1940s Altec 639B

Here is a vintage 1940s Altec 639B magnetic cardioid microphone. This particular example, serial number 1058, is in perfect working condition. Also known as the “Bird Cage” mic, and produced for distribution by both Altec and Western Electric, the 639 series has become a must have all purpose microphone for studios of all sizes. A great utility mic for most applications, it interprets most source audio in a very pleasant manner.

This unit was recently purchased from a recording studio where it remained in frequent operation until recently. It appears that the top 2/3 of the screen was restored along with the “Altec” badge. It is partially cosmetically refurbished many years ago, and it looks great. It does not include the original cable.

This Altec 639B features: a 40Hz – 10kHz frequency response, (6) selectable directional patterns including cardioid, bi-directional, omni-directional, and three additional directional characteristics, each having full frontal sensitivity, but with varying degrees of rear sensitivity.

The 639-type Microphone is a general purpose magnetic microphone with directional characteristics which are selectable by means of a switch. The three basic patterns available are non-directional, figure eight, and cardioid.

The open circuit output level is 84 dB below 1 volt/dyne/cm, 64 dB below 1 volt/10 dynes/cm.Power Output Level 56 dBm for a sound pressure level (SPL) of 10 dynes per square centimeter, or 76 dBm for 1 dyne per square centimeter when the micr­phone is terminated with a resistance equal to its internal impedance. Experience indicates that the sound pressure produced at conversational level three feet from a microphone approaches 10 dynes per square centimeter.

Substantially uniform from 40 to 10,000 Hertz.Signal-to-Noise RatioThe signal for 10 dynes per square centimeter sound pressure is 78 dB above the thermal agitation noise generated within the micro­phone, and 58 dB above for 1 dyne per square centimeter.

Three patterns, C, D, and R, selectable through a three-position screwdriver-operated switch. At the angle of minimum response, the average discrimination with respect to 0 response is 20 dB over the range from 40 to 10,000 cycles per second.

Average value is 40 ohms. Intended for use with equipment having a rated source impedance of 25 to 50 ohms.

Dimensions: 7 1/2″ high, 4 7/16″ long, 3 7/16″ wide. Weight: 3 1/4 pounds.

The 639A Microphone combines a dynamic moving coil pressure element and a ribbon velocity actuated element enclosed in a housing which serves as a protective guard and as a wind screen. The outputs of these two elements are used independently, or are combined by means of a selector switch attached to the microphone, to yield 3 directional patterns. The moving coil pressure element when used alone has non-directional characteristics. The ribbon element when used alone has a figure eight pattern. The combination of the two elements results in a phasing of the output voltage causing the microphone to have a cardioid directional pattern.

The 639-type Microphone is an excellent general purpose microphone for studio use in broadcasting and recording, and for remote broadcast pickups where it can be given reasonable care in handling. In addition, the 3 directional patterns will enhance operation of the microphone where audience or background noise must be controlled, and in public address pickups where acoustical feedback would ordinarily take place before a satisfactory reinforcement level could be reached. It is particularly suitable for multi-microphone pickups of a large orchestra group where a sense of realism and an apparent increase of volume level is desired. This is accomplished by utilizing one 639-type microphone as a non-directional (position D) microphone for general orchestra pickup and several 639-type micro­phones (positions R or C) as accent micro­phones for soloists and individual groups of the orchestra which the operator may want accentuated.The general microphone will provide the necessary reverberation which is desirable for realism, and the accent microphones will permit the individual groups to be accented for special musical effects. Skillful operation of the mixing of the various microphones will permit the operator to control, over a considerable range, the liveness of the pickup.Designed by the Bell Telephone Laboratories and originally manufactured by the Western Electric Company, the Altec 639A Multi-Pattern Cardioid microphone has, for years, enjoyed an unprecedented acceptance by all phases of the audio industry. Indeed, it may be stated that the majority of all wide-range sound recording from its initial appearance on motion picture soundtracks to the present achievement of magnetic tape has employed one or more Altec 639 microphones in virtually all phases of production. The immediate selection of pickup patterns provides the professional engineer with as many as six varying directional characteristics; the built-in two-stage windscreen, rugged protective housing, and numerous attachment and mounting accessories make the Altec 639 ideal for any application.Unlike competitive units, which employ only a single ribbon and which vary the directional pattern by mechano-acoustic means, the Altec 639 is actually two independent transducer elements within a single housing. The electrical signals from these elements (one, a dynamic moving-coil; the other, a velocity-sensitive ribbon) are used singly or in combination to produce the different pickup patterns at the top of the page. Because no mechanical means are used, the Altec 639 provides increased durability, performance quality, and trouble-free operation, yet the 639’s versatility of application meets or exceeds that of all other multi-pattern units currently available.The 639A provides the broadcast, recording, and public address engineer with performance of professional standards; the difference between the two models is found only in the amount of readily-selected pickup patterns. The 639A provides the three most widely used characteristics of cardioid, bi-directional, and omni-directional pickup.Each directional pattern of the 639A Microphone may be easily selected with a screwdriver or similar tool, in accordance with the indicator marks on the rear of the microphone housing.Low output impedance (30/50 ohms) permits the use of the 639A microphone at a great distance from the associated amplifying equipment without danger of increased noise, hum pickup, or deterioration in the quality of the transmitted signal.