Interested in the Dan Armstrong Plexiglass Bass-style pickup design...

[Chris Turner]

So, I'm playing with the idea of making my own version of this type of pickup, but have some questions about the design. For some reason, I always have a little trouble wrapping my head around the idea of stacked humbuckers, so here goes:



My question is about the magnet polarities. On the bottom coil, you have 1 magnet on either side, and on the top, 1 magnet on one side only. Given that the pickup is sharing the blade (pole), how are these magnets situated?

Are the magnets all set so that the same magnetic pole faces inward (all North or all South facing in), and the coils are wound in opposite directions (or at least the wiring such that the current flows in opposite directions in each coil)?

Any help would be appreciated!



Also, if any of you have any insight as to how the original electronics of the bass worked, that would be great too. As I understand it, the tone control used capacitors to somehow make it so that when the knob was turned all the way down, only the bottom coil was getting any juice. As the knob was turned up, the top coil was mixed in as the bottom coil was mixed out. When the tone knob was dimed, only the top coil was heard.

[Chris Turner]

In looking back at the original page (Dan Armstrong - The Man and his Guitars), I may have wrongly jumped to the conclusion that this is a humbucker at all... color me confused.

[Chris Turner]

No one?

[FunkyKikuchiyo]

Sorry, I just don't know anything about Dan Armstrong pickups. First thing is I'd figure out if that really is a stacked humbucker.... are they noisy? Are the magnets all oriented with the same polarity towards the core?

Every stacked humbucker I've looked at (I admittedly don't look all that closely to them) have opposite wound coils with slugs running between both of them, and the magnets are between the two coils, so one is "charged" north and the other south. This gives humbucking and dual coil characteristics with only one magnetic core.

[David King]

Read through this thread Chris:
http://music-electronics-forum.com/t17585/

I think they aren't stacked HBs but dual-tone single coils.

[Chris Turner]

David: I think you're right about them being just two single coils.

So I guess that would mean that all of the magnets have the same polarity facing inward. In that case, is there any way to avoid hum when both are "on"? It seems like you'd have the choice of either having hum with both on, or having them out of phase with both on.

I think I found some schematics for the original circuit for the bass, too:

[David Schwab]

I agree that it's not a humbucker. It allowed the blending of the tone between the two coils.

To make it a stacked hum canceling pickup, the bottom coil needs opposite magnetic polarity, and then has to be wired out-of-phase. You can do that by either having a bar magnet that runs though both, or with the blade charged like a P-90. I did that with a stacked tele pickup.




BTW, looking at that street address, I used to work for a printing company two streets over on Cantor Ave! I knew Ampeg used to be somewhere in that industrial park. They were gone by the time I worked there.

[Mike Sulzer]

To make it a stacked hum canceling pickup, the bottom coil needs opposite magnetic polarity, and then has to be wired out-of-phase.

Only the second condition is necessary. The cores in the top coil magnetize the strings; the cores in the bottom coil do not do that to any significant degree, and so their magnetic polarity is not a factor.

A Stacked humbucker can be made by winding two coils around the same cores, one below the other, connected out of phase, with the magnet below.

If you are making an active guitar (preamp inside the guitar), a simple old-fashioned stacked humbucker can be a good way to go.

Why?

1. The lower output is not really a problem.

2. You can wind on as much wire as you like without the high inductance moving the resonance too low because the cable capacitance is not a factor.

[David King]

Good points Mike.
The less magnetic field coming up through from the lower coil the louder the output on a stacked HB. You can block that field or divert it or just have a dummy coil with no core.

[Chris Turner]

Good points Mike.
The less magnetic field coming up through from the lower coil the louder the output on a stacked HB. You can block that field or divert it or just have a dummy coil with no core.

So, in essence, if I'd like to keep with the dual-tone single coil design, I'd wanna have it set up similar to the pic above. Stronger magnetic field on the bottom coil, given it's distance from the strings?

Also, in reading the article I found on these pickups (Dan Armstrong - The Man and his Guitars), he used this weird tone setup to get the most that he could out the single pickup design. I'm not sure why he wouldn't have just gone with a 2 volume setup instead.

Anyways, I think I'm gonna give this a shot. I need to pick up some more wire, but as soon as I'm done, I'll post some pics and clips.

[Chris Turner]

Got the bobbin made:

[David King]

If that's copper facing the the coils you will get shorts, etch it off or insulate it with tape, lacquer etc (nothing good can come of copper that close to the coils, between capacitance and eddy currents). Otherwise it's looking great.

[Chris Turner]

If that's copper facing the the coils you will get shorts, etch it off or insulate it with tape, lacquer etc (nothing good can come of copper that close to the coils, between capacitance and eddy currents). Otherwise it's looking great.

The plan was to lacquer the coil-facing sides (couldn't find any one-sided copper-clad board). Will I still run into problems?

I guess I'll find out either way.

The original plan was to go with 1/16" birch ply, as this was also an exercise in trying to use something other than vulcanized fiberboard, but the local hobby store had run out.

[David King]

The 1/16 birch is probably too flexible. FR4 or G10 are going to be much better but harder to cut. Have you considered formica? There's also linen filled phenolic. The thing is forbon is probably going to be cheaper than all of these.

You can try slicing through the copper cladding from the magnets out to an edge to stop eddy currents but you will still have capacitative coupling. Tape is much safer than lacquer, paper tape especially (because you can see it).

[Chris Turner]

The 1/16 birch is probably too flexible. FR4 or G10 are going to be much better but harder to cut. Have you considered formica? There's also linen filled phenolic. The thing is forbon is probably going to be cheaper than all of these.

You can try slicing through the copper cladding from the magnets out to an edge to stop eddy currents but you will still have capacitative coupling. Tape is much safer than lacquer, paper tape especially (because you can see it).

I'll make some cuts in the cladding so that there's no complete circuit. Also, those are just metal slugs, and the magnets will be attached much the same as the original Dan Armstrong pickup.

If I have too much trouble with this one, I'll either just go back to fiberboard or maybe some perfboard.