As the community for those vintage equipment is mainly active outside of Europe, this site is written in English.

On the picture you see a so called "Hartley Self Oszillator". This transmitters were state of the art in the 1920ies after the spark transmitter era. The tube visible on the picture is a "EY-627", an original tube from the 20ies in "globe" form. It is known as "type 27 tube" (usually for AF, but I tested the oszillation up to 50 MHz!). The oszillation is done by feeding some of the HF from the output to the input, so the transmitter starts self-oszillation. Output with this 90 years old tube is 1,5-2W. With two of those tubes in parallel I nearly reach the QRP limit!  The tank-circuit needs very high Q (8mm coil "wire"!), so the oszillation is forced to the wanted frequency by tuning the capacitor. The output coil can be changed in its position. The "breadboard" setup is typical for the 20ies.  I have individual coils for 80m and 40m. The whole setup is verrrrry sensitive! The morse key (keying the cathode) has to be on another desk to avoid mechanical effects. Hand effects, moving your body in the room....all is creating some frequency drift. When the antenna is waving in the wind, the changing capacitance to ground tunes the circuit as well. It is a real adventure to be on the air with this transmitter. I have made a lot of QSOs all over Europe. Don't say the tone is bad or call it chirp: It is a "vintage note"...

And for those who always hesitate:  Don't worry about harmonics: The high Q provides a nice output spectrum well in the regulations.