After dismantling all four brass sleeves, taking the damper discs(52) out and reassembling everything it still works but I learned a lesson...

I thought the dampers were only there to not have near infinite reverb and to give us a choice between short, medium, long reverb.
I also thought the spring tension alone would keep the magnets centered (enough) between the lamination (66).
Not so...

Merely soldering the magnet's end-wire (54) through the holes at the end of the sleeve(46) will make them sag too much, almost touching the lamination.
If I drive the tank hard, the magnets bounce against the lamination.
When going through the center hole in the damper, the end support point is much closer to the magnet, preventing it from sagging through and hitting the lamination.
Dampers in whatever form are necessary...

Last edited: Feb 11, 2020 19:24:54

j_flanders, Nice work! Very interesting. I knew the basic idea of a spring tank, but you've filled in the details.

I got to thinking about those little magnets on the support wires. The ones in my MOD tank are ceramic, probably because it's easier to work with. I wonder whether the magnets in the original reverbs of the early 60s were alnico? If so, I wonder whether that would give a different sound to the tank, as it certainly does with pickups.

If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.

As for my damper story:

Today, I took everything apart again, because it doesn't work without the dampers.

It's hard to tell what the original dampers for this medium decay pan were made of.
It isn't rubber but not really foam either. It's kind of like a mix of the two, softer than rubber but harder than foam.
The patent describes it as 'dead rubber' but I have no idea what that is. Googling only gives me the sports term not the material.

I set out to make some new damper discs.
These things are really small...
Here's a damper disc next to a pick to give an idea:

Without a damper disc, the point where the support wire (and the magnet) vibrates is the hole at the end of the brass sleeve.
With a damper disc this points moves to the damper disc.
You could compare the end of the brass sleeve to a guitar tailpiece and the damper disc as the bridge.

With this reasoning in mind I decided to use a much harder material, but something that is still somewhat damping, hoping for a much longer decay because stock it was really, really short.
I decided to use some fairly hard rubber
I have this box with several kinds of washers/seals in different kinds of material:

With an exacto knife I cut new damper disks from the harder, black, flat rubber ones.

To make the tiny hole in the center of the new rubber disk I wrapped a very thin copper wire around the tip of my soldering iron and with that I burned a clean, tiny hole through the center.

After re-assembling everything, I find it still works...
It now has much longer decay, pretty much on par with my other 4AB long decay pans.

I've played it for an hour or so. I really like it.

All in all it was a lot of fun turning this vintage 4EB2C1B pan into a 4AB3C1B.

Last edited: Feb 11, 2020 19:22:01

Short version:
Before you take those brass sleeves out and apart, mark the top of the magnets (maybe not so easy), or take a photo, or just turn the brass sleeves after assembling until you get a reverb output, decay and tone to your liking.

Long version:
In case someone is trying to replace the dampers here's a very important consideration.

A few days ago when I replaced the dampers I didn't put back those ' end caps' (50) that hold the dampers in place.
Those brass caps are really hard to remove. You can see in the photo below how damaged they are from trying to get them out with various kinds of pliers.
A sort of 'hub puller' would be the proper tool but I don't have one that would work on something so small.
I only wanted to put back those end-caps once I was sure the new dampers were working well.

So today I put (pounded) them back in, reassembled the magnets inside and put the brass sleeves back into the plastic holder/bracket, plugged in my guitar and... very little to pretty much no reverb.

Unfortunately, today I also rewound the input coil (again...) with a different gauge wire, so I'm thinking that's where the problem is.
Unwound it and then rewound it with a wire gauge that has worked before.
Plugged in... still no reverb. WTF

I'm thinking maybe the dampers moved in the sleeves when I pounded in those end caps, and maybe the magnet support wire is somehow stuck and can no longer vibrate freely.
I took all 4 apart (again...) but upon inspection they were fine.
I put everything back together and I do notice a little bit more reverb but it's very dark and has a very short decay.

I always just pushed the brass sleeves back into the plastic holder/bracket. I'm not gonna glue them until I'm sure they don't need to come out again.

While plucking the guitar strings with my left hand I started to wiggle the brass sleeves and did notice some improvement.
Then I tried rotating them, as if they were knobs, and wow, it goes from pretty much no reverb to holy-moly-that's-a-lot-of-reverb.

I must have been very lucky a few days ago when I reassembled everything for the first time and got reverb.

So, there seems to be a top and bottom side (or maybe it's left and right) to these cylindrical magnets, making them only function properly when the top and bottom of the magnets are facing the laminations above and below the magnets.

[EDIT:
After checking with a compass, the north and south poles of the magnets are not facing the laminations above and below the magnets, but the poles are facing sideways.
Between the two magnets, the north pole of one magnet is facing the south pole of the other.

END EDIT]

The interesting thing is that, depending how you position the bottom/top of the magnets towards the lamination, it gives a lot of different kinds of reverb tone: from no reverb, to dark and short decay, to medium, to loud and bright and long decay.

Suddenly I also understand this part of the patent, which until now I didn't know what he was talking about:
The last sentence describes the issue I was having (without realizing it)

Now imagine this:
If the hooks on the springs aren't perfectly in line with the hooks on the magnets, the springs will force/rotate the magnets slightly (or more) out of their ideal position.

The hooks on those springs are clearly made by hand. In some pans (my TAD pans for example) they're really sloppy.
This would mean that it's almost inevitable to have a lot of variation between otherwise seemingly identical pans.

That Chinese (or Korean) worker has 4 chances per pan (2 end hooks on 2 springs) to mess up. He may not even realize how important this little aspect is.

Last edited: Feb 15, 2020 14:49:00

j_flanders wrote:

ldk wrote:

If so, I wonder whether that would give a different sound to the tank, as it certainly does with pickups.

The magnets magnetize the lamination, and it's the lamination that is inside the coil, not the magnets.
Following your reasoning you'd experiment by changing the material for the lamination, not the material of the magnets.

Given that, do you think a neo-mag of an appropriate size work in a spring reverb? I botched my attempted resurrection of a cardboard boxed Gibbs 2-spring reverb unit when one of the magnets exploded as I was trying to use heat to remove the brass tube from the spring. I've found some diametrically magnetized N50 neo-tube magnets with the right ID, though a bit smaller in OD and length. Realtively cheap, so I think I'll try them anyway, but I thought I'd ask opinions.