Problems with spring reverb driver

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
5,207
Location
Between Sussex, UK & Aude, France.
Scenario as follows:

I have a reverb tank that was pulled from an old organ. I made the driver circuit using Craig Andertons design here
http://sendling-info.blogspot.com/2007/01/spring-reverb-design-by-craig-anderton.html

When inserting a sine wave into it I was getting bad distortion, I think because the tank was too low impedance. Because if I put a 100ohm resisitor in series it was OK, but there was practically no level.

I checked at accutronics page & was measuring around an ohm on the i/p coil which indicated that I had the 8ohm tank. So I rebuilt the circuit using this diagram for the driver with a BD135 & BD136 for the transistors
http://www.accutronicsreverb.com/drive5.pdf

Result .......... exactly the same as without the transistor buffer ???

Does this indicate the tank has a damaged primary ??? Do I need more meaty drive transistors ?? Should I give up & buy a digital reverb with a spring emulation ? Will the digital spring emulation splash if I slap the side of the box ??
 
Does Accutronics make a similar tank? Maybe even with higher input impedance? They are really inexpensive, with the most expensive one being maybe $50.

I would think that an 8 ohm tank should measure more like 6 ohms at a minimum...but I don't have much experience with those. Just a guess from familiarity with speakers where the DC resistance of an 8 ohm driver is usually around 6 ohms.

HTH!
 
[quote author="SonsOfThunder"]Does Accutronics make a similar tank? Maybe even with higher input impedance? They are really inexpensive, with the most expensive one being maybe $50.[/quote]

It is an accutronics tank ! I was just trying to be cheap & build the whole thing with bits I had lying around that would never get used for anything else.

I would think that an 8 ohm tank should measure more like 6 ohms at a minimum...but I don't have much experience with those. Just a guess from familiarity with speakers where the DC resistance of an 8 ohm driver is usually around 6 ohms.
They have a table that gives the dc resistance of the tanks. The 8ohm one measures around an ohm !
 
I like Walter's suggestion, but I wonder what your voltage rails are. More is gonna be better I think. A couple of 9V batts seem rather not the ticket for this job....

Maybe try cap-coupling? I dunno but 1 ohm is tough to drive!
 
[quote author="SonsOfThunder"]I like Walter's suggestion, but I wonder what your voltage rails are. More is gonna be better I think. A couple of 9V batts seem rather not the ticket for this job....

Maybe try cap-coupling? I dunno but 1 ohm is tough to drive![/quote]

I have been using a + & - 15v bench PSU

1 ohm is roughly the dc resistance which the table that can be found below would indicate is an impedance of 8 -10 ohms

http://www.accutronicsreverb.com/ioic.htm

Will try the cap thing though.
 
This reminds me of an amp that seemingly was driven OK but still didn't produce sound. Turned out that the link from 'speaker-coil excitation' to 'cone-producing-sound' was gone, so a mechanical defect.
Could that be the same for this tank ?

The 1 Ohm indeed sounds very low.
 
[quote author="Rob Flinn"][quote author="clintrubber"]The 1 Ohm indeed sounds very low.[/quote]

I agree but if you look at the table in the link above it is in line (roughly) with the spec for an 8ohm (imp) coil.[/quote]

I see, sorry, didn't check that link. Indeed there's no reason that the rule of thumb for speakers applies to all-things-coil...

Bye,

Peter
 
Rob,

I assume you are not using a 4136 as the schematic calls for. Maybe the opamp doesn't have enough current drive to the transistors. Are the transistors getting hot? What opamp are ya using?
 
[quote author="SonsOfThunder"]Rob,

I assume you are not using a 4136 as the schematic calls for. Maybe the opamp doesn't have enough current drive to the transistors. Are the transistors getting hot? What opamp are ya using?[/quote]

I used a 5534.
 
Hi Rob,

Did you have a look at the mounting diagrams at the accutronics site??

http://www.accutronicsreverb.com/mntconf.htm

Perhaps you get distortion caused by your magnets are not good enough centered in the gap and touch the lamination??

How was the unit mounted in the organ? Did you change it´s alignment??
Perhaps your unit is one of these factory-adjusted models, that should be used in another alignment??

I have one of these units as well, it´s an 14" double-spring unit made by "Gibbs Manufacturing & Research Corporation" Subsidiary of Hammond Organ Company. The input dc resistance is about 6.5 ohms and the output dc res. about 170 ohms. I´m planning to build a fender-like preamp using this unit. I´d like to use the original tube driver, but unfortunately at the moment I have no idea where I could get the transformer needed for this.
Any ideas?? Perhaps anybody here has some specs of these original Fender reverb-drive-trannies??

Greets,
Sascha
 
Those hammond tanks used a capacitor coupling like in the gibson amps, the accutronics used the transformer coupling. I have a replacement driver transformer from newsensor, with an ohmmeter it reads 1k primary, 1ohm secondary. The last time I added a reverb pan, I used a some oddball transformer that another tech said would work, I think it had a couple ohms on the secondary, It worked. On the internal transducer, I have used a shaving of chopstick to wedge in, it tightens up the sound.
 
Walter,

I looked again at the schem for the Fender Blues Jr and its a constant current IC driver. The driven side of the tank is across a 4.7k resistor in the feedback loop. Looking at the schematic, I guess you'd consider the tank to be cap coupled thru the RC on the inverting input of the opamp that goes to ground. Somebody correct me if I'm looking at it wrongly.

Rob,

I think maybe try a ckt similar to that of the Blues Jr. should be in order before you give up. I think you might still need those current booster transistors in the loop, but I think you can figure that out yourself.

Sascha,

There are a few different companies here in the US that sell replacement Fender-style reverb transformers, but surely there is a supplier in Europe too.

HTH!
 

Latest posts

Back
Top