This is a spring reverb unit I built using Tombola’s spring reverb circuit that I found on I put it in the chassis of my first bass combo amp that I blew up on accident and then added an extra reverb tank with a tank selector switch, and a hinged top so I can mess with the springs. It’s a standalone effect, so it works like a pedal in a signal chain.

If you have no idea what I’m talking about: “reverb” is the effect you hear when you yell something in a large room — the sound gets drawn out/prolonged and fades out. This thing simulates that effect by taking an input signal like an electric guitar, then using that signal to shake some metal springs. Due to physics, the springs continue shaking after the original signal has changed or stopped. The shaking of the springs is converted back into an audio signal and then sent out to an amplifier.



The spring reverb tank was invented by Bell Labs at some point and then patented in 1939 by Laurens Hammond for Hammond Organ Company who produced and sold organs to churches, and for the first time, normal households. Before the reverb was implemented, the problem was that the people in households were expecting the echoey reverb effect you’d get in a huge church from their organ, and in a small house you don’t get that effect, so Hammond adopted the spring reverb and put it in the organs to mimic the effect of a large space. Eventually in 1959 the invention was made smaller, which made it more practical for cooler instruments. Leo Fender started adding them to his guitar amps and it changed the music world forever, like actually — rock put it on the map with 60’s surf, rockabilly, etc., and now it’s used in every genre to some extent. Nowadays almost all reverb is digitally simulated — a microchip processes your input signal and applies an algorithm to it that adds an approximation of the reverb effect. Digital reverb sounds really clear and clean, but it’s so clear and clean that some people think it sounds artificial and weird — similar to the differences between digital audio and vinyl audio, it’s that sense of warmth/presence that vinyl offers while digital audio sounds more processed. I’m not an analog purist but I was so interested in the concept of the electronic signal being converted to a physical vibration and then back into an electronic signal that I built my own :+)

Technical Info

The basic spring reverb setup is made of a drive circuit, which drives the spring, then after the spring, a recovery circuit to bring the volume back up. For a while when I was experimenting with springs, I had tried several different amps that didn’t work, so I found a schematic and then a stripboard version (linked in intro section) that had everything needed for the circuit, including two drive controls, wet/dry, and a tone control. The thing I was missing in my attempts was that the drive circuit needs to be a constant current amplifier rather than a regular amplifier. I built the stripboard circuit with mostly salvaged components, except the power supply, which is a dual supply of 12v — the reverb circuit needs a +12v, -12v, and ground. I didn’t feel like building that so I bought a Velleman K8042 kit and used it with a 120v to 18v transformer I found somewhere (the supply drops some voltage, so 18v in gets you 12v out.)

I had two reverb tanks: the blue one is an Accutronics AMC2BF2 (can’t remember if it’s the 150 ohm or 600 ohm input one) and the other one is a weird Belton one that I pulled from a friend’s old Kustom practice amp. I put in a switch that lets you select one tank, the other, or both. It sounds a lot less muddy than I thought with both tanks at the same time, I couldn’t tell ya why. The Accutronics one actually picks up a lot more noise than the Belton one — I had to move the transformer as far as I could to avoid the 60hz hum, so I put it into an electrical work box, cut the power cord in half and spliced it between the two ends. 

The first Drive knob controls the drive into the reverb tank. The second one (I believe) controls the amount of recovery. These two can be adjusted for different tank impedances, which weren’t really an issue for me with this circuit. In the video above I keep the drives pretty high cuz I like the distortion it gives, but if you dial it in carefully you can get surprisingly clean sound. This thing also somehow sounds great with bass! The EQ knob doesn’t do anything noticeable for me, so I was going to change it, but I decided I’m fine just using the tone knob on my guitar/bass. 

I used a pedal in front of the reverb in the video because the output level from the reverb unit was too low without it and it sounded bad — it’s possible the spring wasn’t being driven hard enough. A booster at the input of the reverb circuit would be the best improvement to the circuit. 

A fun project! If anyone knows an easier way to do all this, let me know — I’m still learning all this, but I’m wondering if the circuit could be modified to be powered with a DC adapter instead, it’d be way easier than doing the whole dual supply + mains wiring thing. 


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