Although I don't have enough valve BC band radios (yet) to make an AM transmitter, I have for quite a while been transmiiting on VHF FM from an Akai tape deck, an kitset hard drive MP3 player or of late a spare computer. Early versions were simple mono transmitters which had wandering frequency that analogue tuners with AFC could cope with, but now I have a kitset small frequency locked stereo (
www.jaycar.co.nz and jaycar.com.au). Requires some delicate soldering of surface mount devices. Runs from a 12v wall-wart. Range about 25yards with a whip antenna and about 100yards when fed up to the TV antenna! Doesn't hum and good quality. Music player is "MP3 Tunes", freeware on the net, does everything, segue, searching and sorting, random play etc. If you're using a really old computer with only DOS then MPXplay by PDSoft, also freeware (I know, I'm cheap!) is good too and also has an 'AGC' function.
Best regards - Marty