# reawakening an old milsurp receiver

#### unclejed613

##### Well-Known Member
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?

##### Well-Known Member
This may be of some help to you. During the early 60s I converted several old ARC 5 transmitters for ham radio use. Originally powered by a dynamotor we would remove the rear rack mount connectors and replace with an octal 8 pin tube sockets and build power supplies for the things. It was just a popular way to go.
As to the FETRONs sorry, I haven't a clue.
Ron

#### unclejed613

##### Well-Known Member
i'm going to try out some of the FETRON circuits on LTSpice, and see what i can expect compared to the tubes. i really don't want to change the receiver wiring at all. i've seen that some of the FETRON circuits have a lot more gain than the original tubes, so that might allow some leeway on using reduced voltage. the tricky one will be the audio output tube, and i suspect i will need a power FET for that one. i need to get some octal plugs. i know they still use octal sockets for relays in industrial equipment, so i think at the very worst case i get some old relays, and remove the relays so i can use the enclosure and plug.

Years ago I bought several octal plugs which included a small project box and the boxes even had grooves cut for perf board. It was for a project at work and I forget what we paid for them. The ones I am seeing now they want 20 for and as you mentioned you can buy relays in the box and yank them out for less. Ron #### BobW ##### Active Member FETRON circuits have been discussed on the Antique Radio Forum, and as far as I can see, the general consensus is that they may work okay in non critical circuits, but in RF tuned circuits you're going to have to match every parameter exactly, including all of the interelectrode parasitic capacitances, unless you're prepared to do a complete realignment of the receiver (and another realignment when you go to sell it later with the original tubes). Personally, I'd save myself the grief and just put in the original tubes. That's what antique radio restorers do. If some are missing, they're not hard to find. Contrary to the popular misconception that you can't buy tubes anymore, there's no problem getting them. You just can't buy them at your local Radio Shack. The tubes in a BC548 can be had for3 - $4 each except for the VT-152 (6K6) which is about$12. You can get them here among other places:
http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes6k4-6zy5g.html

#### dr pepper

##### Well-Known Member
Does Ltspice simulate tubes?, or should I say are component files available for tubes?

#### BobW

##### Active Member
Some people have developed spice models for tubes, mostly people on the audio forums interested in tube audio amplifiers. Modelling a converter tube like a 6SA7 would be a monumental task. Again, for it to be useful, you need to have accurate parasitic capacitance parameters.

#### unclejed613

##### Well-Known Member
so far i have been able to identify the TUBESTER components, which were made as drop-in replacements for the tubes in a collins 75-s3 radio for the most part, the transistors are nothing fancy, but there are some tube types used more than once in the receiver, and it appears that the TUBESTERs have different schematics or different component values depending on what that particular tube's function is in the radio. i sort-of expected that, especially after modelling some FETRON/TUBESTER circuits in LTSpice. one puzzling thing is that the B+ voltage in the radio is 150V, yet most of the transistors making up the TUBESTERs are 30V devices. apparently Collins actually sold some radios with the TUBESTERs already installed, so these devices did what they were intended to do. but i do now have some possible candidates to experiment with.