Altec 633A

According to the Modesto Radio Museum, the first 633 microphones were manufactured in 1937 by Western Electric, and were continued by Altec after WE were forced to sell off their recording equipment manufacturing division by the US government. This model had a long life and was still in production in the late 1950s - the 633 appears in the 1957 Altec catalogue.

From the Altec catalogue of 1957

Type: Moving Coil Dynamic

Sensitivity: -55 dbm/10 dynes/cme

Frequency Response: 35 to 12,000 Hz

Output Impedance: 633A - 30/50 ohms; 633C - 30/50 or 150/250 ohms, selective

Finish: Instrument gray

"The Altec 633 "Salt Shaker" Microphone is truly the accepted standard of the broadcast industry for a rugged, quality microphone both for routine studio use and field application. Its continued selection by experienced audio engineers is adequate proof of the durability of this exceptional design in broadcasting, recording and public address.

The Altec 633 type microphone is available in two models: the 633A with an output impedance of 30/50 ohms and the 633C which will provide either 30/50 or 150/250 ohm output.

Due to the perfect symmetry of the design, the 633 is completely omnidirectional in pickup and response to sounds arriving at an angle of 90 to the front of the diaphragm. When this feature is desired, the microphone is usually mounted vertically or suspended on its cordage. For sounds arriving perpendicular to the diaphragm, the response rises gradually from 1500 to 8000 cycles, providing a high degree of intelligibility for speech reproduction. This intelligibility characteristic can be further increased and a directional element introduced into the performance of the microphone by the addition of the accessory 8B Baffle which fits snugly around the face of the microphone.

Other accessories which add to the flexibility of the microphone include the 9A Swivel, and for those applications where it is undesirable to permanently wire the microphone to its cable, the 311 A Plug may be permanently attached to the rear of the microphone. Complete data on these and other accessories will be found on the microphone accessories data sheet.

This famous microphone will be found ideal for those recording, broadcast public address and sound distribution applications where high intelligibility durability and long life are a requisite."

The frequency chart below shows the on-axis response in blue and the response at 90 degrees in red.

A post about the 633A at the Modesto Radio Museum

The 633A at the Coutant website.

Chris Garges writes about the 633A at Tape Op magazine.