The heart of any radio station control room is the audio control console. This unit controls everything that goes on the air. In the early days of radio these were rather primitive to say the least. Later on they became more sophisticated and user-friendly. In the 1930s consoles were developed that put the controls on a desk-top unit with a sloping panel that connected with a whole rack of amplifiers and other equipment.  The early amplifiers were so large that they had to be housed in a separate “rack” or frame-like cabinet and were connected to the actual control desk-unit by many wires.[6A]
In 1939 RCA came out with a revolutionary audio console that they called a “consolette”. Except for the power supply this consolette had all the amplifiers, relays and such contained within the relatively small, desk-top unit.  This was dubbed the 76 series and RCA touted it's flexibility and ease of operation.
The 76 consolette provided program quality that met FM requirements. It had full facilities for simultaneous auditioning and broadcasting for practically any combination of studios, turntables, or remote lines. It performed all the amplifying, monitoring, and control functions of most large and small radio stations.
The model 76 consolette was an instant hit and found it's way into hundreds of radio and TV stations during the 40's and 50's. It also was less expensive than the bulky, rack dependent units of before. The University of Maryland has a website that has photos of disc jockeys at radio stations...mostly from the 40's and 50's...and the RCA 76 series consoles are seen in many a control room of stations all across the country. [4,5]
The very first station I worked for, KYOS in Merced, had one of these consoles in the main control room and so I learned the “nuts of bolts” of actual day-to-day operations on this very console.  Compared to today's modern consoles, the RCA 76 was pretty simple...yet was very flexible for it's time.
The basic idea of a control console is to have, at your fingertips, all the controls needed to operate a radio stations audio system. There are switches that select which input circuits are used, volume controls, sometimes called “faders”, that allow the operator to adjust the level of incoming audio. There is a “board master” fader that controls the overall output level of everything going out to the stations transmitter. Another monitor is the large “VU”, or Volume Unit meter, at the center of the board...this helps the operator maintain proper output level to the transmitter.
Also there is are “monitor” controls that let the operator choose to listen to either the audio coming out of the board or external circuits like a radio receiver that picks up the actual stations over-the-air signal. Most disk jockeys will want to listen to the actual stations sound so he can be sure he's actually on the air. There is also a “cue” circuit that allows the DJ to “cue” up his records or tapes or monitor a network line or even a remote line coming in from some location other than the studio.
The 76 console had a double row of push buttons at the top on each side of the panel. The right side buttons were used to select the input circuit to the number 5 and 6 fader or volume controls of the six at the bottom center of the unit. Buttons one and two on each row connected to turntables 1 and 2...they could thus be switched to either fader 5 or 6. The remaining push buttons could be used for any incoming audio devices or circuits and they would also connect to either fader 5 or 6. This arrangement made it very flexible depending on how the operator wanted to set it up. When one button was pushed it automatically released any other button selected so you could not have two things on the air at once.
Faders one through four, of the center six, were for microphones. At KYOS an engineer had modified the board so that faders one and two could be used for tape machine inputs. This same engineer had replaced the original fader controls with a more modern type that had a cueing position at the very farthest counter-clockwise rotation for cueing tape machines and turntables This made it very fast and easy for an operator to get ready the next audio event rather than having to do complicated board switching through the monitor circuit.
The left side push buttons were used for the monitor circuits and the far left volume control was the Monitor volume to the control room speaker. The fader control at the far right was the master volume control. The lever switches above the faders were used to switch the audio on each control to either the “audition” channel or the “program” or on-air channel. [1,2]
The audio quality of the RCA 76 console was excellent...in fact I remember the chief engineer doing a check of the frequency response of the console and it was flat from 50 to 15,000 cycles or “Hertz”. This, of course, was vacuum tube technology designed way before anyone had ever heard of a transistor! Some audio purists today insist that tube electronics produce a much smoother and true sound than today's digital audio that sometimes is perceived as “harsh”.
The other major manufacturers of audio control boards during this era were Gates, Collins and Altec. Sadly these companies are long gone from the broadcast field. These older makers led the way from the tube era into the solid state or transistor era. [9,10,11,12,13]
Today's radio consoles look much different and normally use straight-line vertical faders rather than the rotary type of the earlier era. But the basics are the same...switches to control which audio comes into the board and faders to control the volume level and to smoothly transition or “fade” from one source to another. The amplifiers inside are now tiny “chips” called integrated circuits or “Op-amps”. 
[6A] The same 1930 era desk-top console with associated amplifiers mounted in the large cabinet rack.
The Broadcast Audio Console
By Gary Avey
 This RCA's 1930s style console was still in use until recent times from the well - worn look.
 Composite photo of the 76 console showing the open top revealing the tube amplifiers inside, high quality Daven ladder-type faders or volume controls and other components on the underside.
 Yours truly at KYOS in 1959 behind the RCA 76 board. Notice the stick-on labels indicating the various input circuits. As I recall the double row of push buttons on the upper right side brought in the two main turntables and the Mutual and ABC Pacific radio networks as well as various remote lines.
 A very young DJ in the late 40s or early 50s playing 78 rpm records and using an RCA 76 board. (U of Maryland)
5] Here obviously running a country-western record show with an RCA 76 console. (U of Maryland)
 A 1940s era Gates Radio console. (U of Maryland)
 A Gates dual channel tube-type board as used at Los Angeles station KPOL in the 1950s.
 This is a custom-built RCA stereo first-generation transistor console with 22 faders!
 Huge RCA custom TV audio console built for ABC-TV in the 1960s.
 A modern-day board with straight-line, slider-type volume control faders.
 From the 1958 Gates catalog This was a last generation tube console. Larger View >
13] The Gates “Yard” console...just a yard wide with small printed circuit tube amplifiers. I remember seeing this console in KJOY, Stockton's street-level window studio in 1960. Larger View>
 Studioette from Gates...a small but versatile audio board. Larger view >