RCA 77-B

MoMics view.

The RCA 77-B (MI 4042) ribbon microphone is somewhat of a curiosity. It was only made for a brief period of time and appears in the 1939 RCA catalogue. By 1941 it had been replaced by the 77-B1 (MI 4043).

Both models have the same motor, with a long ribbon that is split into two sections. The upper half behaves as a simple figure-8 ribbon and the lower half is connected to an acoustic labyrinth by a pipe at the rear of the ribbon, which gives it pressure or omnidirectional characteristics. The two sections of the ribbon may be considered to be in series from an electrical viewpoint. They are literally the same ribbon clamped in the middle!

However there is an important difference between the two models. The earlier 77-B has two wires connected to the centre clamp which run down to the transformer housing along with connections to the top and bottom clamps. The top, middle and bottom clamps are each connected to separate screw terminals.

The transformer itself may be rotated and connected to the ribbon in three different positions:

1. Top to bottom with both ribbon sections in series to give a cardioid pickup.

2. Top to middle clamp (Figure 8 only)

3. Mid to bottom (Pressure / Omni only)

In addition, the transformer primary winding has a centre tap, so that the output impedance is kept the same despite the ribbon length being shorter in the figure-8 and omni positions.

These changes require the bottom bell of the microphone to be removed and would not have been a quick adjustment during a recording session. The 77-C and 77-C1 models had external switching mechanisms.

The later 'improved' RCA 77-B1 appears to lack this versatility. There may have been reliability issues with the screw terminals.

From the 1939 RCA catalogue

ThE TYPE 77-B Unidirectional microphone can be used to excellent advantage in practically any type of studio installation. For example, in an auditorium studio or in one which provides space for an audience, this Uni-directional microphone can be used to pickup the entire action on the stage because of its cardioid pattern and will suppress, approximately 20 to 1, any undesirable noises originating in the audience.

It can also be used to advantage in small studios where the reverberation time tends to be objectionable, precluding the use of a non-directional microphone. In a given studio, the type 77-B microphone can be used with approximately 1.73 times less reverberation pickup than a non-directional microphone. It is ideal for applications where it is necessary to place a microphone close to a wall or a window, since the pickup of reflected sound from the wall or glass is materially reduced.

Its operation is similar to that of the type 77-A Uni-directional microphone which it supersedes, namely; a single ribbon fixed at the centre, one-half operating as a velocity microphone and the other half operating as a pressure type. The two outputs of the ribbon are connected in series and the resultant vector addition of the generated voltages produces a directional characteristic as shown below. This curve also shows the uniformity of the directional response with respect to frequency.

The microphone's small size, light weight, rugged construction and good sensitivity recommend it as one which no station can afford to lack.

SPECIFICATIONS

• Output Impedances: 50/250 Ohms
• Output Level: 55 VU (–63 dB at 10 bar open circuit*)
• Frequency Response: See curve (40-10,000 Hz *)
• Directional Ratio: 10 to 1 (-20 db)
• Finish: Polished Black and Chromium
• Mounting: ½" pipe thread
• Dimensions overall length (including mounting) 10"; width 3¾"; depth 2½"
• Weight... Unpacked including mounting: 2 lbs.
• Supplied with 30' shielded cable, less plug.

* Figures from an alternative data sheet at Countant.Org

A post about the RCA 77B at the Coutant website.

The RCA microphone timeline at Xaudia

The black and white photograph is a publicity photo for Hank Williams taken for WSM in 1948.

View the RCA 77B manual