Building The TNT

Once I had all the parts for the TNT I was anxious to start building it! I decided I to skip the prototype and just build the final version. I figured the worst that could happen is the rig would not work for some reason, requiring a major rebuild or layout change. In that case I would only be out my $17 board and some hours of labor.  I was willing to take that risk.

After sliding parts to and fro on my kitchen table for a while, I settled on a layout. This wasn’t going to be a perfect reproduction of the original or any other TNT. It was to be my version. I started drilling holes and mounting components. This was fun and oh so exciting! A vintage 1929 rig was beginning to take shape!

Some components needed to be mounted on brass angles. Having been unable to find pre-fabricated brass angles that I considered suitable, I had opted to buy 1/2″ wide by 1/16″ thick brass strip stock and make my own. I was quite concerned about bending the stock nicely, as this wasn’t my first attempt at metal bending. All the others had gone very poorly. I destroyed a number of brass strips and spent nearly three days coming up with a technique that, while somewhat precarious and prone to mishaps, was capable of yielding “good enough” bends.

The method involved clamping the strip to be bent between two thick metal blocks and bending it by before-bend.jpghand. It worked out that the thick blocks with a brass strip between was just wide enough to wedge securely in to a channel in an equipment rack I had in my shack. The bottom portion of the rack holds my homebrew 1500-watt amplifiers used for DXing. A guide along one edge of the stock was also found necessary in order to assure a proper bend. This was accomplished by screwing a board across the front of the rack and using a piece of think aluminum bar stock laying on top of the board. The pictures after-bend.jpgexplain this setup better than words. Even with this bending jig, approximately one in three bends was “good enough” so I needed plenty of “extra” brass stock! I really need to invest in a metal bending brake one of these days! It takes a surprising amount of force to bend a 1/2″ x 1/16″ brass strip while getting a good crisp 90 degree angle. Prior to bending, the strip was clamped in place such that the bend would be the desired distance from the end for one side of the finished angle. The strip was left several inches longer than needed for the other side of the angle to aid in bending. It was cut to length after bending.

drill-jig-empty.jpgThe next hurdle was getting holes drilled in the brass angles properly centered and at a uniform distance from the end. Several attempts to mark, center punch, and drill either by hand or using a drill press proved unsatisfactory as the results were not repeatable enough from piece to piece. I wonder how they did this in the 1920s? Builders were probably more skilled at such crafts back then! Obviously, I needed a jig to hold the brass drill-jig-empty-closeup.jpgpieces in position for drilling. After several attempts to create such a drilling jig, I found a workable approach using scrap wood, woodworking clamps, and some pieces of brass strip. Let’s not talk about how many attempts it took to get the jig clamped to the drill press table in exactly the right position to center drill-jig-with-stock.jpgholes in my brass angles! Again the photos explain this setup better than words. There were times during this whole brass bending and drilling adventure, I was very tempted to take my brass strips and a drawing of what I wanted to the local machine shop and have them do the work! But that wouldn’t be in the spirit of 1920s rig building, so I pressed onward until I found a method that works well enough. I won’t say I am completely happy with my brass angles, but I am convinced they are the best I can do with my current shop tools.

When the rig was complete enough to be able to test it, I plugged one of my mighty Cunningham CX-301A tubes into the socket and fired it up. I discovered the rig did not want to work in the 80 meter band but would oscillate fine several hundred kilohertz higher than I wanted. I was very surprised at this. As a test to see if the rig was OK but needed modifications to the grid coil, I temporarily wired a 110 mmfd variable condenser across the grid coil, essentially making the rig a TPTG (tuned plate, tuned grid). It worked perfectly in that configuration, so it seemed I would need to modify the grid coil.Actually I was going to have to wind an entirely new coil, since I needed more turns than the present coil had.

Meanwhile, having realize how dismally low the output was with the ’01A tube, I decided to purchase something a little more beefy. I couldn’t afford the much desired UX-210, but I did find a reasonably priced Majestic G-71-A which should be a nice intermediate output tube.When my new tube arrived, I found to my dismay it would not fit in the bayonet socket I had used! The base of the tube was slightly too large in diameter to fit the Federal tube socket. Everyone I asked about this seemed surprised, but nevertheless it did not fit. I liked the Federal socket because it was in very good condition and would also offer the ability to use short pin (UV) versions of vintage tubes. However, this tube fit problem was not a disaster. Albeit in poor condition I had one of the much loved and sought after Air Gap sockets (not of the bayonet variety). It so happens the footprint, post spacing, and mounting hole spacing on the Air Gap is identical to the Federal, so I was able to swap one for the other with no changes to my rig.

Upon plugging the G-71-A in, the filament didn’t light. I thought it must be a bad tube, but then realized the fancy-ass modern “bench” supply I was using to power the filament had gone into current limiting. Huh? It turns out the G-71-A filament requires 1/2 amp, not the more typical 1/4 amp for a ’71A tube. This required a power supply swap. My next surprise was that the filament in this tube produces a dull reddish orange glow, not the bright yellow / white I was expecting. While tuning up the rig, I had to re-adjust the grid tuning condenser, which ended up with the plates fully open. Interesting. I removed the condenser from the circuit entirely, making the rig a true TNT configuration again. It worked perfectly. At first I thought this must be due to differences in the tube, but later discovered it also works perfectly in this configuration with the CX-301A. In fact, it was not the tubes at all, but difference in the tube sockets that caused it to not work as expected with the Federal socket! Fascinating.

The ’71A was a nice upgrade. Most of my HF antennas, including 80 meters, had been wiped out in an ice storm late last winter. As a result I was not successful in making any contacts with the rig but it appeared to be capable of producing about 2 watts out with reasonable note quality.

At this point it was time to tidy up some details. Primarily the meter and grid resistance needed to be painted. Something had to go wrong eventually, and apparently this was the time. I spent two hours trying to get the Jewell 54 meter apart, when suddenly it let go and flew across the room! The scale and pointer were bent. This was a much prized part, as I’d had my heart set on a Jewell meter all along, and had finally found this one after months of searching. I managed to straighten out the bent parts but the pointer is missing some paint, making bright silver spots on it. I was devastated! Nevertheless I decided to press on and restore this meter as best could be done under the circumstances. Soon thereafter I learned that my skills at painting panel meters leave much to be desired. This shouldn’t really come as any surprise, given my often demonstrated lack of ability with painting of any kind.

It was at this point I realized I was frustrated and discouraged. I really needed a break from the project or else this condition might be permanent. Having many other things on my plate for the summer I  carefully wrapped the rig in plastic so as to keep evil dust away and set it aside for a while.

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