[Jeffhs]

There were tube-powered FM car radios in the '60s. My dad had one in his 1963 BelAir, IIRC. It was a "Titan" automobile FM radio (an adaptor, actually, as I will explain) that worked by converting the output of its FM tuner to a fixed frequency in the AM broadcast band. The Titan units, at least the one my dad had in his car, converted FM signals to 850 kHz (kc when these devices were new). This worked out fairly well in areas that did not have local AM stations on that frequency, but in regions that did have local AM on 850 the adapter may not have worked very well. In our area, near Cleveland, there is a local station on that frequency, but I think these FM adapters' modulator systems may have been broadbanded enough to allow the adapter's output to be heard on any frequency between roughly 840 and 860 kHz. I think most folks who had these adapters simply tuned the AM car radio until they heard the output of the unit, with little regard to the actual frequency the car radio was tuned to.

The output of the Titan adapters was monaural, however, not stereo. I am not sure if there was a stereo version of the Titan FM unit my dad had, or even an FM multiplex adapter jack anywhere on the adapter. I would think the latter would make a rather clunky arrangement, though, with the car's standard AM radio, the Titan adapter, and an additional adapter for stereo FM. I also wonder how many folks would tolerate having two such adapters under the dashboards of their cars, to say nothing of the cabling among them.