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Converting a Tube AM radio to FM

There were many tube radios built from the early 1930's well into the 1960's that work well, but receive only AM broadcasts. The AM band has fewer and fewer stations, and are generally "Talk Radio". How easy would it be to convert one of these radios to FM? 

Well, FM is fundamentally different than AM, so it's not just a matter of changing the frequency at which the radio operates. I have devised circuits that you can plug into your working radio (replacing 1 or 2 tubes) to convert it to FM operation. They do not require modification to the radio itself, so will not affect the radio's value as an antique. These can be built by someone with an intermediate understanding of analog electronics. 

First version, circa 2003 FM_Converter

The second version was quite complicated and I have not documented it, but I discuss it in the third version.

Third version, circa 2018 FM_Converter_2018

Fourth version, circa 2021. This is the one to build! FM_converter_2021

Background information: Ap note on the Philips TDA7000 chip (it's brilliant)  Philips_ap_note  Here's the 7088 data sheet Data_sheet

Another way: the FM5, an FM radio module for retrofitting tube radios from Radiofil in France RadioFil