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Thread: First Rebuild- Electromuse Eye-Beam (for lap steel)

  1. #1
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    First Rebuild- Electromuse Eye-Beam (for lap steel)

    The Electromuse Eye-Beam: My Very First Pickup Rebuild
    by Ralphie B., age 55-1/2

    Introduction
    This pickup was installed in an unbadged lap steel, ca 1940s-1950s. Depending on your attitude, it may be either the crudest or the most elegant design possible- a chunk of magnet wrapped in wire.

    The pickup looks like a double-rail humbucker- but it isn't. The magnet is alnico, cast into an I-Beam shape about 1" wide; its jagged ends infer the piece was snapped from a longer bar. The i-beam is magnetized "across the bar" (one flange is North, the other South) and the coil is wound "along the bar", between the flanges. (I believe some would call this orientation "wrong", with most of the magnetic flux travelling parallel to the strings.)

    The guitar appears to share all electronics and hardware with the Electromuse "canoe paddle"- the only difference being body wood, shape, and finish (pine with straight sides and hallucinogenic blue paint job vs. mahogany paddle with natural finish).

    Original Construction
    Unfortunately, I neglected to take "before" photos of the pickup and lap steel.
    However, some discussion and description can be found here http://music-electronics-forum.com/t25965/
    and this site has photos of an Electromuse "canoe paddle" autopsy Electromuse1p8

    Condition as Found
    The pickup appears to have been "potted" in beer, and the magnet had rusted severely, breaking inner coil wraps. As originally assembled, I can't see how the pickup could produce a healthy signal. The magnet was loosely wound with a fairly small cluster of gossamer wire, centered between the flanges to resemble some kind of cocoon.

    Rebuild Philosophy
    I'm not sure if this project should properly be called a rebuild, a refurb, or a rehab. Rather than restoring the piece to original condition, my goal was to assemble a reasonably robust, quiet (noise-wise) pickup with output compatible with modern amplifiers, using on-hand and readily obtainable materials. I also sought to retain as many of the original electronic components as possible (21K volume and 140K tone Allen-Bradley potentiometers; tone capacitor- labeled .05uF but reads .075uF).

    Some Differences from Original Design
    1) Insulation
    The Eye-Beam used heavy 2" wide cloth tape to insulate the coil from the bare, jagged-edged magnet and to cover the coil.
    Using a rotary tool, I ground the magnet ends smooth. Since the pickup was killed by rust, I thought it prudent to spray the magnet with Rustoleum appliance enamel. Wanting maximum possible space available for the coil, I used a combination of clear plastic tapes for electrical insulation and thin 1" wide cloth "drugstore tape" for for coil wire padding and protection.

    2) Grounding, Shielding, Lead Wires
    The magnet was not originally grounded, nor the coil shielded. I grounded the magnet and shielded the pickup with copper foil. I also foil-shielded the control cavity.
    The Eye-Beam originally used a bare lead wire for Start/Ground and a blue lead wire for Finish/Signal. I used a bare lead for magnet/shield ground, white lead for Start/Signal+, and black lead for Finish/Signal-.

    3) Strain Relief
    As originally built, the Start lead wire was spliced to the coil wire at the middle of one end of the magnet; the coil was then loosely wound over the splice. The Finish lead was soldered to the coil wire with no strain relief. I thought this configuration seemed prone to breakage, and sought to anchor the leads in a less vulnerable fashion.

    4) Coil Wire Gauge and Turns
    As found, this Eye-Beam was wound with few turns of thin wire. I aimed to wind within the range 2300-3200 turns of 38 AWG, for a DCR of 600-822 ohms. If anyone is interested, I'll post an explanation of the calculations and hunches that led to this decision.

    5) "Super-Charging"
    Expecting a fairly weak output from this pickup, I planned on experimenting with hanging small neodymium magnets onto the alnico magnet's flanges to increase output. As it turned out, the pickup's output was more than strong enough for my needs.

    6) Circuit Wiring
    I replaced the ratty "once white" 12-1/2 ft pigtail-to-plug with a 1/4" phone jack. This was a difficult decision, because: a) I had to chisel petrified pine to fit the jack; and b) I thought a new pigtail in blue plaid braid would look too kewel. However, practicality (almost) won out. Thinking the pickup's impedance might be low enough to directly drive a mixer's balanced line-in (typically ~10Kohm input), I used a TRS jack wired T=Sig+, R=Sig-, S=Shield. When a standard "mono" instrument cable is plugged in, R is shunted to S and it ends up being "normal" wiring.

    -----------------------------

    THE REBUILD STORY, as told in pictures, to follow.

    -rb

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    Last edited by rjb; 09-28-2011 at 05:51 AM. Reason: "Technical difficulties" Added "potted in beer" crack.

  2. #2
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    The Rebuild Story, Part 1

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    1 The magnet originally had jagged ends and severe surface rust. Here it is after a session with Mr. Rotary Tool. I then sprayed it with black Rustoleum appliance enamel.


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    2 Magnet end view. The I-beam is magnetized across the bar; one flange is North, the other South. I think some would call this orientation wrong, with magnetic flux flowing mostly parallel to the strings.



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    3 Magnet was charged with this torture device. Stacks of 3/8D x 1/16 neo magnets on the jaws are contained by holders made from tomato cans.



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    4 Bare and white lead wires anchored to magnet; magnet covered with clear plastic and white cloth tapes; coil Start soldered to white lead. Bare lead contacts bare magnet, coated with Ox-Gard; white lead is insulated from magnet by clear tape.



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    5 Grammar school science project, with no help from parents. Wary of spinning a huge hunk of metal, attached by carpet tape to a sewing machine or hand drill, I built this contraption from scrap and surplus. Coil DCR initially read 612 ohms, but dropped as low as 598 ohms over a few days. (Resistance drop as wire spread out and "settled in"? Meter inaccuracy?)


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    6 Coil winder construction detail: Faceplate magnet holders were molded from Sculpey.

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    Last edited by rjb; 09-28-2011 at 07:34 PM. Reason: Note Re: DCR

  3. #3
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    The Rebuild Story, Part 2

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    7 Pickup temporarily taped for electrical and sound tests; I might decide to add more winds. Note the scraps of wire insulation used as wedges- this was the original mounting method.



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    8 New harness with original pots (21K volume, 140K tone) and tone cap (marked .05uF, measured .075uF). Also: Ratty pigtail replaced with ” TRS phone jack (so can plug directly into mixer line-in), control cavity shielded with copper foil.



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    9 As a result of tests, original tone cap was replaced with 2 low-reading .022uF caps in parallel for .040uF. With coil inductance of ~190mH, this results in Fres with the tone pot at 0 of 1.8KHz (B.Brozman’s "magic frequency for Nationals").



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    10 Super-charge neo rocket-booster power-packs. Expecting weak output from this pickup, I had made these experimental "magnet boosters" to attach to the alnico I-beam’s flanges.



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    11 Boosters installed (replacing original wedges).



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    12 Houston, we have a problem. The boosters didn't quite fit under the cover, and raised the pickup too close to the strings. I do think they might've worked with 1/4"D (vs 3/8"D) neos, but it's a moot point- the output is healthy as is.

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    Last edited by rjb; 09-28-2011 at 07:37 PM.

  4. #4
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    The Rebuild Story, Part 3 (Finale)

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    13 Black "Finish" lead soldered to coil with "service loop" strain relief.



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    14 Wrapped in cloth tape and potted in wax.



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    15 Copper foil shield is soldered to bare lead. Insulating tape prevents dreaded closed-loop excess eddy currents.



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    16 Wrapped in sexy black gaffers tape (the closest I had to the original "striped" black tape).



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    17 Beauty is in the eyes of the beholder. Gorgeous, ain't it?


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    18 Installed without guard


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    19 With guard. Note: Ferrous guard is a part of the magnetic circuit; it increases pickup's output and sustain.



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    20 Rehabilitated lap steel chillin' on the back porch.

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  5. #5
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    Impressions/Observations/Evaluation

    I tested the Electromuse with a Fender Champion 30 DSP amplifier:

    - Noise-wise, I think this is the quietest single-coil guitar I’ve ever plugged into an amp.

    - The pickup produces a clear, solid, articulate, signal that is strong enough for my taste.

    - This may have as much to do with the instrument as the pickup, but notes seem to sustain forever. When you pluck a string, you hear a normal attack and sudden volume drop- then the volume remains constant for a long time.

    - Installing the (ferrous) handguard increases output and sustain. I think this makes sense- the handguard “pulls” magnetic flux upward, perpendicular to the strings, thereby increasing output; magnetic forces from above and below the strings balance each other, increasing sustain. I’m sure some of you folks can explain this more authoritively.

    - Installing the handguard also increases microphonics. Tapping on the bare pickup cover produces an almost inaudible “tick”; tapping on the handguard produces a distinct “thud”. I tried a layer of felt between the control plate and handguard, and observed no decrease in microphonics. Solution: Don’t tap on the handguard.

    - I experimentally determined the coil’s inductance at about 190mH. In an article on EQ Bob Brozman LIVE Sound Hints, Bob Brozman calls 1.8KHz “the magic frequency for Nationals”. To get a resonant frequency of 1.8KHz, I replaced the tone capacitor with two “low” .022uFs in parallel for .04uF. With the tone pot set at 0, the pickup absolutely nails the funky “Yowza” tone.

    - Raising the tone pot reveals a somewhat clang-y, “Dobro-ish” tone that Electromuse lap steels are noted for.

    - I don’t currently own a functioning full-range PA. To get an idea what the instrument might sound like if plugged directly into a Line-In, I tacked a 10Kohm resistor across the phone jack. With the tone pot up, much of the “clang” was filtered out, resulting in a cleaner, smoother tone. With the tone pot at 0, the “Yowza” was still present, albeit with a bit less “Yow”.

    - I programmed an Excel spreadsheet to model the frequency response of a single-pickup guitar with standard wiring.
    GuitarBode(Standard Wiring).xls
    Using the values for this pickup and harness, the Bode plots agree with the “ear tests”:

    To Guitar Amp (1Mohm input, 500pF cable)
    Tone=10 +.75dB @12KHz -3dB @20KHz xxx Essentially Flat
    Tone=0 +8.5dB @1.8KHz -3dB @2.8KHz -6dB @3.2KHz Big Yowza

    To Line-In (10Kohm, 1000pF cable)
    Tone=10 No “hump” -3dB @6.6KHz -6dB @12KHz Buzz Filter
    Tone=0 +5.3dB @1.8KHz -3dB @2.8KHz -6dB @3.2KHz Medium Yowza

    Conclusion
    I am pleased and delighted with my newly refurbished pickup and lap steel.
    I bet that in no time, I’ll have worked up a killer rendition of Sleepwalk.


    -rb

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  6. #6
    Pickup Maker David Schwab's Avatar
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    Nice work, and very interesting, as I doubt many have seen such a pickup before.

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    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein


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