Spring-line reverb tank
A spring-line reverb tank is a simple electro-mechanical means of adding a reverberation effect onto an (electrical) audio signal. The unit consists of a number of springs with a transducer at either end. At the input, an electro-magnetic coil is used to turn the (AC) electrical signal applied into a back-and-forth twisting motion on a small piece of magnetic material on the end of the spring. The mechnical vibrations travel down the spring and are turned back into an electrical signal by a similar magnet and coil device (acting as a pick-up) at the other end. Some of the vibrations will 'bounce back' and travel up and down the spring repeatedly, which is what gives rise to the reverberation effect.
By varying the number of springs, and the mechnical properties of the springs, the 'quality' and duration of the reverberation can be altered. Some units use pairs of springs which are wound in opposite directions, as apparently this can mitigate some of the 'sproingy' effects these units can suffer from (whilst others appreciate the 'charm' this brings to the sound of a spring reverb!).
Mounting the tank
Because the springs themselves look like inductors to any nearby electro-magnetic signals that may be floating around, they are very susceptible to picking up interference from power supplies and the like within a modular set-up. This presents a perennial problem of how/where to mount the tank inside/outside the case in order to minimise mains hum from being picked-up by the unit. Trial and error is really the only option, though there are many ideas to be found by searching the forum archives - these threads may be a starting point:
Specifications
Full specs may be found by navigating the manufacturers links below. However there is some commonality in use, so this is a quick summary.
Replacing the Doepfer A-199 spring tank
Manufacturers
Accutronics have been a major player for many years, but have recently been taken over by Belton.
The Belton range of tanks include many similar to the Accutronics units, but also include 'mini' tanks, which are favoured by some Eurorack makers' reverb modules.
These Tube Amp Doctor tanks have recently come to light, with very similar looking tanks/specs to the other makes.
Random schematics
Take care with some of these: some will be designed to interface to guitar-level signals (I'm guessing tens to hundreds of millivolts?), and so may need re-scaling for use with general synthesizer-level signals of volts up to 10 volts or more!
It is also possible some use obsolete or hard-to-source parts!
Useful application notes at the Accutronics site.
A couple of simple and more complex circuits [1]
Scan of a Practical Wireless article from 1972
A more involved circuit using three tanks: [2]
Another random schemo [3]
The Tellun Neural Agonizer which uses two tanks