 consists of the following:
- 3-type 6J7
- 3-type 6J5
- 5-type 6Y6-G
- 2-type 6X5-GT
- 1-type 6SN7-GT
- 3-type 1621
- 2-type 5R4-GY
- 2-type 6SL7-GT
- 2-type OD3/VR-150
Note: The General Electric BA-5 Limiting Amplifier was designed as two distinct units: the BA-5 amplifier itself and a separate power supply unit, as illustrated above.
