As a studio player in the 1960s, Mr. Casher was always looking for effects and techniques that would set his guitar solos apart. He admired the bluesy tones that the trumpet and trombone players emitted, with the help of wah-wah mutes, on “Rhapsody in Blue,” George Gershwin’s 1924 classic, but couldn’t figure out how to imitate them on the electric guitar.

The Thomas Organ company had acquired the rights to distribute Vox amplifiers — a British brand that the Beatles helped to make famous. To promote their venture, Thomas Organ formed the Vox Ampliphonic Orchestra, and Mr. Casher was invited to join. That put him on the premises of the company’s headquarters in Sepulveda, Calif., when its engineers began working to modify the amplifiers into solid state, translating all the tube circuits into transistors. As they did so, they ran across a switch known as a midrange boost, or M.R.B. for short.

“They said, ‘What the heck is this?’ ” Mr. Casher recalls of the M.R.B., which used different frequencies to make certain sounds seem louder. The feature — a switch that musicians clicked — had been invented by Dick Denney, a British engineer and guitarist. “If you really want to say who was the grandfather of the wah-wah,” Mr. Casher says, “it was Dick Denney.”

When Joe Benaron, the chairman of Thomas Organ, found out that installing that same switch in the United States would cost almost $3 a unit, he balked. So the chief engineer, Stan Cuttler assigned a young colleague, Mr. Plunkett, to solve the problem. He did so by replacing the switch with a 75-cent knob much like those used for volume control. Soon afterward, at a Vox Ampliphonic Orchestra rehearsal, Mr. Casher first encountered the device.