The KC5LDO 6L6 Transmitter
The 6L6 oscillator was one of the most popular transmitters ever built. Many hams
started the hobby with a homebrewed 6L6 rig. There were many versions of the
transmitter and most worked well. The circuit I used is very simple and the results
are a very clean stable tone. Harmonics are not a problem with the Pi network output.
The circuit came from a February 1994 issue of QST.
I tried to make my transmitter as if it were made from a commercial kit, using a
"Radiart" emblem at the top. The Radiart Company is no longer in business, but
I liked the sideways lightning bolt above the name. May be silly, but a nice touch.
The steel chassis is 6 X 10 inches and made by Hammond. The front panel is
1/16 inch sheet metal. With the exception of capacitors, resistors and chokes,
all parts are from my junk box. The layout was simple. I took the parts I had and
simply made them fit with the shortest leads possible.
I used the 6L6G because they are big and look great. Newer versions will certainly
work fine. This circuit also works well with the 6V6 tubes, although output will be
less. If you use a 6L6 tube, use a 0-100 ma. D.C. meter. If using a 6V6 tube, you
can get by with a 0-50 ma. D.C. meter. My transmitter runs at 60 ma. A 6V6 tube
is typically 30-35 ma. I use FT-243 crystals and have had no problems.
The variable capacitors came from junked broadcast sets and work well for lower
voltage transmitters. Most are around 365 pf. which will allow tuning into most
The 4 pin plug in coil forms are 1.5 inches diameter made by Hammarland. I used
20 ga. enamel covered wire close wound. 16 turns for 40 meters and 34 turns for
80 meters. If your coil form is a little smaller or larger, just add or subtract a turn,
this is not super critical. Do not touch this coil transmitting! You will get shocked and
get a RF burn.
Power supply requirements are simple. The 6L6 or 6V6 tube needs 6.3 VAC for
the filament. A well filtered 150 to 350 volt D.C. power supply will work for plate
voltage. I suggest at least a 100 ma rating. The voltages can be LETHAL. Do Not
build or operate the transmitter unless you fully understand this. If you not are sure of
something, seek qualified help.
Tune up is simple. If you are using a wattmeter, simply tune for maximum wattage
and the best signal on your receiver and listen for the best tone. Note: Maximum
wattage does not always give you the best tone. Turn the RF gain down on your
receiver and listen for the best tone. If you don't have a wattmeter you can use the
dip and load method, which is my preference.
Plate current is dipped on the meter with C1 and then increase the load with C2.
Do this several times until you have the output and tone you want. Be sure to only
key down for a few seconds at a time. Multiplying your plate voltage times your
milliamp meter reading will give you the input wattage. The output wattage is
approximately one half of the input wattage.
Enjoy your old time rig and I hope to catch you on the air. 73, James, KC5LDO
Click on this link for a real neat Power Supply that works well with this Transmitter
Articles written by James Tobola - KC5LDO
and reproduction, publication, or duplication of this article, or any part thereof,
in any manner is prohibited without the express written permission of the author.
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