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The RCA MI-6203/6204 Varacoustic
polydirectional ribbon microphones (1945)

RCA Varacoustic Ribbon
The RCA MI-6203


The MI-6203 (low impedance) and MI-6204 (high impedance) Varacoustic Micro­phones are high-fidelity ribbon type instruments, which are designed to provide a variety of directional response patterns. The Varacoustic Microphone is especially recommended for public address, industrial sound and stage pick-up. It is intended primarily for indoor use and if used outdoors should be protected by a silk bag or other suitable windscreen. The choice of directional patterns makes it possible to regulate the field of pickup to a considerable extent with a resulting increase in signal-to-noise ratio. Reverberations and unwanted sounds such as audience noise may also be reduced by proper adjustment and placement of the microphone.

RCA MI-6203

Humphrey Bogart, Lauren Bacall
Armed Forces Radio Services announcer Jack Brown interviews Humphrey Bogart and Lauren Bacall for a radio broadcast to American troops overseas during World War II. This image is a work of a U.S. military or Department of Defense employee, taken during the course of an employee’s official duties. As a work of the U.S. Federal Government, the image is in the public domain.

RCA MI-6203

Mr. Schrank observes: “Notice the small diameter mount. This is the same size used on the bullets and some other early RCA models. Only two screws hold it together. There is a slight gap at the bottom, but it is better than most.”

RCA MI-6203

Directional Characteristics
The desired directional response is selected by moving the adjustable slider on the back of the microphone case to any of the marked positions. The positions lettered P, U, and V refer to pressure, unidirectional and velocity operation respectively: the two inter­mediate positions are numbered 1 and 2. The directional characteristics are continuously variable and the slider may be stopped between any of the marked positions. Adjustment of the microphone will be determined by its location and the purpose for which it is used. The response for each position of the slide is shown in Figure 3 and these diagrams should be used as a reference when selecting the directional response setting.

This material was obtained from the Radio Corporation of America publication
entitled RCA Instructions, Varacoustic Microphone MI-6203-D, No. 76332.

Frequency response

Polar patterns

William H. Macy
William H. Macy uses a Varacoustic in Seabiscuit.

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The RCA Type SK-50 Varacoustic
polydirectional ribbon microphone (MI-12034)

SK-50 front

Hear the sound of the SK-50 (MI-12034).

SK-50 rear

RCA Type SK-50

RCA Type SK-50

RCA Type SK-50
The RCA Type SK-50 (MI-12034)

Stephen Sank observes:
“The MI-6203 is literally just a cheaper version of an RCA Type 77-D, with precisely the same ribbon/motor structure. Pattern control is arranged differently with a slide rather than a plate, and there is no voice/music low-cut switch, but otherwise is exactly the same as a 77-D. The LMI-6203 is the rather rare British-made version.”
Dennis Schrank clarifies:
“The SK-50 and the MI-6203/4 are not the same mic. The SK-50 (MI-12034) replaced the earlier model. I believe the SK-50 had an improved transformer, and the two body parts were secured with three screws rather than two. The early ones had a nasty habit of warping and leaving gaps between the two sections. The easiest way to tell them apart is that the early version had the bright red meatball, while the SK-50 has a plain embossed logo. The SK-50 also fits a 58″-27 stand without an adapter.”
Bret Morrison, Grace Matthews
The Shadow—Bret Morrison portrayed Lamont Cranston and Grace Matthews played Margo Lane on this popular radio program from 1946 through 1949. Others who performed these roles include Orson Welles and Agnes Moorehead.

RCA Varacoustic

6203 cover
Download the instructions for the MI-6203.

SK-50 cover
Download the instructions for the SK-50.


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