The "Double-M" Transmitter for 80 meters
I have an interesting tale to tell. Not long ago, a friend of mine came into the possession an Amateur Radio transmitter that was said to be built by a young Jim Larsen, later to be famous as the founder of Larsen Antennas. The set was built pretty much with odds and ends, as was the norm in the 'thirties. If you had a ham ticket and wanted to transmit, better start scrounging parts. I am sure many a radio shop owner would sigh in resignation when a young ham would come through the door looking for advice, parts or both. "OK Bud, sweep the shop out and I'll give you this #27 tube." I can hear the lad say "Gosh Mr. Fernsprecher, it's a deal!" Coming back to 2010, I looked closely at this set and noticed some odd things. One of the glaring ones was that as this was ostensibly a TNT (Tuned-Not-Tuned) transmitter, it seemed to be missing the grid coil. Mike Peebles, Mike McCartor and I haggled and discussed this and other things at length over the circuit and thought that as it was, it wouldn't oscillate. Such as it stood we classified it in the "interesting" category and moved on to other things. Fast forward a couple of weeks. Mike P. tells me of an article he dug up and, lo and behold, there was that very same transmitter. In 1935 Modern Mechanics magazine issued an annual compendium of radio articles including one called "Build this "Double-M" Three-Band Transmitter". In the first illustration is Dad in his bow-tie, puffing a pipe, watching Junior, also in shirt and tie, sending on his pride and joy.
Now in those days it seems you could not construct or run equipment unless you were were wearing a shirt and tie. I don't know what the few female operators did, dress like they were going to the opera? The upshot of this whole story is that this transmitter had a coil coupling circuit that to say the least was unusual. The grid coil is inside the plate coil. What I mean is the plate coil is made from 1/8" o.d. soft copper tubing and the grid coil is an insulated wire that is threaded through the inside of the tubing. Talk about close coupling! While this circuit could be arguably be called a TNT (Tuned-Not Tuned) oscillator, the coupling is so tight that the unit fits more in the category of a "Unity Coupled" oscillator.
Here is the schematic from the original article:
DEATH WISH? Read on...
The death wish part is that he high-voltage DC goes to the plate through the plate coil. That is an invitation for curious fingers to touch and get zapped...BIG TIME! Also the antenna would have a high-voltage DC on it. The rig that I built has a circuit like this:
This is a safer way to build it. Safer, but not safe. While in operation, KEEP YOUR BLOODY FINGERS OUT OF IT!!! The plate coil no longer has high-voltage DC on it, thanks to the DC blocking capacitor C5. The top of the RFC still does have HV so you still need to watch where you put your fingers. The following pictures show the finished unit. L3 along with C8 & C9 are a series-tuned LC circuit so the transmitter can operate into a 50-ohm coaxial system.
Those who would like a copy of the original article can e-mail me at dan.w7oil(you know what's here)gmail.com (9-meg "pdf" file)
Here are some pictures of the finished unit. It puts out about 1.2 watts. I have already made one contact on it with VE7SL in British Columbia, a distance of 209 miles (300km).
A couple construction details:
That's all folks! Click your "Back" button to return from whence you came.