6740 Hot Springs Reverb

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6740 Hot Springs Reverb

Post by PAiA-Scott »

Dear info-person,

I am inquiring as to your Hot Springs Kit, I am interested to know if this particular device can be used via an effects loop on a guitar amp?

Any help is appreciated!


Hey there again,

Just read that I need a amp of sorts to power this device as well, I know I need a power supply but am curious what kind of amp and if the instructions or schematic will specify where to incorporate the amp?

Thanks again!



Hello Brandon.

Thanks for contacting PAiA with an interest in the 6740 Hot Springs Reverb project kit. I have copied some extra, general information about this kit below. Yes, it can be connected in an effects loop, but it is best if the loop signal has a control that sets a mix of the effect signal with the original signal. I think you might be thinking of the external power supply required for the kit, or you might have seen some reference of using it with an extra amp and speaker to work as the acoustically mixed reverb signal.
Thank you.

Sincerely, Scott Lee


The 6740 Hot Springs Reverb project kit is just a building block approach for a line-level spring reverb unit to be patched into a mixer in a studio situation. It is only a small circuit board, a handful of components and wire, and a couple of reverb spring tanks provided as a companion to a D.I.Y. article from Modern Recording and the manual is basically a reprint and not one of our usual assembly manuals. Information for assembly of the board is contained in the article text, schematic, parts list, and wiring diagrams--it's just not the more informative step-by-step instructions we usually provide. The completed assembly is a circuit board wired to the tanks and 1/4" phone jack input and output. It must be connected to a bipolar or dual-dc (+and- 9 to 15V) power supply and housed or mounted in an enclosure or platform. The 9770R-12 is a power supply listed in our catalog that can be used for the project.

The springs and their transducers are in a steel case, but the bottom is open. The transducers in the reverb tanks are likely spots for electromagnetic interference to get in and the signal is weak like a mic so hum or other noise can be introduced without the shielding afforded from being in a grounded metal chassis or having good shielded wiring techniques applied. A metal plate could be used to cover the bottom. The patch cables to the tank RCA Phono connectors attaches the circuit ground to the tanks. Using the expected line-level input signals helps to ensure an optimum signal to noise ratio.

For use with guitar, some extras are needed. When putting it in a amp/speaker, vibration and acoustic feedback should be anticipated and a permanent installation should be tested in stages. The spring tanks should be mounted to reduce this possibility (one spot may be better than another, damping foam, felt or rubber might help in isolation). For the tanks we provide, they are intended for a wall-mount, connector-down installation, although its hard to hear a difference when listening as the unit is tilted one way or the other.

A guitar signal should be boosted to be a line-level signal using a preamp or direct interface or some effect that boosts or at least buffers its signal. Then, since the 6740 is 100% reverb on the output (no original signal remains, only the signal through the springs), a mix function is needed. This can be accomplished by splitting the boosted guitar output two ways and sending one to the 6740 and the other to a second, direct, input which could be a mixer or multiple input amp.

If you don't have devices to do the split and mix, our TubeHead, Stack-in-a-Box, Tube MicPreAmp, 9605K Buffer Amp, Spluffer or similar could be used to boost the guitar and drive the split. The CA18 is a simple mixer circuit that could be used to combine the 'effect' and 'straight' signals. These projects can be built from Craig Anderton's 'Electronic Projects for Musicians' and 'DIY Projects for Guitarists' books listed in our catalog. A simple, but effective guitar-set-up would be to run the guitar to a preamp or boost device and split or Y-cord connect the output from this to the usual guitar amp/speaker and run the other, 6740K signal to a small portable/practice/satellite amp/speaker for the reverb mix. A home-stereo receiver can blend the signals acoustically (ie reverb left and straight signal right with blend via the balance control), or the unit set to mono so both outputs get LandR signal and the line output of a Tape Record/Monitor/Out connector.

-0-0-0-0-0-0- sl


Brian Hensley wrote:
> Hello from Nashville,
> I have been interested in your kit for some time but find no references to any "Build Reports" or sound samples or Wav files that may demonstrate some of the differences in the unit as opposed to normal reverb.
> Could you direct me to a web site or provide some information about the kit that will help me with my purchase decision?
> Thanks for all the great material and writing through the years.
> Best regards,
> Brian Hensley


Hello Brian.

Thanks for writing with your questions about the Hot Springs Reverb project kit. It originally appeared in a 1980 Modern Recording magazine article by Craig Anderton and we've offered a kit for it since then. That's a great idea, bringing the scoop for this project into the current format. I'll try to get something together. A build with photos would certainly enhance the assembly pages which are just copies of the article to include our parts list and schematic--not the usual step-by-step, PAiA assembly manual. It is a simple printed-circuit with just a single quad op-amp IC (4136), etc. But building is a matter of referencing the parts list and schematic, followed by modification of the spring tank wiring, and completed with wiring of the printed circuit board to the tanks, connectors, and an external, user supplied dual-dc +and- 9-15v regulated dc power source. There is not a case or cabinet provided, but the image on the front of the assembly pages show Craig's two-channel prototype with the tanks mounted on top of an LMB chassis and the 1/4" input and output connectors mounted on the front.
So, when you order item 6740K you get the assembly pages, a pcb, a handful of parts and wire, and two reverb tanks. The 'heart' of a single channel reverb project for connection with a mixing board. Note too, when ordering from the PAiA site, a 9770R-12 is suggested as the power source. This is fine, but the manual suggests a 15v type. We hadn't considered this when we made the site association, but there is negligible difference between the operation on a 12 vs 15 v supply, and, the 12v supply we offer has more current available than the 15v type. If only being used for the 6740K, the 9770R-15 would be the best choice.

Many wonder about using the 6740K for a guitar set-up. The simplest way to do this is to input the guitar to the mixer with the reverb attached to provided the Wet and Dry mix. The 6740K output signal is 100% reverb, no original signal is in the output for the comparison which is usually mostly Dry with a bit of Wet. The guitar signal can be split two ways and run through one amp/speaker dry and through another amp/speaker after passing through the 6740K. The amp/speaker volumes set the mix. It is best if the guitar is boosted or pre-amped before this split. An effects loop on an amp may boost or buffer the signal to an externally connected device, but if there isn't a control to mix the return signal with the original, then a 6740K patched on this loop will not have original, dry, signal. A simple, but effective guitar-set-up would be to run the guitar to a preamp or boost device and split or Y-cord connect the output from this to the usual guitar amp/speaker and run the other, 6740K signal to a small portable/practice/satellite amp/speaker for the reverb mix.

Hot Springs is different from other reverbs that just run the signal through a single tank. It runs the signal through two tanks connected in-series and out-of-phase in the negative feedback loop of the driving amp for flatter response and a 'balanced' transmission through the springs. The output of the tanks is recovered from the two outputs wired in-series and in-phase for a stronger signal with less spring signal that isn't from the input signal. The reverb effect has been favorably compared to that available from expensive and large plate-type systems.

I am not aware of any places on the internet where you could get the information you're after, but there are a lot of studio owners and musicians that have this kit and many report they are pleased with it. There is likely someone that has posted about it. I'll save this message as a reminder to contact you if I see anything or get anything on our site.

Thank you.

Sincerely, Scott Lee
Scott Lee