Every ham has a favourite "first", a first receiver, a first contact, a first good key, first DX contact, etc, and for me there is a first construction project. This one inspired me to get into ham radio. While i never did actually build it as a first transmitter ( that one came in 1976, and turned out to be a 2 stage MOPA with a transformer power supply and a 6L6 final at 20 watts) it did introduce me to the idea that building equipment could be done at a simple level with parts readily available, and on a shoe-string budget at that. This project showed me that a ham ticket would be worthwhile to get if i could build these neat things and talk all around the State. You see, i was a CBer in 1968, and it cost me $120.00 just to talk 30 miles. CW or not, if i could build something for a lot less and communicate a lot farther, why stay with CB??
I then resolved that i would become an Amateur Radio Operator, and further, build all of my equipment. As time and re-locations occured ( my dad was a career military intelligence specialist and we lived several years in Germany ), i finally realised my goal the winter of 1976 - 77. Although i wound up making another transmitter entirely, i kept this article in my library, just the clipping, to remind and challenge me, to keep the fires of motivation burning.
Sometime in the early nineties, CQ magazine ran a series of "Classic Homebrew" articles in the "World of Ideas" section authored by David Ingram, whom i had met at the Atlanta Hamfest in 1991 (i love to drop names. Makes me feel so . . . so . . . . . . important.) W5LETs article was mentioned, so i figured, heck, i still have that old article stashed away, why not give it some air time as a classic of a classic on the web?
So, folks, as a tip o' th' hat to both Mr. White and Mr. Ingram, here it is, an re-edited version of the classic Tri-Tet one-tube Crystal killer. Read it for fun, or just go ahead and make one: hey, no transformer, which costs as much as the rig itself these days (although, in my cushy little safety coccoon, i would advocate a nice, beefy isolation transformer at least, to distance the builder some from ground. Or, a nice variac.)
Just . . . . beware of Lethal Voltages. Gotta warn you about that. It's kinda sad, when you think about it, Boy Scouts have built these things for years to earn Merit Badges, and they had enough common sense to already know that. But these days of recounting dimpled chads and folks not following simple instructions, ya gotta make it REAL PLAIN : DO NOT, I REPEAT, DO NOT INSERT THE 117 VOLT LINE PLUG INTO YOUR EYE, MOUTH, OR OTHER BODY PARTS !!! DO NOT PLACE THE 50C5 TUBE INTO YOUR MOUTH AND BITE DOWN HARD !!!! DO NOT INSERT YOUR TONGUE AT OR NEAR THE OUTPUT OF THE FILTER CAPS !!! Sheeesh !! Now that y'all have been warned, enjoy.
A Word about the Schematics and the Drawings: These are re-drawn from the original article. TubePad is used for both. TubePad 5.0 is available from my Website as a free download, just follow the link on the main page or click here.
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The Bare Essentials Transmitter (mostly transcribed directly from the
original text of the article.)
Everybody talks about inflation these days. According to the experts, we're spending too much and driving the price of everything sky high. Washington now threatens us with higher taxes to stop our spree. ( Note: remember, this is written in the pre-Greenspan era, Johnson was still the President !) But for hams on a budget there's a way to have your cake and fight inflation, too. You do spend a little of the green stuff but it won't make much of a dent in your wallet or the national economy. The way out: our Bare-Essentials Transmitter. This little 40 and 80 meter rig takes the prize as the anti-inflationary CW transmitter of the year (1968). You spend only $ 7.00 to get it on the air. There's no chassis. The rig is built on a piece of wood ---- any kind, like the end of an orange crate, will do. And for tie points you use finishing nails. The 50C5 (or a 50L6) tube can be salvaged from an old AC/DC radio. You'll have to spend 11 cents (Jeeepers!) for a tube socket but you won't have to buy a socked for the crystal. It's not fancy but it packs a wallop for its price. Used with a mediocre antenna, it has worked stations all over the U.S. When conditions are right and with a good antenna there is no reason why it can't work some real DX.
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The transmitter consists of a 50C5 crystal oscillator, which operates on either 40 or 80 meters. Since the 50C5 has a 50-V filament, it uses a 400 ohm 20 watt dropping resistor instead of a filament transformer. The power supply for the plate and screen voltages is a doubler which provides about 350 VDC. Two capacitors, two silicon rectifiers and a 1- watt resistor complete the power supply.
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Building the Transmitter
First thing is the chassis, which is simply a 10 x 6 x 3/4 inch thick piece of wood. Take a close look at the pictorial to see where each part goes. The tube socket is mounted on short spacers so that its lugs clear the wood. (gary's note: the xtal seems to be attached by alligator clips soldered to mounting nails -- i would, however, use a socket of some type, if available, as i show in the re-drawn pictorial.) The rest of the parts are soldered to finishing nails in the board. (Note: i would encourage soldering spade lugs on the end of the wires and using screws instead of nails as an alternative. This provides somewhat more of a finished look to the construction. Also, consider the use of tempered clip-board masonite on a wood frame, to facilitate some underwiring as well.) The power supply components are located in the upper right corner of the board. There is no power switch so the AC leads go directly to the nails (or screws). Nails ( or screws ) again are used for connecting points for the antenna and for the key. Another nail, located at the lower right of the board is for the ground connection for neon lamp-NL1. If the AC plug is in the wrong way, the hot side of the line will be connected to the key. This will mean that 117v exists from the key to ground, ENOUGH TO CAUSE SEVERE SHOCK. However, if the plug is reversed, the ground side of the AC line will be connected to the key circuit and a shock hazard will not exist. (gary's note: on open breadboard circuitry such as this, the term "shock hazard" is relative. There is ALWAYS a hazard if one is not careful, so be careful, OK??) By connecting a water-pipe ground to the nail, the neon lamp will light if the AC plug is in the wrong way. (so, view the neon light as a WARNING light.) Observe carefully the polarity of the silicon rectifiers and the electrolytics. The coils are wound on plastic pill bottles. The 80 meter coil is wound on a 1-1/2 inch diameter by 3-1/2 inch long bottle. The 40 meter coil is wound on a 1-3/16 or 1-1/4 inch diameter by 2 inch long bottle. First drill a small hole in the bottom of each of the bottles for the mounting screw. The plate winding (primary) for the 80 meter coil is 45 turns of #24 enameled wire, closewound. The 40 meter primary is 23 turns of #24 enameled wire also closewound. Both secondaries are #20 solid hookup wire : the 80 meter is four turns, the 40 meter three turns. Before winding the primaries drill four small holes (two at the top and two at the bottom of the primary winding) in each form. Then thread the ends of the primary wire through these holes. The secondaries are wound over the primary coils and are held in place by twisting the ends together.
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On the Air
At this point, this is where I deviate from the printed text. There are some things I would endeavour to point out. 1- This project uses a compression cap for tank tuning, which, I suppose, isn't too bad, and is definately cheaper than a standard air variable, but i would use the latter as a precaution. Also I would provide some provision to tune the antenna coil, even if it is simply proximity tuning by sliding the coil. Because there is exposed wiring, be very cautious tuning the compression cap because of the high voltages exposed and open. Dropping a screwdriver could result in some interesting fireworks and aroma. This is why I advocate the use of Masonite and underwiring when at all possible. One method of tuning this kind of rig comes down from the old Novice Class crowd who spent all their money just gettin the receiver and decent key and couldn't afford a milliamp meter: Take a No. 47 bulb, and solder to it one or two loops of wire the diameter of which matches that of the output tank coil. This serves as an absorbtion device. Next, attach the coax connected to a resonant dipole ( one which has a known feedpoint impedance of about 50 - 75 ohms ). With the light-bulb coil coupled closely to the tank primary, rotate C4 till the bulb starts to glow. Slowly adjust C4 to maximum brightness. This proceedure can be done with a 50 ohm 20 watt resistor serving as the load, and then replacing the resistor with the transmission line when the output tank is tuned to maximum brightness of the "Absorbtion indicator" assuming the antenna impedance is similar to the load resistor used. Still another, maybe simpler way is to connect a 5 watt lightbulb to the antenna output coil, and tune C4 to maximum brightness. Then connect the transmission line. If you have a 0 - 100 mA meter, just connect it in the B+ line, bypassing RF around it with a couple .01uF caps, and tune for minimum current. If you have an SWR meter, you can read the forward power and slide the antenna coil back or forward to a point where maximum forward current is indicated, then re-dip C4 to minimum current on the mA meter. ( the good ol' peak and dip method.)
This little rig should realise about 5 watts input, 2 watts out. You might have to experiment some with the best type of transmission line and antenna. An antenna tuner might come in handy. Return to top