Trying for uniform volume of Rickenbacher B8 pickup

Mike Bonnice · From: Arizona, USA

I'm trying to equalize the output of all eight strings of my pre-war Rickenbacher B8. The volume of string 1 is low compared to strings 2-4, and even 2 is a little low.

The design of the horseshoe pickup causes the output to be weak on strings 1 and 8. In fact, the center strings (4 and 5) will naturally have the potential for higher output than the others. The reason is that the horseshoe magnets have their strongest field strength at the tips of the horseshoe and their weakest near the bend.

One tip of the horseshoe is north and the other is south. The lines of flux go between the top of the pickup and the bottom. The ideal pickup would have all strings exposed to a constant north field on one side and a constant south field on the other side. However, with a horseshoe magnet, magnetic field needs to switch from north to south at the bend. The field goes through a neutral zone at the extreme end of the bend. The trouble is that the neutral zone begins somewhere before the bend, where the field drops off before it switches from north to south.

Strings 1 and 8 are tucked into the bend of the horseshoe where the neutral zone begins. Hence, they are weaker.

All pickups need pole pieces and magnets. The horseshoe magnet separates these features; in modern pickups the pole pieces are the magnets.

The strength of the signal in a pickup is also related to the distance from the string to the pole piece. To improve the output of string 1, I'd like to adjust the pickup so the pole piece is closer. I've tried to equalize the output by using the height adjustment feature of the Rickenbacher pickup, but the benefit is limited by getting so close that the string hits the pickup, and that fact that all the strings are also getting closer. I can drop the low side and sacrifice string 8 for the sake of string 1, but then string 8 hits the magnets.

In the pre-war B8 pickup, all the pole pieces are the same height. I have a post-war NS 6-string and the pole pieces are taller for the high strings and shorter for the low strings. Evidently, Rickenbacher learned the lesson over time.

Re-charging the magnets does not seem like the path to improving the uniformity of the output across the eight strings, unless it has the ability to cause the neutral zone to be smaller. Otherwise, all the strings get a boost in field strength, but strings 1 and 8 are still in the neutral zone.

I'm considering these two options to get uniform output and would appreciate some discussion:

- File off some material from the pole pieces of the stronger strings. This would allow me to pick string gauges that give good playability (I've adjusted stings and found a range of usable tension from too stiff to too soft), then file the pole height to give uniform output. I would likely file off poles 3-5.

- Magnetize the pole pieces, demagnetize the horseshoes and retain them as decorative and as hand rests. Each pole piece would presumably be the strength it needs to be to match the string gauge and distance from the string. I don't know how to do this except by pressing out the poles, magnetizing them, then pressing them back in. I have no idea if they will be strong enough to give a good signal. Maybe I can replace them with permanent magnets?

Other thoughts?

Tom Pettingill · From: California, USA (deceased)

I'm certainly not an expert on horseshoe pickups, but I'd be very hesitant to do any testing or make any modifications that were non reversible.

Depending on how much you want to get tied up into it, the ultimate option might be to have a knowledgeable winder like Lollar wind and install a new bobbin with staggered height pole pieces. The original bobbin can be boxed up and saved for posterity.

Short of that I'd be tempted to get some tiny neodymium magnets to play with. Maybe something like like 1/8" x 1/32" thick or maybe 1/16" x 1/32" stuck to the top of the pole piece would do the trick. You don't want too much and those little neo magnets are strong.
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Rick Aiello · From: Berryville, VA USA

8 string Ricky's are notorious for having poor string balance ... Especially the ones where the factory crammed in two extra pole pieces into a six string bobbin.

The 7 stringers ... With the mounting screws used as pole pieces ... We're fine.

Anyway ...

Don't try and pull out the pole pieces ... 9 out of 10 times ... You will break the bobbin or short the coil.

Most are very tightly embedded in the Bakelite bobbin ...

They won't stay magnetized anyway ... They are not heat treated steel, certainly not cobalt steel like the horseshoes are.

The simplest way to get even distribution across all strings ... Would be to have Jason Lollar build you a custom bobbin with alnico poles that are oriented to 'jive' with the horseshoes' ...

I've retrofitted horseshoe units with neodymium and custom bobbins ... There isn't enough clearance for that with the original bobbin though ...

A custom Lollar unit would be my first "go to" option ...

And keep the original ... In a ziplock baggy ...

Bill Creller · From: Saginaw, Michigan, USA (deceased)

All this is the reason I had no interest in an 8 string bakelite Very Happy

The sevens work fine for me....

Rick Aiello · From: Berryville, VA USA

Mike ... I just re-read your initial post. I've never seen a Pre-War B8 . Could you post a photo ? I've only seen Pre-War B6, B7 and B10 s ...

If you never had the magnets recharged ... I'd do that before anything ...

Bill ... Yep, The seven stringers have that string right in the middle of the gap and the first and seventh strings are inside the "comfort zone" ...

The ten stringers have extra long magnets ... Big SOBs

I remember Jason L. worked on an Ricky upright bass that had horseshoes the size of toasters ...

Anyway ... Some of the eights are OK ... the mid-late 40's magnets were quite strong and could supply the outer strings with enough juice ... The T-logo and early BD8s ...

Some really suck ... especially the ones made in the later years when they ran out of the good cobalt steel ... and were really weak , even when saturated.

Mike Bonnice · From: Arizona, USA

Here's a photo of the guitar. I can take pictures of details if that would be interesting or helpful. For sure this bobbin was intended for six strings; I can see the windings extending off the end of the bobbin and the pole pieces barely within the extent of the bobbin.

I emailed Jason Lollar and he said he could re-magnetize the magnets and/or make a custom bobbin having adjustable pole pieces.

Does anyone know the actual technology that is used to re-magnetize the magnets?

Rick Aiello · From: Berryville, VA USA

Wow ... What a beauty. Bolt on bridge like my prewar D14.

That's a very rare guitar !

As far as remagging ... An electromagnet that is capable of delivering a Magnetizing force of about 25,000 amp/ turns ... With adjustable pole shoes to accommodate various magnet length is needed to magnetize alnico poles, ferrite poles and cobalt steel.

Neodymium-iron-boron and Samarium-cobalt magnets require much more powerful capacitor discharge chargers.

The horseshoes will be placed individually on the magnetizer's pole shoes ... And zapped for 1 second ... No biggie.

Sounds like Jason will take good care of you ... Good luck !

Ron Whitfield · From: Kaaawa, Hawaii, USA

Now there's something you don't see every day. More pix would be nice.
Thanx for posting. The only way I beat the similar problem you face was to change the tuning.

Mike Bonnice · From: Arizona, USA

I'm curious how re-magnetizing the horseshoes will improve the uniformity of the volume from string to string. If the field strength of the magnets is increased, don't all the strings get a boost? String 1 will get louder, but so will all the other strings, so string 1 will still be the lowest. Anyway, this is my hypothesis.

Regardless, what is most convincing is some data. Is there data that shows an improvement in balance with re-magnetizing? My measurements show changes in volume with string height, string gauge, wound versus plain strings, but I haven't made any measurements that show changes with magnetic field strength (because I haven't been able to vary it).

Rick Aiello · From: Berryville, VA USA

You have to realize how the unit actually functions.

The two horseshoes are set up to repel ... the strongest points on each magnet are at its 4 corners.

A fully magnetized cobalt steel horseshoe measures about 200 Gauss ... midway between the upper and lower flange ... probe at the center.

1 Gauss is 1 line of force per square centimeter.

When the two opposing magnets are brought close to each other in the repelling configuration ... the field is greatly distorted ... instead of the greatest number of lines of force running straight down from point to point ... they are "pushed back" into the interior.

So you have lines of force running straight up from the flanges ... and more lines of force being pushed toward the magnets bend ... via repulsion.

The stronger the magnet ... the greater the repulsion ... the further the lines of force are pushed back toward the bends ... increasing the total number of lines avaliable to the outer poles.

I've had prewars come through here ... depleted to a little as 25 gauss (midgap). Therefore, very little repulsion ... greatly decreasing the interiors flux density.

To be extremely technical ... in any string through that utilizes this "repulsion" method ... IE Rick Horseshoes, Fender Trap/Boxcars, Supro String Thrus ... No two slug pole pieces have the same flux density. They all vary due to their relative distance from the source ...

When the source magnets are at their max ... the differences are just less drastic ...

I've spent hundreds of hours with my gaussmeter ... working on different ways to increase efficiency using NIBS ...

I hope to return to building replacement stuff and new units someday ...

Rick Aiello · From: Berryville, VA USA

PS ... If this stuff interests you.

A Gaussmeter and an electrogagnet are very expensive for the casual investigator ... and having access to many units to study is hard to come by ...

I spent tons of $$$ and remagged 100s of different units for forum folks ...

But you can build models and run simulations with software ...

I use this one ... http://www.vizimag.com/

Mr. Green

Mike Bonnice · From: Arizona, USA

Rick,

This stuff interests me immensely. I made Gaussmeters using Hall effect linear ICs and have mapped the flux on the surface of the magnets. When the magnet is isolated it reads 278 G over string 4 and 108 over string 1. When the pickup is assembled I get readings on the order of 430 Gauss over string 4 and 360 over string 1. Evidently the repulsion and pole pieces increase the flux and reduce the non-uniformity, but it's still not uniform.

I happened to find the Vizimag web site just before I read your most recent reply. I sent an email to the admin asking if he thinks that I have enough info to construct a model, and asking if it would be capable of answering my questions.

The electromagnet is a barrier; I don't know the technology or the cost, although it might be fun to play with. However, I have been playing with using permanent magnets from the hardware store to affect the Rick horseshoes (I first played with the magnets on my NS in case I made a permanent mistake). I think I can magnetize and demagnetize Rick magnets with the permanent magnets, and I can restore a flux value that is at least as high as it was before I altered it.

Rick Aiello · From: Berryville, VA USA

The 200 gauss measurement above is without a bobbin.

When you decrease the air gap distance via pole slugs ... the flux density exponentially increases.

I just went down and measured my '38 A25 6 string.

Pole number one reads 920 gauss ...

Pole number three reads 1250 gauss ...

Those are fully charged magnets ...

It's my best sounding Ricky ... I have better sounding steels though Winking

Anyway ...

I don't recommed using NIBs to remag a Ricky horseshoe.

They are ok for little alnicos and ferrites ... but the large surface area of a Ricky horseshoe .... best to use an electromagnet with the appropriate pole shoes.

I've had alot come through here ... where folks tried to DIY ... and totally "screwed the pouch" ...

The NIBs are so strong ... you'll get some screwy oriented domains ... regions on one flange that are oriented at freaky angles to the norm ... lowering the efficiency.

The magnetizing force must be applied uniformly ... across the entire 1.5" (or 1.25") pole surfaces.

Here is one of my electromagnets ... this one is the easiest to use ...

http://www.magnetsource.com/Solutions_Pages/magnetizrsetc.html

Have fun ...

Mike Neer · From: NJ

I second the suggestion of having a bobbin made for your steel. Listen to Rick. That is a sweet guitar--it deserves it.
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Mike Bonnice · From: Arizona, USA

This just crossed the boundary from hobby to professional! There's no way I'm going to get those kinds of numbers with hardware store magnets, and I don't think I'll take the plunge for the heavy equipment. So, it's time to trust the pros and get a new bobbin and remag.

Mahalo plenny for your extra effort.

Rick Aiello · From: Berryville, VA USA

I just ran down to the basement and got those readings fast (my 2 yr old Max and 1 yr old Luigi can get into alot of mischief in a very short amount of time) ...

So the third pole reading is abit too high ... I just shoved the probe between the string (a big 0.022 plain) and the slug ... so that diminished the air gap.

Air gap distances are huge variables ... lines of force "hate" air ...

But I wanted you to get an rough idea of what a charged unit reads out at.

Things like "flare" ... where some Ricky's flanges are not even close to parallel ... and overall magnet length ... play big roles in variations between units.

My prewar D14 (cast aluminum body with bakelite necks) has very long magnets (almost touching) and are almost perfectly parallel ... some of my B6s have alot of flare and have a big gap between magnets ...

And don't get me started on alloy composition ... some prewars only max out at 150 gauss (midgap/no bobbin) while others can be as high as 230 gauss. Mid to Late 40's 1.25" units are much more uniform (and quite strong).

Anyway ... I've enjoyed this little trip down Memory Lane Laughing

You're in the best hands with Jason Lollar ... I'm sure you'll be very pleased Mr. Green

Bill Creller · From: Saginaw, Michigan, USA (deceased)

I moved a couple pickup pole pieces in one of Bobby Ingano's bakelites at Joliet one year, when he asked me to do it, but I told him how risky it was. It worked OK, but it's a bad idea.

David Matzenik · From: Cairns, on the Coral Sea

Wow! And I thought I was just playing a guitar! Laughing

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Richard Shatz · From: St. Louis

Rick Aiello · From: Berryville, VA USA

Around $500 ...

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