I got this unidentified spring reverb tank with what looks like a driver. It’s not an Accutronics tank and there is no indication where it comes from, but a little detective work showed that it might have been pulled out of an old Ace Tone (the precursor of Roland) powered mixer (like this one).
The circuit, which has seen better days, is transistor-based with three 2N5089 (I think it’s the low-noise version of the classic 2N5088) and two 2N5855. Rather than trying to build a driver for a spring tank with unknown impedances (probably futile), I’d like to see whether the driver at hand works. I therefore need to:
Sort out a way to power it
Figure out the location of the input and output
There is a reading on the tank itself that says “6V”. Do you think that I might be okay with switching supply at 6V or should I have a regulator for the job (like
L7806 or LM317)? How much current do these spring reverb tanks typically draw?
I also tried to line up the back and front images of the circuit board to trace the circuit to locate the input and output. I have uploaded a file here where you can play with the transparency between the images (the images do not line up perfectly, but you get the idea…) The black and red wires of the 4-pin Molex connector are power and ground, just like the wires soldered on the back of the board (no idea why). So I guess the other two are the input and output of the circuit. One of them connects to a 100k trimmer and then to a 100nF capacitor, while the other to a 56nF capacitor. I obviously need to dig further, but on the first look, the former is probably the input?
Nice find. Elliott Sound has some really useful information on spring reverbs. Here’s a link: Care and Feeding of Spring Reverbs
In order to know the current that your tank draws, you’ll need to work out its impedance. Is there any code at all printed on the tank?
It looks like it should be pretty simple to draw a schematic from your PCB – there are only a handful of parts, and the traces are all very clear – which would make it much easier to say with some degree of certainty what’s going on regarding which leads are input and which output.
For testing purposes, a switching supply should be alright, but in the long run, you’ll need a regulator. Spring reverbs are prone to picking up noise. Good supply and in particular, good grounding are critical in keeping that to a minimum.
I’ve read the Rod Elliott article, but without any markings on the tank whatsoever, I have no idea of its input impedance. Since it’s not an Accutronics tank, I cannot even guess the model by measuring the input resistance. This leaves me to try the actual circuit, which is probably easier than building one of my own and hoping it will work with this particular tank. All the parts are through-hole, and I don’t see anything exotic in terms of components, so I am confident that I can repair it if needed and make it work. I guess I will have to bite the bullet and do all the tracing before I try anything though!
Once you’ve done that, it should be obvious which side is the input and which the output (I think that your hunch is probably correct – it makes sense that the trim pot is there to adjust the input level, but better to know for sure), and we’ll also be able to make a more educated guess as to what level and impedance the circuit is expecting.
