The RVB-1 was a stand alone Reverberation effects box produced by Estey beginning in 1961. It was a solid state device with three transistors powered by 9-volt batteries. It had high and low input and output jacks, and a reverb level control knob.
At the heart of the design was a miniature spring reverb tank that John M. Hanert of the Hammond Organ Company had just patented. Hammond immediately put many different tank into production for other amplifier and organ companies to buy and install in their own circuits.
Estey engineers adapted the effect to several amplifiers in the 400 Series guitar amplifiers as well as their premimum organs. The RVB-1 was designed and sold to provide reverb to anyone who had an amplifier built prior to 1961!
The circuit was assembled on a cone board, similar to the board assemblies of the vibrato sections of the later Custom Series.
Production of these was probably mostly in 1961-1962. There were still some of these units in inventory at the Harmony, PA plant in late 1966.