Odd old Thunderbird pickup

[marku52]

Anybody seen this before? It supposedly came out of a 70's Thunderbird bass. It's not the usual sidewinder, kind of a perverted P bass style. Anyway, here is some analysis I did for its owner.

An unusual pickup design—two coils, each serving two strings, like a P bass. But each coil is on the opposite pole of a central magnet shared by both coils. And each coil has an secondary magnet mounted to the outside of the coil, which energizes one of the two pole pieces that each coil has. So, a total of three magnets, four pole pieces, and 2 coils.

The two coils should be wired out of phase to cancel induced mag field noise, and since they are on opposite poles of the magnet, the string signals will still be in phase, like in all humbuckers.

In pix1, we see the central magnet, where the yellow paint is indicating the N pole of the magnet. The 2 pieces of copper foil are to ground the steel pole pieces, so they will not pick up noise. The foil wrap touches the steel at the interface between the central magnet and the pole piece . One coil was where all the damage is, and the other is at the other side of the magnet where you see the 2 steel pole pieces.

In pix2 you can see the secondary magnet, its purpose is to magnetically energize the lower of the two pole pieces (it isn't touching the central magnet, so it isn't energized by it—hence the need for another magnet). Why not a single steel pole piece? I suspect it is an attempt to reduce eddy current losses in the pole piece, tho I doubt it is necessary, and a single pole piece would make the pickup much easier to build.

In coil1 you can see the 2 pole pieces, the secondary magnet, and the copper coil wires.

Specs:

Central magnet: Ferrite, .2” X .55” X 2.35” ~140mT pole strength
Secondary magnet: Ferrite, .2” X .2” X 1.2” (about half as long 'cause it only covers one coil) ~110mT pole strength.
Pole pieces: cold rolled steel, .062” X .7” X 1.135”
Bobbin: molded plastic, .385” tall, .56” wide, length over all 1.5”. Winding area .3” tall,.285” wide, 1.26” long. Pole slot 1.13” long X .185” wide.
Coil: wound with AWG 44 solderable. Intact coil measures 5.50Kohm, L 2.628H, Q 2.641 at 1kHz. (this is with the iron inside, raw coil would be lower L and Q). I estimate between 4500 and 6500Turns. You'd have to wind one and sneak up on the final value.

Probably a pretty powerful and yet bright pickup. The overall R would be about 10K (2 coils) and about 5.2H. Not much losses due to the 2 pole pieces and the ferrite magnets (much less lossy than alnico)
Maybe not very warm sounding, ferrites tend not to sound that way.

[David Schwab]

That's a newer Thunderbird Plus pickup, not an old one. They didn't use the plastic covers in the 70s.

Here's a set that someone chipped off the plastic covers and installed in chrome covers:

[marku52]

That's it all right. What years did they use them? Do you know what they sounded like?

[big_teee]

Anybody seen this before? It supposedly came out of a 70's Thunderbird bass. It's not the usual sidewinder, kind of a perverted P bass style. Anyway, here is some analysis I did for its owner.

An unusual pickup design—two coils, each serving two strings, like a P bass. But each coil is on the opposite pole of a central magnet shared by both coils. And each coil has an secondary magnet mounted to the outside of the coil, which energizes one of the two pole pieces that each coil has. So, a total of three magnets, four pole pieces, and 2 coils.

The two coils should be wired out of phase to cancel induced mag field noise, and since they are on opposite poles of the magnet, the string signals will still be in phase, like in all humbuckers.

In pix1, we see the central magnet, where the yellow paint is indicating the N pole of the magnet. The 2 pieces of copper foil are to ground the steel pole pieces, so they will not pick up noise. The foil wrap touches the steel at the interface between the central magnet and the pole piece . One coil was where all the damage is, and the other is at the other side of the magnet where you see the 2 steel pole pieces.

In pix2 you can see the secondary magnet, its purpose is to magnetically energize the lower of the two pole pieces (it isn't touching the central magnet, so it isn't energized by it—hence the need for another magnet). Why not a single steel pole piece? I suspect it is an attempt to reduce eddy current losses in the pole piece, tho I doubt it is necessary, and a single pole piece would make the pickup much easier to build.

In coil1 you can see the 2 pole pieces, the secondary magnet, and the copper coil wires.

Specs:

Central magnet: Ferrite, .2” X .55” X 2.35” ~140mT pole strength
Secondary magnet: Ferrite, .2” X .2” X 1.2” (about half as long 'cause it only covers one coil) ~110mT pole strength.
Pole pieces: cold rolled steel, .062” X .7” X 1.135”
Bobbin: molded plastic, .385” tall, .56” wide, length over all 1.5”. Winding area .3” tall,.285” wide, 1.26” long. Pole slot 1.13” long X .185” wide.
Coil: wound with AWG 44 solderable. Intact coil measures 5.50Kohm, L 2.628H, Q 2.641 at 1kHz. (this is with the iron inside, raw coil would be lower L and Q). I estimate between 4500 and 6500Turns. You'd have to wind one and sneak up on the final value.

Probably a pretty powerful and yet bright pickup. The overall R would be about 10K (2 coils) and about 5.2H. Not much losses due to the 2 pole pieces and the ferrite magnets (much less lossy than alnico)
Maybe not very warm sounding, ferrites tend not to sound that way.

I notice a gap between the dual blades.
I wonder what the purpose is for that, and what material is between the blades to keep them spaced apart?
Kind looks like a blade P-Bass Pickup under a cover?
T

[marku52]

I think they were trying to reduce eddy current losses in the pole pieces. The two poles are separated by the potting epoxy. But the separation means they have to have that secondary side magnet to energize that outside pole. Complicated, probably not really necessary.

Has anybody heard one of these? Are they any good?

[David King]

Like a P bass pickup except it looks like maybe the coils are full width (both coils go under all four strings)?? That makes no sense either.

[marku52]

no, look at my photos, they are half coils, 2 strings each. the center magnet goes full length tho.

[David King]

Got it! Thanks.

[David Schwab]

That's it all right. What years did they use them? Do you know what they sounded like?

I think they started in one form or another in the late 80s. They still put the ceramic pickups in he current basses. Some people like them, some don't. The one I tried out sounding kind of dull.

[big_teee]

I think they started in one form or another in the late 80s. They still put the ceramic pickups in he current basses. Some people like them, some don't. The one I tried out sounding kind of dull.

You would think with all the ceramic magnets and being two independant pickups wired in series?
That they would be real bright.
Or maybe they did the Overwound, too much wire thing.

[David Schwab]

You would think with all the ceramic magnets and being two independant pickups wired in series?
That they would be real bright.
Or maybe they did the Overwound, too much wire thing.

I think they are over wound. I have not heard any new Gibson bass pickups that sound good to me.

I had one customer who was recording an album with his band. He had a new Gibson Thunderbird and replaced the pickups with some of my Sidewinders, which are more like the Bicentennial Tbird pickups. He sent me before and after on a studio track, and the new pickups were nice and bright, and you could hear what he was playing. The old pickups are too dark. The Epi Tbirds are the same way.