RCA 77-B1 U.S. Army variant

The RCA 77-B1 replaced the short-lived RCA 77-B around 1941. Both models share the same motor, with a long ribbon that is split into two sections. The upper half behaves as a simple figure-8 ribbon, while the lower half is connected to an acoustic labyrinth by a pipe at the rear of the ribbon, giving it pressure or omnidirectional characteristics. From an electrical viewpoint the two sections may be considered to be in series. They are literally the same ribbon clamped in the middle!

The polar pattern of the earlier RCA 77-B was adjustable by rotating the transformer housed in the base of the microphone. The later 'improved RCA 77-B1' appears to lack this versatility, potentially due to reliability issues with the screw terminals.

The example pictured here is the U.S. Army variant, MI-2199, which came in a bronze finish, whereas the standard version, MI-4043, came in a black and polished chrome finish similar to the earlier RCA 77-B.

Technical Specification:

Output Impedances: 50/250 Ohms

Output Level: -66 VU ( Sound pressure 10 dynes per cm. sq. output terminated into a matched load)

Directional Ratio: 10 to 1-20 db.

View the RCA 77-B1 manual.

A page about the RCA 77-B1 and MI-2199 at the Coutant website.