Anachronistic Audio: the Cathedral Radio

In the beginning of 2014 I had the amazing experience of being an Artist-in-Residence at the Autodesk/Instructables Pier 9 workshop. During this time, I embarked on an exploration of audio related projects re-imagined in the style of the Medici workshop of the 16th/17th century. After creating a few instruments and effects, I needed an amplifier.

Obviously speakers and such didn't exist back in that era, so I decided to model the amplifier unit after the mid-20th century cathedral radios- the project goals would be great sound and a stunning visual appearance leaning towards the art deco and art nouveau stylings of these old wooden radios. I also planned on including a 1/4" input as well as have built-in bluetooth connectivity.

As I often do, I first turned to Google image search to start aggregating design ideas for my radio. I accumulated a full folder of design and shape ideas, and starting picking the features that I liked the most.

From the beginning I was sure that I wanted to follow the shape of the Philco 90, a classic cathedral radio, but harness the power of the laser cutter to make some more intricate trim pieces and give it my own styling. Woodworking is also not my strong suit, so I would try to make the actual construction part as simple as possible.

It was also important to me that the radio have some sort of visualization, which ended up being a CRT tube with the deflection coils wired straight into an amplifier- but more on that later.

Plan 1: Use this incredibly awesome green phosphor CRT pulled from a piece of Soviet 1960's equipment. I actually did a full board layout of the necessary power supply and control circuitry, but I felt the tube would make the radio too wide. Also, being an older electrostatic tube meant it required a bulky high voltage transformer which I wasn't too keen on mounting in the radio.

Plan 2: Raspberry Pi running lo-fi visualization engine on an LCD screen, then put some sort of ground glass or bezel in front of it to make it look more analog/older. Meh, this would be the easy way out.

Plan 3: Harvest the CRT and power supply from a portable black and white TV. Now we're talking. I had about 8 of these laying around from a previous project, and by clipping the wires to the horizontal deflection coil and wiring them in to the amplifier I made an incredibly responsive and easy visualizer. Now all I had to do was break the TV down to it's components and fit them somewhere inside the radio shell.

With some ideas in place, I took to CorelDraw to lay out the various pieces of the radio. I used the outline trace feature (basically the same as Live Trace in Illustrator) to trace various parts of the images from the google search to make vector versions of the features I wanted.

I also spent a while on Parts Express picking out the full range speaker and tweeter that I wanted to use, and added the proper cutouts and mounting holes for these.

Finally I cut all of the pieces out of cardboard first to see what it would look like and how it would stack together. The 120 watt Epilog laser cutter can really scream through cardboard and 1/16" plywood, so I did a fair deal of revision and testing before nailing down the design. It basically came down to a backing piece that the visualization screen, speakers, and knobs would mount to; a front trim piece that would cover up the support cross-members and any glue/veneer discrepancies, and various trim pieces for the front that would hide fasteners.

After all of the speaker and amplifier components arrived, I cut a prototype face out of 1/8" plywood to test fit everything. Much smarter to do this now than make mistakes on the fancy wood.

As I mentioned before (as well as in previous instructables), I'm a pretty bad woodworker. However I decided to push myself to make this radio out of nicer materials than the usual stained baltic birch plywood. I headed over to MacBeath Hardwoods to find some nice veneer which I would (hopefully) skillfully apply to the front of the plywood to make the radio look more stylish.

I decided on cherry for most of the trim pieces, sides, and back because I figured it would stain to a nice rich mahogany color. For the front, I bought a gorgeous bookend-cut sheet of Mappa. Being on the lighter side, I thought it would be a nice contract to the trim pieces.

Now, this is slightly insane, but I read about this technique on some woodworking sites online. Basically, you apply a coating of Titebond III and spread it as evenly as possible using a brush / foam brush / roller to both the veneer and the backing board. After it completely dries, you position the veneer on the board and apply heat using an iron which will activate the glue. By ironing the whole piece and smoothing out any ridges, you should end up with a nice smooth veneer application.

Strangely, this actually worked quite well. I found it easiest to first make a huge tic-tac-toe board of glue that avoided the edges of the board and smear that around uniformly with a brush. Next you make a box around the edge and brush outwards in every direction. You want to avoid getting any glue on the front face of the veneer, 'cause it isn't coming off once it get on there. Also the veneer like to curl like crazy so you can either dampen the front face or clamp or weight down the edges as the glue dries.

Contact cement is another way that this is usually done, but it's basically a single shot at getting the alignment right- once the two pieces touch they are never coming apart again.

I clamped the freshly glued and ironed pieces flat against a table until everything was fully cured. The radio required about 4 pieces of the cherry veneered plywood at 36"x24", and a single similarly sized piece of the mappa. The bottom was just standard 1/8" birch plywood.

Well, this was certainly stressful after the hours and hours it took to lay up all of the veneer. After I was sure the design was right by making lots and lots of cardboard and plywood tests, it was time to cut the actual wood.

I made a few test cuts to dial in the laser settings and then went for it all in one go. It was super important to apply masking tape to the pieces as shown in the second picture before lasing them so that the beautiful veneers wouldn't get burned or sappy.

As a non-expert woodworker I try to test anything that I can't CTRL-Z before committing to it, so I tested a few pieces of the veneer to make sure they would glue together properly. This also let me determine how difficult it would be to stain the pieces after they were attached, as glue won't adhere pieces of stained wood.

At this point I was ready to assemble the front panel, but I didn't have a way to mount the CRT. I took some rudimentary measurements of the the tube and CADed up a quick bracket that would hold the tube and give me four mounting holes. Then I cut it out of sheet steel on the waterjet and test fit it to the tube, with passable results.

OK! Now it's time to actually attach the components and fire this baby up. Before I screwed anything down, I used a countersink bit to open up all of the laser cut holes and allow the tapered heads of the machine screws to lay flush against the front panel. This was important so that the trim pieces could be glued on and also sit flush.

When I first fired up the system, the bass response was pathetic. Of course the speaker was running in open air so that was to be expected, but I decided to go ahead and build a box around the full-range driver to create the proper volume of air behind it. As you can see in the third picture from an earlier test-fitting, I also added a port hole of the right size that would live behind the more decorative paisley shaped features.

With a quick zip and a zap, I laser cut a simple box and glued it on behind the main full-range speaker. This also gave me a convenient place to attach the amplifier and crossover, since the floor space was being rapidly consumed by the CRT and associated electronics. I also modified the front panel to include a small port hole that would fit an off-the-shelf plastic fluted port. Basically you buy a 6" tube with a flared front and cut it to the proper size for your speaker/box specifications then press-fit it into a properly sized hole.

After testing again with the speaker boxed in, the sound was awesome.

In addition to the amplifier and crossover, I would need a few other components to bring this radio to life as a freestanding unit. Since the main power input was 110VAC over a standard IEC cable, I would need a 18VDC power supply for the amplifier which could be easily harvested from a PC laptop. Also 5VDC would be necessary to power the bluetooth electronics, as well as 12VDC for the LED lighting (a last minute addition to add a nice warm glow through the front panel openings). Finally, a solid state relay would be used in conjunction with a switched pot to turn the CRT on and off so that is wasn't just sitting there burning in a line on the screen when not in use.

To generate the 5VDC, I used a small off-the-shelf buck regulator- these are like $5 on amazon. Using a multimeter, I turned the tiny trimpot on top until the output voltage was roughly 5V.

The tiny bluetooth board was hooked up to an 1/8" audio jack and attached to the back of the front panel.

To power the amplifier, I used a spare 19VDC Dell laptop power supply that was just attached to the inside of the radio with VHB and hard-wired into the AC power switch.

The last few pictures above show the installation of all of the electronics and CRT. The silver box in the third picture is an RCA-RCA audio hum filter that I installed because of the switching noise from the CRT power supply.

I also installed the potentiometers in the front panel for volume control and intensity control of the visualizations after verifying the correct leads for proper direction with a multimeter.

In the final picture, you can see the deflection coils of the CRT spliced to wires which are hooked up to an output channel of the amplifier. This directly drives the deflection of the electron gun with the magnetic field generated by the voltage of the audio passing through the coils on opposite sides of the monitor. It's kind of like a what an oscilloscope waveform would look like if was retriggering at 60Hz and couldn't lock the display on a signal.

This is a good one- the huge magnet on my unshielded full-range speaker actually caused its' own deflection on the equally unshielded CRT screen. Without any signal, the scan line was pulled drastically up on side!

My original plan of attack here was to install as many neodymium magnets as needed on the opposite side of the CRT to cancel out the fields from the speaker. When the package showed up from Amazon the magnets had somehow escaped though, so I ended up just cutting a piece of steel mesh to attach to the bottom of the speaker box. Having this layer in between the speaker and CRT took care of most of the inadvertent electron attraction.

Now that the guts were all functional, it was time to install the back panel. I used a drill and jig saw to cut the holes in a prefabricated speaker rear panel. Pictured here are the fused IEC jack for power, the 1/4" audio input, and an AC switch to turn the radio on and off.

I also wired in the switch mechanism of the 1/4" jack to act as a bypass of the audio when there was nothing plugged in. If the jack is empty, the bluetooth receiver audio is sent to the amplifier. Otherwise, the audio coming from the 1/4" jack input is used.

I knew that bending the wood for the sides of the radio was going to cause me a huge amount of grief, so I tried to do a few smart things that would make it easier. Unfortunately, some of these things ended up being part of the problem.

First, I laser cut a mockup of the front, back, and bottom of the radio and some wood slats that would help frame out the curve and give me something to attach the plywood to as I progressively bent the plywood. Using the brad nailer and some Titebond I attached these to the notches that I had cut in the front and back of the wood and let it dry/cure. Two pieces of plywood would be attached following the curve of the outside edge of the front and back and would meet in the middle at a not-too-pronounced seam. Then I'd sand it all flat and it would be beautiful. It all seemed like a good idea at the time...

When it came time to work on the actual radio, I dampened up some 1/16" birch ply that was cut to size and attached it to the bottom edge of one side. Working up from here I added glue to the edges of the front and back and across each of the slats and starting bending the plywood.

1. Plywood doesn't like to bend much even when wet, and doesn't like to hold it's form as it is bent.
2. Originally the plan was to use wiggle wood, which would have been much smarter, but the original plan was also to use a single piece of wood to curve over the top and the wiggle wood likely would have snapped at this point.
3. Never underestimate the usefulness of duct tape.

I was also making the mistake of trying to do this late at night by myself, so I don't have a whole lot of pictures of the process. In the first one you can see roughly how I was gluing, bending, stapling, and clamping the side as I bent it up towards the top. What I quickly realized was that there was not nearly enough purchase with the staples on the thin slat pieces, so they wanted to just pull out. Likewise, the clamps held the plywood nicely to the slats, but the tension of the curve caused the slats to pull loose from the radio! ARGHGHGH!

Eventually I started strapping the whole half of the radio down with duct tape after failing to find a ratchet strap or two anywhere in the shop. It seemed to be holding, so I left it and hoped the glue would dry and hold it all together. The next morning, I carefully undid the duct tape and was thrilled to find that it didn't really look bad at all.

At the last minute I decided an inner glow would be nice, so I picked up a few feet of amber LED strip and a small 12VDC power supply and rigged it ever so carefully inside the radio. There was no need to be more careful with this, I just made sure an adequate amount of light shined through the few places in the front that it could escape.

I tied the 12V power supply to the same AC switch that turns on the other 19V power supply for the amplifier so that when you threw the switch, both sound and light came on.

This step was even more painful than attaching the first side due to limited maneuvering space and the pressing desire to not pneumatically fire a staple through any internal wiring.

Again I wet the plywood, and did a combination of gluing, bending, stapling, clamping, and taping until the whole things was closed up.

Then I gave it an inappropriate two-finger salute which was been censored for the more polite of viewers.

As with most of my projects, I brought all of the trim pieces home for my lovely wife to stain. She's much more careful and thorough than I am, and I wanted these to look really good so I waited patiently through multiple days of staining and sanding.

The final stain bill was a combination of a dark red Minwax wood finish and a final coat of Emmet's "Good Stuff". All of the trim pieces were sanded with 220 grit and a finishing sand with 800 grit, blotted with a tack cloth, and coated twice with the stain and once with the wood finish.

For the center trim piece that covered the speaker cabinet, I ordered some twill speaker grille cloth from Parts Express. These guys have about every conceivable pattern to match classic loudspeakers and guitar cabs, so I picked one that seemed like it would match the radio nicely. Using the actual piece as a template, I cut the cloth to size and attached it on the back with hot glue. This piece then also got stained like all of the others with Erin being careful not to get stain on the grille cloth.

The trim pieces were then glued on and clamped to hold them in place while the glue set up. At this point it was finally possible to see what the radio was going to look like in its final state!

I was especially careful not to mar the beautiful finish of the wood with the clamps or extra rogue glue. Also, note that you can't easily glue to a stained surface so the front and back panels are untreated at this point.

For attaching the veneer overlay to the sides, I used the same procedure as before: apply Titebond to both surfaces, brush until coverage is uniform, allow to dry, press pieces together and iron. The veneer was a single piece which I hoped would look better than having two pieces coming to a point.

Unfortunately here's where my "horizontal slats" idea came to bite me. The veneer was so thin that even with the 1/16" plywood in between you could still see distinct horizontal bend lines from the squared off edges of the slats. Doh! I should have rounded them over. Even though I had tried to sand the plywood to a nice smooth curve, the veneer was very unforgiving and the stain only exacerbated this problem. Likewise, the point at the top made a weird lumpy crease in the veneer. If I were to do this again, I'd definitely find a better way to build the frame for the curve, or possibly score lines on the bottom of the plywood to allow a smoother bend.

After the glue was totally dry, the sides were stained in-place.

With the exception of the problem with the smoothness of the side curves, the radio came out beyond my expectations. The speakers sound great and the visualizer is fun to watch (especially while playing music with synthesized sounds). Bluetooth audio and plugging in audio devices both work perfectly.

I haven't yet had the chance to cut the metal bezel for the CRT, but it's on my list to put the final touch on this radio.

If you happen to tour the Pier 9 Workshop in San Francisco, look for this radio in the Artist-in-residence desk area!

Added: Here are the CorelDraw files in case you want to play around with the design. Please use "I made it" and show me some pictures if you try it out!