Shown above is the Western Electric 8B Speech Input bay. The left bay includes an 8-C microphone preamplifier followed by a 9-A line amplifier and a 203-A TU meter panel. The right bay includes a 350VDC power supply, input patch bays, metering, and a precision clock. Batteries were used to power the filaments and supply bias voltage to the tubes. A large bank of batteries in series provided 24VDC to operate the filaments. A charging system rectified current from the AC power line to keep the batteries charged. Bias voltage was provided by dry batteries held by small wooden boxes with a hinged door mounted directly on the amplifier panels.
Two basic types of microphones were used with the speech input equipment, carbon and condenser. The carbon microphone was basically an adaptation of the telephone transmitter element and required a small bias voltage for operation. Carbon granules contained in a small “button” in direct contact with a thin metal diaphragm changed varying pressure on the diaphragm into a tiny electrical current. This current feeds the primary winding of a transformer that steps up the voltage to drive the grid of the first audio amplifier tube.
Condenser microphones required a high-gain multi-stage amplifier to be located near the pickup element, which is basically a capacitor having a very low capacitance. Because of the very high impedances involved (in the megohms), the leads between the amplifier and pickup element need to be very short in order to avoid hum pickup and to minimize signal attenuation. Power to the amplifier was provided remotely. The signal from the pickup element was amplified to a level near that of the carbon microphone output.
The Western Electric dual turntable is the granddaddy of the DJ turntable. It represents one of the first commercially available turntables featuring an electrical (versus acoustic) pickup for compatibility with the 8B Speech Input equipment. In addition to broadcasting, it was also used in motion-picture theaters to supply the sound track for first-generation “talkies” where the sound track was not recorded on the film, but supplied with the film on 78 rpm records. Obviously, it took a special skill to synchronize the record with the film to “lip-sync” them.
The panel containing the clock is part of the World’s first Primary Frequency Standard built by General Radio in 1928. The precision clock is driven by a vacuum tube amplifier that receives a 1000Hz tone and drives three equally-spaced electro magnets around a gear-like wheel that is geared down to drive the clock motor. The 1000Hz tone is derived from a precision 50kHz crystal oscillator that is in turned divided down to 1000Hz and distributed throughout the building through an audio distribution amplifier. This enabled all the clocks in the building to be precisely synchronized. This was critical for announcing the time and for taking hourly meter readings.
