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Building the Main Triangle
The tubing was ordered from Henry James, and arrived in about 5 days.
I used an art matte board to make the drawing, working out the head angle, seat angle, and bottom bracket height ( or "drop" of 7.5mm from the wheel axle height as it's measured on the drawing). For angles (73 for head, 72 for seat), I used an office supply store protractor. The board is 40" wide, so the wheel centers went off the drawing a bit since the wheelbase ended up at 41 1/4". I carefully took into account the fork length I wanted (long reach brakes at full pad extension). You can see a line that represents the front brake bolt, and another above it for the bottom of the head tube. They take into account the fork crown measurements and headset stack height. I didn't do any trail measurements, since I know that a rake of 40mm or so would work out fine here.  The slope of the top tube was however low I could get it using my 320mm seatpost at the max height mark. Top tube length was measured from center to center, as if the tube was parallel to the ground, like on a classic bike, and was 23". When I finally put the tube in, I simply cut it to fit the space provided.  Seat stays are not even drawn in, since they will simply be cut to fit. The most important cuts and brazing would be undertaken first. The headtube/ downtube connection was critical for the proper head angle and bottom bracket height. Any mistake here would affect everything downstream.
For initial cutting of the tubes, I used aviation tin snips, which gave me a rough "fishmouth" shape. I refined it further with the grinder, and then the half round file. After some filing I would put the headtube against it and look for gaps with some backlighting. Then I would check the angle against the drawing. My goal, which I didn't always achieve, was to have not even a thin crescent of light visible in the gap.
Here is what the downtube looked like, after I finished mitering it to fit against the headtube. It took almost an hour of work before I was satisfied. Some later miters that I did would take 10 or 15 minutes.
In the photo to the right, I'm holding the downtube/headtube connection together and comparing it to the drawing. It had to be as accurate as possible. This is the most important joint of the whole frame. If this is done wrong, everything else downstream will be off base too. This is the one joint I don't want to have to cold-set. Trying to straighten it could ovalize the headtube.
Look carefully; you can see the circle that represents the bottom bracket, and the faint lines of the seat and head tube. I mitered the downtube so that it matched the drawing exactly, when fitted over it.
Here, the headtube is resting against the downtube, with another tube inside it, weighing it down and balancing it firmly against the downtube. I moved the rather heavy balancing tube back and forth until the weight was centered and it was stable. Having extra tubes of various sizes laying around the shop was very helpful at times like this.
I put a spot of brass on each side of the joint first. (It's better, however, to spot the top and bottom first, since the tube can twist a bit while the first spot cools when done as in the above picture.) This held it securely enough for me to be able to remove the balancing tube. Next, I checked the tube angles against the drawing-  it was spot-on. Then I clamped it by the headtube so that the down tube was vertical, with the non-brazed end upward. This would put less stress on this tenuous connection while I heated the joint up again. Then I went around the joint, "tinning" it.... putting a thin layer of brass down, making sure it flowed into the crotch of the joint.
Brass fillets: Testing
Initially, I made many joints, improving as I went along. Each joint I would test, by bending it to failure. In all my tests, the joint deformed, but held without peeling away, and the thinner  walled tube bent close to the joint. This told me that the joint was good, and that I must be on the right track.
 
Fillet size:  It seems to me that the size of the fillet does not need to be very  big. If the tubes have 1mm wall thickness, and the root depth (surface of the middle of the fillet to crotch) is 3 or 4mm, that should be more than enough. The brass is about 65 psi strength, and the tubing is 110 psi strength (for this nonhardended tubing).
 
 I built this frame with "low fuming bronze" rod (1/16") and Welco #17 flux which I got from my local welding supply shop. Now I use Henry James Gasflux rod and paste flux. It performs noticeably better, and my fillets improved since switching.
Here is a joint (of the handlebar stem) that I think came out okay. Little pits are visible, where flux and impurities probably  bubbled up to the surface. Some ripples are visible. My control of the flame is adequate, but far from exemplary.
 One way to improve the joint would be to use the Gasfluxer. It puts flux in the flame itself. I can't justify the expense though, as I build only one or 2 frames a year .
Finishing the joint with a Dynafile would be ideal. These costly tools have a narrow sanding belt that whirs along a tip, kind of like a chainsaw. They eat up the brass, but are much easier on the steel, so undercutting the joint is minimized. With this tool and lots of practice, one can create very  beautiful fillets.
 
I wanted to make a lug for the seattube, where the top tube attaches. This would provide extra beef for the seatpost clamp, and at the seatstay junction too. I used a piece of 1 1/4" .058 tubing, which I purchased from Aircraft Spruce awhile back (for my folding bike project). It slipped over the tube at the butt; I had the tube extending out of it a few millimeters. The seat tube was a 1 1/8 AVR (mt. bike type). When reamed, it would take a 28.6mm post (common mt. bike size).  I cut it so it would have a front and rear tang that would point down the tube.
Here it is ready to braze. I will use brass, since I intend to use brass for the top tube to seat tube fillet, and the seat stay connection. If I used silver, it would all melt away during the brass brazing.
All brazed! I'm not sure how well the brass flowed inside. It was difficult to carry it thru to the top, so I added some brass from the top. The fit probably wasn't sloppy enough for brass brazing; silver would have flowed better. Next time I will use silver, since brass brazing over a silver brazed sleeve like this will work fine.
 On the right photo, I cut excess tube off the bottom bracket end, and mitered it to fit the bottom bracket.  I didn't want to shorten the tube by cutting the butted end, since that was where the seatpost would fit (it would be loose in the thinner walled section of the tube, lower down). I needed to check it so that it was absolutely square- 90 degrees angle. Using a #0 torch nozzle I tacked it, I put dabs of brass at the "lips" of the fishmouth first. Then, I tinned lightly all around, making sure brass flowed into the crotch of the joint. Then I built brass up along either side of the bottom bracket, but not where the chain stays or the down tube would connect, since the tubes were not attached there yet.
How I learned to miter: It all comes down to practice. Using the aviation tin snips saved time at the grinder. I had 3 different sized half round files. I went back and forth; filing and holding the tubes together, looking for gaps, and trying to eliminate them... all while fitting the tube at the proper angle. When the opposite miter was finished and brazed, I had the added requirement of getting the tube length right, and making the miter perfectly parallel (or perpendicular, as in the bottom bracket to downtube miter), in addition to all the rest of it.
I cut my hand badly on the sharp ends once; now I wear gloves. As you angle the file, the shape of the curve narrows. File strokes are supposed to be one way (away from you). Again, practice is the key for good miters
This is the miter of the down tube, where it meets the bottom bracket and seattube area. It is a very important and time consuming joint. First I had to miter it against a spare lugless bottom bracket that I had (the other one was already brazed to the downtube). It had to be square with the bottom bracket, and at just the right length. I compared it to the drawing frequently.
 After I was happy with the length and bottom bracket fit, I had to cut another miter in it for the seattube. I would file some, and then lay both tube assemblies against the drawing (with some spacers underneath the smaller tubes). When I saw no gaps, I cleaned it with sandpaper and set it up to braze.
There is a lot going on in this photo. The seat tube with its bottom bracket attached, is balanced against the downtube. One heavy tube above it provides a counterbalance A smaller tube inside the bottom bracket aids with balance, and provides a way to check for square. Below is another image of the same setup.
Make sure that the left-hand threads of the bottom bracket will be on the right! I had to learn to be careful the hard way, a few frames back... The F you see on the tube is my note for "fixed cup".
I measured the distance from the top of the head tube to the top of the seat tube. It had to match the drawing exactly. That is pretty much where it was when I did this setup, because of my careful miters, but I had to adjust it a bit before brazing. I anticipated a some cold setting afterwards. Also, I got a visual line up of the head tube and the seat tube. I had to climb up on a ladder to look down and make sure they were lined up and parallel. I gently pushed the tube in position.
When I was finished fillet brazing, I did have to cold set the seat tube to come into parallel with the head tube. They were just a bit off when I sighted them with one eye closed. I clamped the lower part of the seat tube (close to the bottom bracket) in my bike stand, slipped a spare tube over the head tube, and pulled hard. It took many pulls to get it right, and a lot of force. These tubes are really strong. They are the modern day equivalent of Columbus SL.
 Afterwards, I had to pull the seat tube away from the head tube about 1/8 inch. This measurement needed to be accurate, as it would reflect frame angles. This pull was pretty easy, since I already had plenty of leverage due to the position of the tubes, and I didn't need to use any "cheater bars".
Now I needed to fit the top tube in the space provided. First, I cut the top tube on both ends with the sawzall, so that the butted ends were about equal. Then I mitered the end that fits against the seat tube lug. This took about 10 minutes. I had spare 1 1/4 tubing to check it with (that's what I made the lug out of). As I filed away, I would check the angle against the seat tube here.
Mitering the front part of the top  tube against the head tube took much more time. I had to get the length of the tube just right, as well as the miter. If I cut too much off, I could only start again with a new tube. So I was very careful, and when things were getting close, I would file only a few strokes before I checked the fit again. Notice how I have it against the drawing here.
When I began brazing, I fluxed everything well, and tacked both ends of the tube first with a good sized dab of brass. That would keep the tube from moving as I started to lay the fillets in.
Looks like this came down strapped to a meteor. When I put brass down here, much more heat was required against the lug than the top tube, since the lug is so much thicker. I pretty much put all the heat on just the lug; enough of it went to the tube to make them equally the right shade of red.
I wanted to copy a seat stay arrangement I had on my Ritchey tandem.  Below, I am preparing a cross tube to be brazed onto the seat tube lug. It will clamp the post, and accept the seatstays.
This is the 1/2" .058 cromoly cross tube brazed against the seat lug,  and you are looking down inside the seat tube. I was generous with the brass, which holds the cross tube in the lower part of the photo. The seat stays would attach to the cross tube, and a seat binder bolt would go through it.
There is no way to get around this next step. The seat tube must be reamed with an adjustable reamer. Distortion has made it impossible to put the seapost in without using a hammer.
The seat post needs to fit in easily, but with no play. I used plenty of cutting oil, and turned the adjusting nuts of the reamer about 1/8 turn for each new cut. When I  was getting close, I would always check the fit of the intended seatpost after each cut. Reamers cost about $50, and your local bike shop can probably order you the appropriate size.
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