Spring reverb is a classic effect that has stood the test of time. Early reverb use dates back to the 1940’s when the Hammond company began using it with their organs. Spring reverb first gained popularity with electric guitar in the 1960’s thanks to the tube-based spring reverb units created by Fender. Not a whole lot has changed since then. While there are some pretty good digital signal processor (DSP) pedals that attempt to achieve the same effect, there really is no substitute for an authentic analog spring reverb effect. In this article, I explain the basics of how spring reverb circuits work and options for selecting a spring tank.
Audio Signal Processing
Real analog spring reverb is achieved by passing the signal from your guitar through two or three springs. Before that happens though, your instrument’s signal must first be split into dry and wet paths. The wet pathway refers to the signal that passes though the springs, while the dry pathway refers to the signal that bypasses the springs. Why is this necessary? Because the wet signal by itself lacks attack. While a 100% wet signal can sound beautiful, the lack of attack makes it unsuitable for most lead guitar playing. To overcome this, the dry, unprocessed signal is mixed back in with the wet signal to get the classic reverb sound we are familiar with.
Spring Reverb Drive Circuit
Before the audio signal enters the springs, it must be amplified in a way that will adjust the signal to match the specifications of the spring tank connected to the circuit. The portion of the circuit that performs this task is known as the drive circuit.
Spring Tanks
The spring tank (aka spring pan) is a metal case that holds the springs in place. The wet portion of the audio signal typically enters the spring tank through an RCA jack. The audio signal drives the coil of a transducer, which moves the magnets attached to the springs. This creates wave impulses that travel the length of the springs until they reach the output transducer at the other end of the springs, creating a delayed output signal. The audio signal can then leave the spring tank through a second RCA jack.
Two of the most popular spring tank brands are Accutronics and Mod Electronics.
Recovery Circuit
The audio signal leaving the spring tank is weakened, so it must be amplified to a level that is compatible for mixing with the dry audio signal. The part of the circuit that accomplishes this is called the recovery circuit. The recovery circuit must be carefully designed to avoid the introduction of unwanted noise.
Choosing a Spring Tank
Because spring tanks are connected to the circuit via RCA jacks, changing out one spring tank for another is an easy tank. That is great for those of us who really want to customize our tone. Below you will find specification options and how they affect your tone.
2-Spring vs 3-Spring Tanks
Two spring tanks have a slightly more uneven reverberation, which is often described as a flutter. Three spring tanks have a smoother reverberation. If you want a vintage reverb sound, two spring is the better choice. If you want a smooth, full-sounding reverb, then you should go with a three-spring tank.
Reverb Decay Time
Decay time is how long the reverb tail persists after a note is played. Spring tanks are available in long decay, medium decay, and short decay models. Long decay is usually 3 – 4 seconds in duration. Medium decay is usually 2 – 3 seconds in duration. Short decay is usually 1 – 2 seconds in duration. Medium and long decay times are often favored by guitar and keyed instrument players.
Spring Tank Length
Spring tanks are typically classified as long (16.25 inches), short (9.25 inches), and mini (5.875 inches).
Spring Tank Impedance
In general, you want the input and output impedance values to match the specifications set by the circuit. Matching the specs exactly will guarantee the tank will work with the particular circuit you have, although often times an exact match isn’t necessary as long as it isn’t too different. For example, if a circuit calls for an 800 ohm input impedance tank, you can go with a 600 ohm input impedance tank without any problems. You can probably go even lower than that, although some distortion may occur when the drive/dwell is maxed out.
In the Bulinski Effect Pedals ASD-1 and ASD-2, a 600 – 800 ohm input impedance tank is specified, but you can actually go much lower. That is possible because there is a trimpot on the circuit board that can be used to adjust the spring drive (dwell) range.
