for a while, i've had this WWII vintage BC-348-M receiver i picked up at a thrift store. somebody was trying to modify it to work with a power supply that used an octal plug (it doesn't look as if this model ever had a genemotor in it) and so it has an octal socket on the back instead of the original military rack connector. i was thinking of doing a solid state conversion on it, and hit upon the idea of using FETRONs to replace the tubes, so i wouldn't need to rewire any of the internal circuits. i'm also thinking i might also be able to get away with using a 24 to 48 volt plate supply, but at this point i want to concentrate on building the FETRON devices. fortunately, i have a box of about 50 or so jfets (2N5484). using FETRONs means i can do a solid state conversion without further damaging the value of the radio, and i could always put the tubes back in if i wanted to sell the radio. i've found some FETRON documentation, and i can see that the JFET cascodes seem to be pretty straightforward, and it would be relatively simple to fabricate them. there was another line of solid state drop-in tube replacements called TUBESTERS, and i found this schematic page: https://www.qsl.net/kh6grt/page4/tubesters/TSUB.pdf i will have to sit down and compare the pinouts to tube base diagrams to see which tubes are being simulated. it seems such devices might be an alternative to operating tubes in vintage equipment (which eventually will degrade the tubes, which, except for common ones used in audio amplifiers, are no longer being manufactured). some vintage equipment uses long obsolete types such as 6K7, 6J7, etc... so those tubes are very hard to come by. the other advantage to running FETRONS would be saving the energy used to heat the tubes, so battery powered antique radios could be run without the filament battery. i might even try my hand at fabricating octal, 7-pin, and 9-pin plug bodies with my 3d printer. just wondering if anybody here has actually made or used any such tube replacements, and if so, did they work well? were you able to get the radio to work well at "plate" voltages in the 30 to 50V range?