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Odd reverb tank, is it usable?


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12/29/2004 10:29 PM
clint b Odd reverb tank, is it usable?
I have a reverb tank and am wondering if it can be put to use. I got it out of an old Motorola console. It looks much like an Acutronics tank with the four springs set up like two pairs connected in the middle. It has "REVERBERATION UNIT TYPE IV" PATENT PENDING HAMMOND ORGAN COMPANY.  
 
What makes this tank different is it seems to have stereo inputs. Even odder is that each channel is run through a small light bulb on a small circuit board on the side of the tank. The input was paralelled from the speakers through a 7.5 ohm power resistor (10w to 20w?). The speakers were 8 ohm. The total DC resistance is 23.2 ohms... 8.2 for the resistor, 15.2 for the light bulb and 3.2 for the input coil. Times 2 because its stereo. (OK, I know the resistances don't add up.. That's what the meter reads!) The input seems to be center tapped. The output is DC 173 ohms mono. Note to self... check meter!  
 
Would this be of any use for a guitar amp? If so, how?  
 
What's up with those light bulbs?  
 
Thanks,  
 
Clint
 
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12/30/2004 8:39 AM
Mark Lavelle

That particular tank isn't familiar, but Hammond organs used "post power amp" reverbs for years. As you noted, they tapped into the speaker output to drive the tank directly. The return side went into its own amp, with an additional speaker attached to that.  
 
The light bulbs act as limiters, which makes the reverb a little less dependent on the dry volume level. I guess the tank itself could be stereo, but I've never seen such a thing. Does it really have two transducers on each end?  
 
Since the input is low impedance, you could probably drive it with a standard Fender-style circuit. The recovery side should be about the same, too, unless Hammond diverged wildly from their other reverb circuits...
 
1/4/2005 1:50 PM
Ben N
Just a note
"...Hammond organs used "post power amp" reverbs for years. As you noted, they tapped into the speaker output to drive the tank directly. The return side went into its own amp, with an additional speaker attached to that."  
 
Ditto Ampeg, e.g the Echo-Jet and Echo-Twin, and Gibson. O'Connor also discusses a variation on this in TUT-1.  
 
Ben
 
12/30/2004 10:16 AM
Mark Hammer
Re: Odd reverb tank, is it usable?
That is totally whack, my friend.  
 
If you check out the specs on the various Accutronics tanks, you'll see that some have input impedances that are more like dynamic microphones, and some have input impedances that are more like speakers. Those with low speaker-like impedances can not be driven by just anything. Whatever device that drives them has to be able to sink lots of current into a low impedance load without blowing up.  
 
In this particular instance, it would seem like the designers fixed that by having the speaker output stage do double duty as a reverb driver. That doesn't mean they HAVE to be used in that manner though. It is common these days for folks to drive reverb tanks with the various headphone power amp chips out there, such as the LM/JRC386. Alternatively, some op-amps, like the LM833 and NE5532/5534 are quite comfortable in the presence of low-impedance loads. Forrest Cook has some nifty reverb circuits that use these two options. http://www.solorb.com/elect/musiccirc/reverb/index.html  
 
If it is indeed a stereo unit, check out the schematic and/or kit for Craig Anderton's "Hot Springs Reverb" that PAiA sells. http://www.paia.com/hotsprgs.htm  
 
There are a few modifications I'd make to that circuit (the op-amps being the first), but the essentials of it are quite sound, and should be capable of making a decent sounding reverb for your amp or studio.
 
12/30/2004 12:49 PM
clint b
Mark, and Mark,  
 
Thanks for the info. And yes it is whack! It is stereo input and mono output. The input has three wires. One goes to ground and the other two went to the right and left speakers. It's center tapped. I have the small amp module which drives the separate speakers for the reverb. I do not have the main amp which powered this little module. The power for this modules seems to need B+, Screens and preamp, ground and two wires for the heaters. There are five wires on a Molex like plug that I have only one end of. It has a 2.5 by 5 by 1.5 approx. chassis. It's point to point wired with no corcuit board, mostly on the sockets. The tube complement is a 12AX7 and 2 ECL82's. It is cathode biased with a 480 ohm 5w resistor and a 10 MFD bypass cap.It has a push pull output transformer. I think the speakers were 8 ohm. I would like to see if I can fire the amp up... Any suggestions (or guesses!) on the voltages needed?  
 
Oh ya, it's called the "Vibrasonic"  
 
Thanks,  
 
Clint
 
12/30/2004 2:26 PM
Mark Hammer

Back in the days when formula 1 cars were probably the only thing that didn't have bench seats, some car radios actually came with on board spring reverb, generally a single spring, ultra-short delay type. Bizarre little things about 6 inches long. Ironically, the interiors of the cars they were in could probably contain a whole Hyundai or Toyota from today. Why one needed to make them sound "larger" inside is beyond me. On the other hand, I still can't figure out why almost every single piece of film music from Bollywood also has reverb on it.
 
12/30/2004 4:17 PM
Carl Gigun

The input is center tapped? Kind of sounds like a balanced mono input. Stereo inputs wouldn't share a transformer/transducer.  
 
-Carl
 

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