Product Details:

Vactec Vactrol resistive opto coupler, neon lamp - light source. Photocell 80¦ (full bright) to 9M¦ (full dark). Used in Fender and Gibson amps. 3/8" x 1-1/8". Made in 1976. NSN: 670-0028-000.

In the early 1960s, Gibson and Fender started using resistive opto-couplers to modulate the tremolo effect in guitar amplifiers. Both companies were assembling their opto-couplers (RO) from discrete lamps, photoresistors and coupling tubes. While Gibson used cheap but slow incandescent lamps as light sources, Fender replaced them with neon lamps. This Vactec RO uses a neon lamp as used by Fender in the late 60's. It increased the maximum frequency response and reduced controlling currents. At the same time, nonlinear modulation resulted which was desired by some as a sound effect.