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. The 5th even has custom boards, making it even easier to reproduce.
History of my FM conversion projects
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. FM_converter_2021
NEW Fifth version, 2025. THIS is the one to build. FM_Converter_2025 FM_Converter_2025_Schematic
Background information: Ap note on the Philips TDA7000 chip (it's brilliant) Philips_ap_note Here's the 7088 data sheet Data_sheet
There are better chips than the TDA7000 series. Check out the Si4800 series, used in the 2025 version Si4825 Si4825_Ap_Data Si4836 Si4836_Ap_Data
Data sheets for the PLL chips 4046 9046
Another way: the FM5: an FM radio module for retrofitting tube radios from Radiofil in France RadioFil (Note: as of 2025 the project seems to have been removed)