One day while visiting Andy's shack (WA4KCY) I spotted his beautiful AM Transmitter using 4-125's in a windowed cabinet. Right then I knew that I had to build one. Inspiration had struck. It took about four months of work, but here it is. I hope you enjoy it and maybe you too will get inspired to build a transmitter.
The hardest part of building is getting all the parts you need, including a good cabinet. It is hardly worth trying to buy one commercially since they are so very expensive. I found one from a friend that had some surface rust on it. It had been in a basement for a long time and just needed some care. I brought it home and sandblasted it until I was sure all the surface rust was gone. Then I painted it hammertone blue which resembles the Hallicrafters blue. The hammertone is good to use on an old cabinet because it hides any imperfections. I was pleased with the way it turned out.
Next came the power supply. I knew that I wanted to run 2500 volts on the 813s so a suitable transformer had to be secured. Andy helped me with the task. I bought a transformer and choke from Andy. I already had the caps I needed. It uses a 100 watt 70K bleeder for stabilization. I used plexiglass shelving so that I could look at components that get hidden by metal shelves. Angle brackets hold everything.
Andy's transmitter uses a Millen 90801 as exciter. I wanted to do the same. The only problem is that they can hardly be found anymore. But I did locate a Millen 90800, the earlier production model that Millen used before the 90801. When it came I was shocked. It had dirt dobber nests in it and lots of surface rust, but otherwise it could be salvaged. I decided to strip the entire chassis and start all over with it. I sandblasted the cabinet until I got all the rust off and then painted it with wrinkle paint as per original. Then I rebuilt the exciter. It worked. My only problem was that I did not have any of the coils that the exciter used. I wound a grid and plate coil for 80 meters and started trying to find some other coils. Again, they are scarce. But I got lucky and found a fellow who had a complete set in the original boxes. I got 160 through 10 meter coils from him and that part of the project became complete. Below is a picture of the Millen in the cabinet.
Next came the building of the modulator. Again, my friend Andy helped here. He had a modulation transformer that would work well for the rig. So I bought it from him and began the process. The modulator design is from Jim Taylor (sk) and is a dandy one. I had no trouble building it and getting it going. Final smoke tests on it proved that it had a little stray hum that no one would ever hear on the air, but I could hear it on my signal tracer and see it on the scope. So, I got my trusty scope to work and everywhere I found a sine wave in a DC component, I did some more filtering until I got a nice flat line. I can honestly say that there is no trace of hum anywhere in the modulator, at least none that I can see on the scope or hear with my ears. The modulator is built on a heavy stock steel cabinet.
Building the Final was a slow process because I had such problems in locating a B&W 850A. I knew I wanted to make the final as efficient as I could so an 850A was a must for me. Finally a piece of luck. I went to a hamfest and did not find an 850A, but I did meet a fellow who said he had one. I pleaded with him to let me buy it and he was quite helpful. He told me the thing was at home and that he would call me. He did call a few days later and actually brought it to me. But it was in sad condition. He had taken it out of an old amp. All the plexiglass standoffs were broken but everything was there, so I rebuilt it. I re-silverplated the coils and fashioned new spacers for the coil and put it back together. It works fine. The final also uses a tuned circuit input from National, an MB 40.
When I finished building the project, it was time to test it in real life. Lou, K4MNY is a longtime friend and lives close by so I knew he could hear everything in the signal. So, we got on and tested together in a QSO. He helped me adjust the levels on the modulator to good quality and checked to see if the signal was narrow enough to suit us. Sure enough, it was. Subsequent tests and QSO's with Lou and Andy have confirmed that the 813 Transmitter that I cloned from Andy's 4-125 is working just like the design. And that's the way it is supposed to be.
Click here for a schematic on the modulator and the final.
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