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.

Description
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 Muff Wiggler forum archives - these threads may be a starting point:
 * A-199 Spring Reverb
 * noisy A-199

Replacing the Doepfer A-199 spring tank
The Belton tank used as standard in the Doepfer A-199 is a BMN3BB2C1B, which breaks down as:
 * BMN3 Belton, mini, new, short 3-spring
 * B - input impedance 190&Omega;
 * B - output impedance 2575&Omega;
 * 2 - medium decay
 * C - input insulated, output ground
 * 1 - no lock
 * B - horizontal mount, open down

The two most important ones are the input and output impedances, and any replacement should try and match these two closely in order that the circuit functions as originally intended (different input impedances have different input drive level requirements, so if one of a different impedance is used, the tank may be either under- or overdriven, most likely resulting in poor performance, either a weedy signal and/or distortion of some type).

The 'connections' specifier, the 'C', is unlikely to be critical: in the Doepfer circuit the input is grounded in any case, so using a 'B' type (input grounded/output insulated) will not cause any problems; use of an 'A' (both grounded) or a 'D' (both insulated) will not cause any problems from an electrical standpoint, but may not be advisable from a noise point of view, i.e. leaving the case floating ('D'), may make it more susceptible to hum pick-up, and grounding both sides ('A'), may have implications for noise due to the very small signal levels at the output.

As for the last two characters: the '1' = 'no lock', the only place there is likely to be one with a lock (which simply holds the springs safely for transport) is in an ancient tank on one of the auction sites; the final letter, the mounting orientation, refers to the preferred mounting method to give optimal performance - I'm certain most people ignore this, and I've never seen an issue reported for which the root cause was not having the tank correctly orientated (apparently the 'bobbins' either end, with the spring-mounted magnets inside them, perform better some ways than others).

Thus an example suitable replacement would be an Accutronics 9BB3C1B, but any v BB wxyz, where v, w, x, y, z are any of the possible alternatives for that character position (with the proviso on the 'x' connection-specifier, the 'C', noted above), will be fine!

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.
 * Another brand of tanks has recently emerged - MOD reverb tanks. These are apparently meant to be clones of the early Accutronics tanks: they are made for CE Distribution, who also seem to be the same people as Antique Electronic Supply ('tubesandmore.com'), and Mod Kits DIY, as well as Amplified Parts - the similarities in the webpages are obvious, and the technical page mentioned in the intro above is available at several of these sites, and also contains a summary comparison between the Accutronics, Belton, and MOD tanks.

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
 * Scan of a Practical Wireless article from 1972
 * Triple Spring Reverberation Unit, a more involved circuit using three tanks.
 * Another random schemo
 * The Tellun Neural Agonizer which uses two tanks