Reverb Tanks: An Orientation Experiment

[nickb]

Well I got curious. I have a lot of reverb tanks on the shelf and I started to wonder about the whole orientation thing and how much it really matters.

I compared a 8BB2A1A (open side up mounting) while operating both open side down and up. In case you're not familiar, the effect of gravity on the springs moves them off center and this is compensated for by changes at the manufacturing stage to ensure that when mounted as specified, the spring's magnets are centered between the driver laminations.

My first thought was that it was to prevent the magnets from hitting the laminations when driven to max. Well that turned out to be wrong as when driven flat out the springs barely move at all. Thinking about the distribution of flux in the gap I then guessed that it might be a distortion issue. If the magnets see an asymmetrical flux as a consequence of being off-center then you'll get even harmonic distortion.

I drove the line with 6Vpp at 1KHz and measured the (nearly all 2nd ) harmonic distortion in both orientations 0.43% and 0.34% - very repeatably.

So unless there is some other effect I need to consider, I conclude that for all practical purposes the orientation doesn't matter at all.

[waspclothes]

Well I got curious. I have a lot of reverb tanks on the shelf and I started to wonder about the whole orientation thing and how much it really matters.

I compared a 8BB2A1A (open side up mounting) while operating both open side down and up. In case you're not familiar, the effect of gravity on the springs moves them off center and this is compensated for by changes at the manufacturing stage to ensure that when mounted as specified, the spring's magnets are centered between the driver laminations.

My first thought was that it was to prevent the magnets from hitting the laminations when driven to max. Well that turned out to be wrong as when driven flat out the springs barely move at all. Thinking about the distribution of flux in the gap I then guessed that it might be a distortion issue. If the magnets see an asymmetrical flux as a consequence of being off-center then you'll get even harmonic distortion.

I drove the line with 6Vpp at 1KHz and measured the (nearly all 2nd ) harmonic distortion in both orientations 0.43% and 0.34% - very repeatably.

So unless there is some other effect I need to consider, I conclude that for all practical purposes the orientation doesn't matter at all.

Hi Nick, my understanding is that the "orientation" spec for the tank just refers to how the internal section is suspended inside the tank.

The internal section is suspended by 4 springs, one on each corner of the tank. Where these springs attach to the outside of the tank, there are many different holes for different mounting points. The different holes move the tank up/down/left/right to account for sag due to gravity.

Maybe there's something else to it, but I haven't seen it.

[52 Bill]

Maybe there's something else to it, but I haven't seen it.

I'm my experience, I have never had any problems with using a tank that had the "wrong" mounting specs. I'm sure that there is a technical reason, but in practical use I don't think it makes much difference.

Now orientation in the amp with the output side of the tank too close to the power transformer will make a huge difference.

[nickb]

+1 Bill. Now that is the kind or orientation that really makes a difference!

[Enzo]

The physical mounting orientation, as I understand it, is indeed about how the inner assembly hangs. Most of the time it doesn't matter, but under high vibration conditions and such, the innards could wind up whacking itself against the pan walls. So it is more about potential issues than some critical thing.

[Mick Bailey]

I recently had a combo where I thought the tray had been mounted upside-down - open side up - and no cardboard top. With the cables stuffed in the back of the amp the bag sagged against the springs. so I thought I'd mount it open-side down and put a stiff cardboard lid on the tray. Bad move - no reverb. When I pulled the tray out I could see the whole suspended innards flopped downwards just enough to make contact. Never seen this before and mounting position has never been on my list when replacing a reverb tray.

[MarkusBass]

According to Accuctronics the orientation of the reverb tank is coded in the following way:
DIGIT #7 - MOUNTING PLANE
A = Horizontal Open Side Up
B = Horizontal Open Side Down
C = Vertical Connectors Up
D = Vertical Connectors Down
E = On End Input Up
F = On End Output Up

Mark

[Chuck H]

Well I got curious. I have a lot of reverb tanks on the shelf and I started to wonder about the whole orientation thing and how much it really matters.

I compared a 8BB2A1A (open side up mounting) while operating both open side down and up. In case you're not familiar, the effect of gravity on the springs moves them off center and this is compensated for by changes at the manufacturing stage to ensure that when mounted as specified, the spring's magnets are centered between the driver laminations.

My first thought was that it was to prevent the magnets from hitting the laminations when driven to max. Well that turned out to be wrong as when driven flat out the springs barely move at all. Thinking about the distribution of flux in the gap I then guessed that it might be a distortion issue. If the magnets see an asymmetrical flux as a consequence of being off-center then you'll get even harmonic distortion.

I drove the line with 6Vpp at 1KHz and measured the (nearly all 2nd ) harmonic distortion in both orientations 0.43% and 0.34% - very repeatably.

So unless there is some other effect I need to consider, I conclude that for all practical purposes the orientation doesn't matter at all.

Well... A repeatable difference of roughly 25% is considerable though, idn' it?

Did the output remain the same? Was it analogous to the distortion content? I wonder if a 25% difference in output is very noticeable without measuring for it.

What if you REALLY drive the tank, as Accutronics usually recommends. By my numbers you powered the transducers to about .2W. Probably enough (?) but I know the tanks will handle A LOT more.

Great opener though for checking this criteria. It's interesting to me that there's a considerable difference at all. I've tried tanks in incorrect orientation when looking for the quietest mounting location without hearing an immediate difference myself. I must not have "golden ears".

[DRH1958]

It probably is a long term consideration. Just because it doesn't matter right now doesn't mean 10 years down the road it won't. See this and scroll down to "Mounting Considerations".
https://www.tubesandmore.com/tech_co...d_and_compared

[Jazz P Bass]

I had a Crate amp with a reverb issue.
As it turned out, the code letter on the tank indicated Horizontal, Open Side Down (B).
It was mounted Horizontal, Open Side Up (A).

I simply moved the 4 little springs up a notch on the side of the pan .

[TomCarlos]

Interesting thread.... I never really thought about this so it is a new subject. I have two Peavey Keyboard amps and the tanks are mounted vertically, on the side walls of the cabinets. If we consider gravity as being an issue, I would hope the top end springs are much stronger than the bottom side!

[DRH1958]

According to the article link in post #9, the issue is the springs being centered in the air gap of the transducer. If oriented in the wrong position, they will sag away from optimal position. If you really look at how a tank is constructed, adjusting those mounting springs will not change this. It only changes the position of the inner pan that the whole assembly is mounted in relation to the main outer pan. It will not have an effect on the position of the springs in the air gap of the transducer. This is a factory set parameter.

[Chuck H]

I just want to say that I have a hard time believing that the transducers are actually designed for tiny differences in pressure due to how the springs are hanging. How would they do this? Do the rubber dampers have unequal wall thickness or something? This assumes that those dampers are precise and marked for this criteria right down to the different spring weights and that it's all fastidiously observed during construction for each pan type. Really?!? I think it's more likely that all the transducers are made with the same parts and that orientation design has more to do with just keeping the springs from hitting things. As Jazz observed by moving the tray support springs. I might be full of crap since I don't actually know anything about these specifics. That's why it's interesting to me that nickb actually measured any consistent difference at all.

[DRH1958]

I can only go by what is in the article, but somewhat agree with you as to how they can compensate for such tiny differences. Like I said, I think it is a long term consideration. I restore old Hammond organ and PA amps for guitar and have many of the old Gibbs tanks, made before Accutronics even existed and do observe the springs sagging due to hanging there for 50 years. I have many of the same style tank and there are several differences in how much reverb signal is present when changing out the tanks. Whether this is due to old transducers or spring sagging, I don't know. I DO know that there is a lot of difference in reverb signal between tanks of the same design.

[nickb]

Well... A repeatable difference of roughly 25% is considerable though, idn' it?

Did the output remain the same? Was it analogous to the distortion content? I wonder if a 25% difference in output is very noticeable without measuring for it.

What if you REALLY drive the tank, as Accutronics usually recommends. By my numbers you powered the transducers to about .2W. Probably enough (?) but I know the tanks will handle A LOT more.

Great opener though for checking this criteria. It's interesting to me that there's a considerable difference at all. I've tried tanks in incorrect orientation when looking for the quietest mounting location without hearing an immediate difference myself. I must not have "golden ears".

Yes, Chuck you are right in general. In this case 0.35% THD is nothing in a guitar amp. And another 0.1% THD onto that is still nothing. It might even be that the extra 2nd harmonic is appealing but I'll let you test that one and you can tell us what you think. It will be a double blind trial right? Sorry, I couldn't resist, help me!

Really drive the tank? The recommended drive is 6.5mA (rms I think, it's not clear from the datasheet) for 3.5AT for the 150 ohms coil. With 6Vpp applied that comes to 14.1mA rms so I'm driving it almost double. Presumably you hit saturation if you go much higher but I don't have an amp that has low enough THD to test it with right now and my signal generator only goes to 6Vpp.

As Bill said, it's far more important where you locate the output coil relative to noise sources that anything else.

The adjustment in the MOD reverb tanks I have, seems to be done by the orientation of the magnet mounting wire into the plastic end piece. Presumably there is a small offset and by rotating it into each of the four possible positions you set it up for the four orientations.