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For many years, the subject of low power tube transmitters has come up periodically on the Usenet newsgroup rec.antiques.radio+phono. You can access the archived posts through Google Groups. There are many, many circuits available for low power tube transmitters. The circuits range from single-tube “phono oscillators” included in some phonos from as far back as the 1930s to multi-tube “real” transmitters that have been published over the years.
Beware that virtually all of these designs overlook the most important point: matching the tube impedance to a short antenna. Typically, the power input to these transmitters may be as much as several watts, but the designs fall far short when a short wire is connected as an antenna. You can pump several watts through the tube and get a 50′ range because of the gross mismatch to the short antenna.
A good transmitter design for Part 15, whether tube or solid state, must focus on the best possible transfer of power to a 3 meter antenna, not just power into a 50 ohm load (this is not even close to the impedance of a 3 meter wire or whip or whatever!!!).