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While the distortion produced by a vacuum tube audio amplifier might be useful in producing the sound of an electric guitar, such an application is different than that of a transmitter.
Using vacuum tube circuits in a transmitter because tubes produce greater content of even harmonics is virtually pointless. A radio receiver is tuned only to the fundamental r-f carrier and some of its modulation components, and ignores their r-f harmonics. But regardless of that, those r-f harmonics can interfere with other radio services. And the modulation produced by the higher-order audio harmonics fall outside the r-f bandwidth of the receiver, and are likewise ignored (even if they are generated by the transmitter) — yet they can interfere with the reception of adjacent channels.
A transmitter/receiver system should add as little distortion as possible to program audio. At a given r-f power output, the most important parameters of a broadcast transmitter are the fidelity (linearity) of its audio amplification and r-f modulation processes. The lowest-distortion transmitters used by licensed commercial AM broadcast stations invariably use solid-state circuits throughout their audio, modulation and r-f sections.
The best place to generate a desired “sound” for a broadcast signal is in the audio/modulation processor used to supply the program input waveform to the transmitter. That is the function of commercial audio processors such as supplied by Orban, Inovonics, Omnia, etc.