Godda@mn spring reverb tanks

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jamos
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Godda@mn spring reverb tanks

Post by jamos » Thu Jul 25, 2019 6:54 pm

OK, question first: if I can't get an ideally matched reverb tank for my needs, what should I compromise on: impedance matching or orientation?

I've been trying to get a spring reverb tank to work with my PulpLogic spring reverb driver; the blue Accutronics tanks are like antennaes (they pick up all sorts of stray noise, especially 60-cycle hum.) I would like to replace the blue tank with a tank in a metal case. In looking at the Accutronic/Mod numbering system, my ideal tank would be an 8BB2C1D. Does this exist? Not that I can tell. The closest I can find is an 8BB2C1A (impedances match but must be rotated 90 degrees) or an 8DB2C1D (mounting direction matches but input impedance is 600 ohms rather than 150).

Any thoughts? I hate hum.
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Triglav
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Post by Triglav » Fri Jul 26, 2019 8:30 am

In my (limited) experience, the metal case doesn't help that much with mains hum. Orientation and location of the tank is the most important thing.

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Post by Jaytee » Fri Jul 26, 2019 8:43 am

I know the tanks all come with recommended orientations, but I think the conventional wisdom around here is to ignore those recommendations and just put the tank wherever it’s quietest.

Edit: but also worth mentioning that you’ll probably never be fully rid of hum/noise. Just the nature of the beast. Like you mentioned, it’s very much an antenna.

On another note, using good quality shield cable to/from the tank will also help.

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Post by GrantB » Fri Jul 26, 2019 10:17 am

The only thing that's worked for me is putting the tank as far away as possible from anything electronic, especially power transformers. I've gone as far as building external power supplies for rack units, as their own PS induces noise in the spring.

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Post by jamos » Fri Jul 26, 2019 10:59 am

Thx all. I was afraid of this.. been spending too much time on this and I was/am looking for a cheap and easy solution. I did find that rotating my existing tank 45 degrees, by hand, seemed to damped the noise... so I built a little jig to hold it that way and once completed, it still hums...
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Post by jorg » Fri Jul 26, 2019 2:58 pm

A few tips from my experience:

I agree that orientation (mechanically) doesn't seem to affect the operation noticeably. I have two tanks arranged as stereo, bolted open-face to open-face. One is definitely "upside-down" per datasheet, but both work fine.

Bolting them face to face (and they are metal cases) does appear to provide some good shielding. My setup has virtually no hum.

I drive them with a little Radio Shack stereo speaker amp, about 5 Watts, through a resistor that is about 5x higher than the nominal input impedance of the tank. I recover the signal with a simple home-built op-amp with a gain of 50. This gives a very flat frequency response out to about 7KHz, and nice clean output. It's a hi-fi stereo spring reverb.

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Revok
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Post by Revok » Fri Jul 26, 2019 7:31 pm

The tank orientation isn't crucial in new production tanks. Impedance is important though. Is the blue tank not powder coated metal?

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Post by jamos » Fri Jul 26, 2019 10:55 pm

Revok wrote:The tank orientation isn't crucial in new production tanks. Impedance is important though. Is the blue tank not powder coated metal?
Good to know... no, the blue tank is pure unshielded plastic.
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Jim the Oldbie
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Post by Jim the Oldbie » Fri Jul 26, 2019 11:44 pm

Too bad someone didn't come up with a humbucker pickup for these things, back when the world at large still cared about them.

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Post by GrantB » Fri Jul 26, 2019 11:55 pm

Jim the Oldbie wrote:Too bad someone didn't come up with a humbucker pickup for these things, back when the world at large still cared about them.
AKG did

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Post by Revok » Sat Jul 27, 2019 11:35 am

The Craig Anderton design has always looked interesting. It uses two normal tanks so no need for an unobtainium reverse wound/polarity tank.
https://www.paia.com/manuals/docs/6740pages_200.pdf

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Post by weasel79 » Sat Jul 27, 2019 3:03 pm

GrantB wrote:
Jim the Oldbie wrote:Too bad someone didn't come up with a humbucker pickup for these things, back when the world at large still cared about them.
AKG did
is that why the bx series was so popular? what about diy-ing one? or just modifying the pickup?

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Post by jorg » Mon Jul 29, 2019 10:30 am

Revok wrote:The Craig Anderton design has always looked interesting. It uses two normal tanks so no need for an unobtainium reverse wound/polarity tank.
Cool! I love it! I might try taking my stereo unit and phasing it that way in mono, just to see how it sounds.

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Post by Jarno » Mon Jul 29, 2019 12:14 pm

Also, use good cables to hook up the reverb tank, a shitty RCA cable that has been on the bottom of a moving box for a couple of years will not cut the mustard.

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Post by jimfowler » Mon Jul 29, 2019 12:47 pm

At the risk of derailing, here's a pic of the guts of a BX20 I'm repairing for a friend's studio. It's a wonder to behold.

- Jim

Picture file

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Post by GGW » Mon Jul 29, 2019 1:46 pm

Image
This is a wood 3+1u rack in transition. I have a PulpLogic reverb driver with a blue plastic Accutronics tank mounted to the inside back. I use sticky backed copper foil to shield the inside of my wood cases. I know this stuff to be used for shielding electric guitar cavities. It's on eBay and comes in wider widths. I hope you can see from the picture, but I shielded the case and then, stuck the foil over the plastic tank as well. The tank is now within a grounded copper shield. I don't know if I got lucky with the orientation, but the tank is about as quiet as I think I can expect to get.

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Post by jorg » Mon Jul 29, 2019 1:56 pm

jimfowler wrote:At the risk of derailing, here's a pic of the guts of a BX20 I'm repairing for a friend's studio. It's a wonder to behold.
How many springs are in that tank? Looks like maybe 4?

That's a monstrous magnet - I can only make out one gap for a transducer though. Interesting that the coiled part of the spring seems to go right into the gap.

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Post by fuzzbass » Tue Jul 30, 2019 5:41 am

weasel79 wrote:
GrantB wrote:
Jim the Oldbie wrote:Too bad someone didn't come up with a humbucker pickup for these things, back when the world at large still cared about them.
AKG did
is that why the bx series was so popular? what about diy-ing one? or just modifying the pickup?
I doubt this would help. The pickup is not the antenna here, it is the connection from the return transducer to the return amp. The signal is very weak and near the noise floor. Effective steps to reduce noise: use a high quality coaxial cable (I use RG174), limit the distance, and route it away from noise sources such as mains power, mains step down transformers, noise generating modules, switching power supplies, clock modules. It won't help at all plastering copper shielding all over the inside of your case, if the noise source is inside the case. Better to focus on where the return transducer is positioned, and how its signal is handled en route to the reverb driver.

Blue Reverb is going to be noisy because the factory supplied return cable is just hook up wire leads, twisted.

Amplified Parts has an 8BB2C1B tank.

https://www.amplifiedparts.com/products ... od-8bb2c1b

The only difference here is the mounting plane.

Input and output impedance must match the requirements. Grounding config must also match, but you can easily modify any pan to any grounding config. Other factors (orientation, decay time and spring count) you can choose to suit taste. Don't worry about mounting plane, focus on the placement of the return transducer.

Spring reverbs are never noise free, primarily because the return signal is so weak. But you can minimize the noise with careful handling of that signal.
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Post by jamos » Tue Jul 30, 2019 3:43 pm

Thanks all. I did many of the things suggested with my first tank: I wrapped it in mumetal sheeting, grounded that, and made an output cord of RG174 (not easy to solder to those screwy little connectors.) That dropped the noise level about 3 db(!). And to top it off, the repeated removal of the output cord during the build process eventually caused the output connector to fail! So I was back to square one.

More parts coming next week. Second try.
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Post by jamos » Sat Aug 03, 2019 12:03 am

OK, final report. I replaced the blue Accutronics tank with a MOD 8BB3D1B tank. The impedances are a close match, the grounding configs are the same. The MOD tank is a steel open tank, with RCA jacks. I built an adapter cable from Hosa phono cable.

The results are shown below (screenshot from Adobe Audition, don't judge me.) The second bar up is 60 Hz and you can see the various harmonics; I don't know what that crap below 60 Hz is.) The first chunk is the original tank, the second two are the new tank, ungrounded and grounded.

Image

Based on separate analysis the ungrounded MOD tank is about 12 dB lower than the blue tank, and grounded is perhaps 20 db lower. (I wouldn't bet much on the accuracy of that 8 dB gap but I did hear crackle when removing/attaching the ground wire, so there is definitely electrical potential there.) In either case the hum is now lost in the overall background noise, so I am happy.

In addition, the MOD reverb sounds much smoother and less clanky, and is far less physically sensitive (the blue tank would boing when inserting patchcords!) Plus the MOD tank costs $19.67 and the Accutronics tank combined with the crappy isolating panel is about $20, so it's a no-brainer.
Last edited by jamos on Sat Aug 03, 2019 11:01 am, edited 1 time in total.
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Post by JimY » Sat Aug 03, 2019 4:32 am

Nothing wrong with Audition! Those are great results.

A point of interest maybe, I have an unbranded tank from an old Farfisa organ and the spring receiver coil is on one side of a laminated core that extends an equal amount to the other side. I wonder if that is for an optional hum-cancelling coil? I've never seen a humbucking one, but can't see why it couldn't be done like that.

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Post by EATyourGUITAR » Sat Aug 03, 2019 12:06 pm

I am planning a new case with spring reverb. I thought hard mounting the flange to the wood was normal but now I see rubber isolation mounts. should I use the rubber mounts or hard mount? should I put copper shielding everywhere? I was planning on using the doepfer with a long spring horizonal accutronics tank. if you think the MOD tank is better then let me know before I buy it. should I just DIY my reverb driver and recovery? I know doepfer is single sided PCB.

reading this thread inspires me to mount a recovery circuit to the wood very close to the tank with a mesh or solid shielding over the entire thing. suggestions on specific opamps or circuits would be great.
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Post by Revok » Sat Aug 03, 2019 12:25 pm

Copper shielding is probably going to be a waste of money. The rubber feet are good but are mostly aimed at combo amps where feedback might be an issue. There's no downside to using them so why not. If your case or wherever the tank is being mounting doesn't vibrate from sounds or touch then I wouldn't sweat it though.

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Post by fuzzbass » Mon Aug 05, 2019 6:52 pm

Revok wrote:Copper shielding is probably going to be a waste of money. The rubber feet are good but are mostly aimed at combo amps where feedback might be an issue. There's no downside to using them so why not. If your case or wherever the tank is being mounting doesn't vibrate from sounds or touch then I wouldn't sweat it though.
I typically use the rubber feet, but only because they usually show up for free with a tank. They exist to isolate the springs from loudspeakers in guitar amps.

In my experience, Mod tanks are equivalent, but not superior to Accutronics tanks. I would not be surprised if one factory supplies all the world's reverb coils.
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Post by fuzzbass » Mon Aug 05, 2019 6:57 pm

EATyourGUITAR wrote: suggestions on specific opamps or circuits would be great.
Further reading:
https://sound-au.com/articles/reverb.htm
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