I converted an old hobby grade tube RF generator, a "Superior Instruments" model TV-50A (it's not that superior
), to an AM micro transmitter. It has 6 bands of RF frequencies, including the MW AM b'cast band.
The 12AT7 RF osc and modulator are original, the rest I built new. The 6AQ6 triode, similar to half a 5751 triode, with a mu of 70, serves as the audio gain amp and feedback mixer. The 6C4 is just a cathode follower, to feed the 12AT7 modulator triode, which has a low impedance input circuit. I used geranium diodes instead of the 1N4148's shown on the diagram.
The feedback loop is described at
http://www.wa2ise.com/radios/amxmit.html