Just for fun this weekend I built a one tube Hartley Cw transmitter. I used a 01a tube however a 45 or a 2A3 could be subbed for more power. I used a clementine wooden crate for the chassis fresh from the grocery store and recovered the bottom with Masonite also making a front panel. A few minutes with a brush and some flat black paint produced the 1920s look. While it was drying I looked in my junk box for suitable parts. Found a 1920s variable cap, proper tube socket for the old 4 pin tubes, a heater control rheo from an old battery set. Some very old resistors which I paralleled to get around 12k for my grid leak and paired that with an ancient .003 paper cap for a gridleak assembly. Wound 14 turns of #16 wire on a three inch form with a centre tap. Found a RF choke of some description and a second .003 cap. Wired it all together Sat with a key jack in the cathode to coil centre tap lead . Fired it yesterday. Tunes both 80M and 40M . Keys well . Is fairly stable after a couple of minutes. Power input with a 01A tube was just over 4 watts. Will be trying it out this week. Out of pocket costs about $5. No schematic required because a Hartley osc should be committed to memory as it was part of our ham exam back in the day. Quick review is grid to grid leak assembly to top of the coil to centre tap which is connected to cathode(which with these old tubes is also one side of the filament) and bottom of the coil is grounded . Another cap capable of passing RF goes from grounded coil lead to plate. Plate power is fed from the power supply side of this connection through an RF choke to B+. If Rf choke is not available put enough scramble wound turns on a piece of small wooden dowel that if you were an electron you would be too tired to bother going through it. Wire up filament voltage (5 volts for 01A), supply B + and enjoy.
A second coil , (I used 1 turn ) is wound at the top of the main coil and provides ant and ground connection. Simple and works every time with no fussing. Parts values (fixed caps need to be able to pass RF and resistor can be anywhere from 5K to 1 meg and it will still function) are not critical. It is amazing what will work.
don
Please note, this is VFOed so you can roam free no crystals needed and is stable enough to use.