Even further progress

 The two-layer neck guitar is progressing further. Now, it's a very nice, compact and well-sounding instrument. There are still improvements to be made, but now that it's playable, I hope to play more and modify less.

The neck is very flat (as Rick Toone's Exoskeleton neck) without any wood on the back. I might add a piece of wood to make the neck thicker and more familiar, but for the time being, playing the thin neck is fun and works surprisingly well. My thumb still creeps up over the edge of the neck, and that's not the proper way to play it. As long as you play with the "correct" classical grip with your thumb behind the fingers, having a thin neck feels fast and precise.

I've mounted two of my "current transformer" or CT pickups on the guitar. There's not enough room underneath the strings for a traditional pickup. The CT pickups work in the same way as Lace's Alumitone pickup: The part beneath the strings is the primary coil, which is connected to the secondary coil (the current transformer), which is at the side of the main aluminium beam. Magnets are small cylindrical neodymium magnets mounted in hex blind screws. Since there's only one or few primary windings under the strings, such pickups can be made very flat. This in turn allows me to keep the whole instrument rather thin, since I don't need deep pickup cavities.

On my neck pickup, I use a brass coil with only one primary winding (made from a 20 x 3 mm brass bar) connected to a secondary coil by an iron core (the green florist's wire). The secondary coil is home made using a sewing machine bobbin. The bridge pickup has about 15 primary windings running through an AS-104 current transformer. They supplement each other well, the bridge pickup being quite snappy and sharp, while the neck pickup is more deep and soft.
 
Both pickups have low impedance; in the range of microphones, which is why I am using an 500 ohm -> 50.000 ohm line transformer before plugging the guitar into my amp.

Also added is a string retainer at the headstock end, keeping the strings from rattling. A bracket for jack, volume pot and two mini jacks for the pickups makes it easy to plug it into an amp and to switch the pickups. For intonation, I've added a couple of screws to the saddles going into the end piece that holds the tuners. By turning the screws, the saddles are moved back and forth.

I still haven't added piezo pickups, partly because I haven't figured out where to put the pre-amp and controls, partly because the CT pickups function so well.

Next step might be a thicker (6 mm rather than 4 mm) plastic body, this time bent with a little more care. The one I'm using now has been bent into shape using a heat gun and brute force. Another improvement could be a piece of wood on the plastic body that acts as forearm support - the curve of the body makes it hug the torso of the player (which is nice and gives good control of the guitar), but there's nothing on which to rest my arm, and that becomes a bit tiresome.

There's only one aspect that I'm not quite satisfied with: The bow of the neck is not easily adjustable. This is due to the strength of the beams - even without tightening one against the other, the pair of them are enough to counteract the pull of the strings, so there really isn't much to adjust. A solution could be a thinner back beam, but I am not sure they are available in the necessary width.

Further progress on the two-layer neck

Improvements since last post:
Saddles modified -> lower action
Transparent plastic body (styrene-acrylonitrile) added.
Two screws at the bottom side of the head end holding the two layers of the neck together.
Row of neodymium magnets in M4 blind screws for current transformer pickups.

Future improvements:
String guide - pressing down on the strings before the zero fret, since they tend to rattle.
Row of magnets for current transformer pickup in bridge position.
Screw holes making the saddles lengthwise adjustable for intonation.
Piezo pickups and amplifier.
Jacks, switches, pots etc. I plan to put the electronics on the plastic body rather than on the beam itself.

Progress with the two-layer neck

Still much fitting required, but it's playable and tuning works well. Neck tension needs attention; the two beams don't have sufficient contact at the headstock end.

Two-layer aluminium neck

Among the drawbacks of my old T-beam designs were the lack of a truss rod or similar system to control the bow of the neck.

A way to help this - but still use stock aluminium beam for the guitar - could be having a neck in two layers - an upper flat beam, tightening itself by pulling on a similar lower beam.

I hope the picture explains it, but in case it doesn't: The design uses a screw being retained by a bracket attached to the lower side of the top beam. The screw goes into a threaded hole in the end of the lower beam. When tightened, the screw will draw the top beam downwards, counteracting the pull of the strings.

With this build, I'll try if it works. The upper beam is only the length of the neck. It has frets mounted directly into it. The lower beam extends into a narrow body.

On the bridge end, I plan to mount a "stick-and-screw" tuner system. The prototype shown here works decently. I imagine, that using a brass bracket rather than the aluminium one here as well as harder steel for the screw threads will improve that.

The nut end contains a brass bar for holding the string ends, plus - on the bottom side - a bracket for the screw, that tightens the neck against the strings' pull.

Today, I cut the fret slots in a home made miter box and mounted the frets. The glue (two component epoxy) is hardening at the moment.  Tomorrow, I shall see how well it turned out.

Next steps will be making the brass bar and bracket for the nut end plus drilling the screw hole in the lower beam.

The Bobber - the black SG is further reduced

My black SG has had to go through further alterations. I am trying to see, how much you can modify a stock guitar into something ergonomic. Since cutting away is easier than adding on, i've carved a "shark bite" for thigh rest. It allows the guitar to be balanced in a classical playing position with the neck pointing upwards.

On the bass side, I carved a concave area for the strumming/picking hand. It does away with the sharp edge and makes it easier to hold the guitar.

Both were covered in damp-shaped leather, fastened with epoxy glue and copper tacks.

The shark bite made it necessary to relocate the controls, so I added a series/parallel switch mounted in a slanted copper pipe end cap. The jack enclosure is from copper tube and brass plate. The volume and tone knobs are located in what's left of the original control cavity.

The tip of the bass side horn was replaced with a brass one, while the length of the treble side horn was reduced considerably and topped off with a brass cap.

I am going to abandon the new coarse tuner system for now, so next steps on this guitar might be tidying up the headstock brass plate and remount the old high e coarse tuner.