My New 6L6 Transmitter
Carl W. Davis W8WZ
I have always wanted to build my own transmitter. I got my first Amateur Radio License when I was 14 years old. My grandfather was still alive then, and he helped me build a Hartley Oscillator for 20 meter CW operation. Unfortunately, he passed away before we finished the project's power supply and I never did get that transmitter on the air. That rig was later lost in a flood. I have not had any formal training in electronics. All I know is what I have learned from my grandfather, from books while studying to go from no-code tech, to technician plus, to general, to advanced, to extra, and what I have learned from listening to more learned hams on the air. I have found listening to the AM operators on 75 meters to be a great source of technical information, as well as a source of entertainment.
I have always been interested in tube radios, since tube theory is what my grandfather taught me about. My first HF rig was a Drake TR-3 lent to me by a local radio club. Since then, I have always had a piece of “hollow state” gear in my shack. Before I graduated from high school. I bought an old HW 101 that was not working, and fixed it by removing parts from the boards, testing them, and then replacing the faulty components. I had a working HW101 on the bench next to it and would use that as my default parts tester. That's not the best method, but it did work for me. I have also built some kits. In addition to solid state projects on circuit boards, I built a 1 tube 1 watt 40 meter CW transmitter from www.glowbugkits.com and I built a 1 tube QRP transmitter based around a 12K5 tube from a kit produced by NN8G. Kit building and fixing older gear is great, but I still wanted to actually build a transmitter from nothing but a schematic and some parts. This winter, I decided to finally do something about that goal.
After reading through some articles in old magazines that had belonged to my grandfather, as well as searching the Internet, I decided to build my CW transmitter around a 6L6 tube. Since 40 meters is my favorite band, I decided to make my transmitter for use on that band. I wanted a radio more stable and “user friendly” than a basic Hartley Oscillator, but I knew that building a VFO would make this project more difficult than I wanted it to be. So, naturally that meant that the transmitter I was going to build would be rock-bound with those specifications in mind, I sorted through the various schematics I had found. I decided on a schematic that I found online at www.io.com/~nielw/6l6/6L6.htm. I like this design because it is designed to be as easy on tubes and crystals as a single tube transmitter can reasonably be. Since parts are hard to get, and because I'm more concerned about longevity than RF output, being easy on crystals and tubes seemed like an important quality to have in my transmitter. Also, since parts are so expensive and hard to find, I liked this transmitter's low parts count. That web page shows the transmitter built breadboard style. I decided to use a metal chassis instead. I decided to do that for several reasons. First, I felt safer with 450VDC being on insulated wires under a chassis and behind a faceplate instead of on an open buss screwed to a board. Second, I already have a 1-tube transmitter made from a kit that uses a PC board, and one that is made breadboard style. Using a metal chassis for this rig will give me one “glow bug” using PC board, Breadboard and point-to-point chassis construction. I figured I could then use them when I teach ham radio licensing classes again to demonstrate the different types of radio construction. Also, I think a homebrew transmitter with a full chassis just looks cool!
Once I had chosen a schematic to follow, I made a list of all the parts I would need. I then searched through the “junk” boxes in my closet and found which parts I already had. Then I set about acquiring the parts that I lacked. I was very fortunate to have QSO with WB0ZNY on 75 meter AM. I mentioned my winter project to him, and he offered to send me what he could from his parts collection. Tom's assistance with parts helped lower my cost of my transmitter quite a bit and his advice on construction was priceless. I got the rest of my parts by placing “Want To Buy” ads on www.qth.com also from Antique Electronic Supply, Surplus Sales of Nebraska, and Radio Shack. My total cost for the project was just under $125.00. The project could have been completed at lower cost, however I wanted to use top of the line parts, a vintage ammeter, and a full metal chassis and faceplate. I also needed to buy hole saws to drill out the openings for the meter and the tube socket. I included the cost of those tools in my total project cost. Also included is the cost of shipping when I ordered parts.
After I had collected all the parts, I set about assembly. I had read enough articles about building transmitters by this time to have a good understanding of how to proceed. I laid the parts out on the chassis to make sure everything would fit. Next I figured out where I wanted to place the capacitor, coils, tube, and connectors. Then I began drilling out all of the holes needed to mount the parts. Since this is how kits arrive, I figured that was a good first step. Building this transmitter from scratch gave me a great appreciation for mechanical engineers. For me, actually doing the mechanical lay out and tooling was the hardest part of this job. Even though I measured twice and drilled once, I still found that some of my holes didn't exactly line up as I had intended. After I got the major components mounted, I followed the schematic and built the transmitter with point-to-point construction. I found that if I highlighted the schematic as I completed a section, it helped me remember where I left off when I came back to work on the project later.
I built the power supply first. I used an old cake pan for the chassis and decided to use solid-state diodes as a full bridge rectifier, and computer grade electrolytic caps in the filtering section. I thought about using a more vintage power supply design, but I thought that the higher reliability and lower construction costs of using the design I chose out weighed the nostalgic effect of using a tube design. Although, my transformer does have a 5-volt tap, so if I decide to modify the rectifier circuit in the future, I do have the option of using a rectifier tube. I debated weather to build the power supply in the same chassis as the transmitter and just hard wire the connections, or to build a separate power supply and use hookup wire to connect the two units. I decided to build a separate power supply so that I can use this supply with any other transmitter I may choose to build in the future. The supply gives me high voltage of 450VDC and 6.3VAC for the 6L6's filament. It is fused, has an on/off switch and a bright red neon lamp to tell me when it is on. Other than those luxuries, it is just a straight rectifier/filter design using a center tap transformer that another ham, whose call I regrettably forgot, sold me for only the cost of postage. When I got the transformer in the mail, I was not sure which wires carried which voltage. The color had worn off the primary windings so they appeared light gray or dirty white instead of black, there were another set of wires that looked sort of black or gray also. The 1957 ARRL Handbook had great information on testing and identifying transformer values. I was able to use my Ohm Meter to find the Primary leads, then learn that the green leads carried 6.3V, the red leads were the high voltage leads, and the red/yellow checked lead was the center tap for the high voltage secondary winding. I also learned that the other set of leads provided 5 volts for the rectifier tube. Once I had borrowed an isolation transformer, I tested this transformer with my voltmeter and found that the data in the ARRL Handbook was exactly correct.
I used rubber grommets in each hole in the chassis to protect the insulation on my hookup wire. I also decided to insulate my negative DC voltage from the chassis ground instead of using the traditional method of connecting DC negative to chassis ground. I thought this technique would make it more difficult for an accidental short to take place, thus adding a little bit of protection for my hard to replace high-voltage transformer.
Once the power supply was built, I started working on the transmitter. I sent e-mail to W0VLZ; he was very helpful and supplied me with the tube socket pin connection information for the 6L6.
I thought about winding my own tank coil. However, I have never liked winding coils, and I decided to buy a section of B&W coil stock from Surplus Sales of Nebraska and use that. I did wind the pick-up coil myself, and was amazed when it worked!
For hookup wire, I used wire from a discarded computer power supply that was a gift from my friend Fred, KC9KLL. This wire worked perfectly! It is stranded silver wire, and came in 8 inch lengths insulated in many different colors of plastic. This allowed me to follow the standard usage of green wire for my filaments, red wire for my positive DC; black for my negative DC and another color for my general hookup wire.
Once the transmitter was assembled, I anxiously powered it up. To my joy there was no great flash of smoke, or explosion of flame. However, to my disappointment, it didn't transmit any RF either. I called my friend Fred, KC9KLL and he came over to take a look at it. He found that my low watt soldering iron had left a cold solder joint or two, and he graciously fixed those for me with his high watt gun. I then tried again and found that it was showing oscillation as soon as it was turned on. Fred's eye saw that I had wired the key jack backwards and so the key was always closed. Fred fixed that for me too. Since it was showing the proper levels of plate current, and responding to the closing of my Speed-X straight key, we assumed it was putting out RF, although I had no RF meter that worked at that low of a level to verify our thoughts. I decided it was time to invest in a QRP wattmeter and later learned that the reason no one answered my CQ calls was because I still had no RF output. I was puzzled about how my transmitter could be oscillating, showing proper levels of plate current, yet still not be transmitting any RF. I figured there had to be some bad connection between the antenna connection and the tuned circuit.
I decided to follow the schematic right through the radio and look for my problem. I found it when I got to the pickup coil's connection to the capacitor. Instead of connecting the non-ground side of the capacitor to the coil, I had connected the grounded side. Once I had fixed that problem, I had RF output of about 8 watts! When I called Fred over to see my working radio, he suggested adding a .01 capacitor at the B+ input to ground for added filtering. Once that modification was made, the radio was deemed an official transmitter at my station.
Building this radio was a wonderfully fun way to spend some winter nights. I enjoyed the process of building a transmitter from scratch, and I enjoy having a transmitter that I built in my shack. I feel like I have accomplished a ham radio goal that I have had since I got my ticket 14 years ago. I could not have finished this project without the help of the hams mentioned in this article plus others who have helped me with their technical knowledge over the air and in person.
The radio operates fine. Like any radio of this design there is a very slight chirp at times. It chirps more with some crystals than it does with others. Until I get some more FT-243 crystals, especially one for 7.040, I operate on 7.050, 7.005, 7.222 or 7.150 because those are the only rocks I have for 40 meters.
Hams in the “old days” built their own transmitters to save money. Today that is not the case. I could have bought a much better transmitter on e-bay (such as an HW101) for about the same amount that I spent for this transmitter's parts. Nevertheless building this transmitter represents for me, what ham radio is all about. I built this transmitter to apply the technical skills I had learned, and to learn new things, and thereby improve myself. In addition to those things, I had fun! I hope that in the future I can build even better transmitters, maybe a plate modulated AM transmitter will be in my future someday. Until then, I am grateful for the chance to use this rig and for the knowledge I gained while building it. I am especially thankful to all the hams who helped me, and for those of you who answer the CQ on 40 meters that sounds a bit like a Cuban Islander, but is actually being sent by a W8 in Indiana. Thanks es 73. Carl W8WZ.