Low power 40M one tube test rig

[VE3LYX]

Just before spring hit I designed and built a one tube (6l6) AM rig. I did one test with it but needed to reduce HUM. I did that but hadn't put it back on since with summer activities in full swing. In this recent cool a rainy weather I fired it up a couple of days ago. I heard quite a few on 7290 and 7295 but they were working on getting super audio and were running 100 watts at least if not more. I didn't want to butt in with a minimalist rig. Anywhere else on the 40m band where it might be good to test with reasonable chance of AMers listening willing to reply?
don

[Opcom]

Too bad you are not around here in the TX-OK area. There is a little RT Saturdays, and people sometimes show up with the small stuff, 15-20W carriers, and the like, and so if you were in Dallas area, Austin, or Houston, someone would hear you. We always talk with the low power experimenters, as well as those using 100W 'plastic radios' who sometimes hear us and switch to AM to give it a try. yeah it means most of us eat static while one talks to the low powered station, and we try to hear them, so that is some fun there, and we do not mind at all.

Why not contact one of those stations you hear, tell them where you are, and ask if they or a closer friend would make a schedule to try out the situation?

Did you have the schematic of the 6L6 rig and even the power supply? It would be interesting to see a one tube rig. One thing, if you get hum when you are hearing your own signal, it could just be an artifact of your setup.

[VE3LYX]

You are right , it is fun!
I think the first response was from Buffalo New York area. He had my call right but said I had a hum on my audio .I shutdown and went on a hunt for it. I had the filament transformer close to the tube with the transformer core facing the glass. I should have mounted it the other way at least but it was too late then as there was no room. I did however have a steel tube shield and tube shield base. I installed it using the tube socket screws and it seems to have eliminated the hum. I of course discovered that trying it by hand first. Hi voltage is an Ac line rectifier doubler.
The circuit is pretty simple. A normal 6L6 Hartley osc , screen voltage from plate supply through dropping resistor. One coil on the osc and an identical coil at the output. A dual gang 365 variable cap tuned by a single control. So the grid tank(VFO) is tuned and the plate tank is tuned by the same control. I wondered if it would work and surprisingly it does fairly well. Modulation is by Take-away using a loop absorbing Rf from the plate tank into an old #3 carbon hand mic. I have gotten pretty good at this type of modulation with other rigs (TNT 45 tube for one.) This was supposed to be  "Pretty" version of that work.
I would jump in on 7290 except I think when folks are dedicated to getting broadcast audio and are involved in testing and discussing that interrupting with a rig that will only have "audio" and that is all is unwise and probably unwelcome. The PW term I am sure would be immediately used.  I have worked a couple of New Jersey fellows and also western Ontario with the 45 TNT and the loop mod system so If I hear them on I may jump in. The Western Ontario stuff I did by sked on or about 7140. To me and I know it may just be me but I find this type of Ham Radio very interesting.
don
I drew up a schematic but it doesn't copy well. I didn't use one to build it as it is just a Hartley osc (and we all should be able to whip one of them out from memory) with an identical  coil in the plate circuit as well . One section of the dual gang cap tunes the osc and the other tunes the plate or final tank. Since the coils are identical (even interchangeable) and I included a 100 pf in the lead from the plate to the tank to match the 100 pf grid leak in the osc they track together. Antenna is shunt fed with a .01capfrom the top of the tank coil. Loop is just a single turn free standing loop made out of #12 insulated wire placed just above the tube socket level at the bottom of the plug in coil and about 1/4 inch away from it. Carbon mic plugs into that. 

[VE3LYX]

Here is a schematic you asked for. It should expand alright if you have that ability. Not the best but I don't have a scanner anymore. Because of the mod method IE it is in the final tank and not the oscillator section to eliminate or reduce pulling a singe feed ant is required. I use a long wire but I suspect but don't really know for sure a windom would also work. I didn't include the PS since it doesn't matter how you decide to make it as long as the right voltages are present and it is well filtered. I will be trying it anytime I am in the shack. usually for about 5 minutes. I am set up on 7290. The carrier should be easily detected as it is full power till I engage the mic and the loop.(using mic's press to talk) Propagation from this antenna should be best east and west from me. I am on the central north shore of Lake Ontario.
don
Had an evil thought. Maybe I should build one with a 1625 since I have a dozen of them. However mod method works best a lower power IE 10watts or less. Have never tried putting the AM signal so produced through a PA. Still a bit amazed (or stunned) that it actually works. It is a design (Use of a carbon mic and an absorption loop, not this particular transmitter design)  from the 19 teens I accidently stumbled on.

[N0WEK]

That's a pretty cool, absolute minimalist rig!

Have you checked the harmonics out?

Nice work, I enjoy your experiments with these rigs!

Greg

[VE3LYX]

Thanks Greg,
Just starting to check into all this. I had thought , hoped, that having a tuned tank in the output would give it good manners that way. Before summer hit I had just done a quick test and repaired the hum thing. When it turned cool last week and poured rain I got bored and went in the shack and started back at it. Have tested for a few but need another day to get serious.  Seems fairly stable and seems to be only where it is supposed to be, so far.
don
Last night I disconnected the centre tap output and made a #12 insulated wire 2 1/2 turn loop fastened at the (top hot end) of the output coil. I grounded one end and connected the other to the coax connector centre to see if  could run coaxial line off the transmitter without detuning the output coil. I works quite well, in fact power out if more then the previous set up and it didn't detune the system changing it.