Original HT-37 VFO Circuit
A couple of things before I start:
First, this is not my fault. The Radio Gods are to blame. I innocently tried to by an HT-37 tuning capacitor on e-bay, but the seller sent me the entire VFO unit. The only thing missing was THE TUBE. Clearly, that was a sign, right?
Second, this is a work in progress. That is why my diagram (below) is a bit ugly. I am looking for your input and advice on how I might do this better. I will understand if religious principles prevent some of you from participating in this endeavor.
I am trying to solid-state this device WITHOUT major surgery, and without adding any reactive components that would change the resonance or tuning range of the original. The original circuit tunes from 5 to 5.5 MHz and that is fine with me.
I started out by just sticking a J-310 FET into pins 1, 2, and 5 of the tube socket. I put 12 V on the drain and the thing oscillated right where it is supposed to. That was a good sign.
Here is what I have done so far:
Bill's initial solid state conversion of HT-37 VFO
Mechanically, my effort has been very simple. At first I tried to fashion a more serious male socket for the FET using two broken 7 pin tubes. This didn't work well.
So then I just ran three short wires up through the center hold of the tube socket to the connections for pins 1,2, and 5. I superglued the J-301 to the chassis and made some non-reactive connections: I put a 47 ohm resistor on the source, and a 220 ohm resistor on the drain. I grounded the drain for RF with a .01 uF cap to ground. I added a 100k resistor and a diode on the gate. Oh yea, I put a couple of ferrite bead on the FET gate lead. (See pictures below.)
Three lead up through the center hole
A rare look inside an HT-37 VFO
The original thermatron circuit has an output bandpass transformer, a 3900 ohm resistor and a coupling cap. I left them in the circuit, but they are not doing anything.
The output from the source of the FET looks pretty good. I can see some VHF on the trace, but I suspect this is from my FM broadcast nemesis at 100.3 FM (one mile away). On a receiver, I can hear some AF noise on the signal, but this may be the result of the RFI from THE BIG 100 -- WASHINGTON'S CLASSIC ROCK.
So what do you folks think? What else could I do, or should I do?
I think you have lost your mind! nuff said.
ReplyDeleteBill, I see this as a journey of continuous improvement with the 1 step to remove the heat generating tube. The next step would be to either swap in a second J310 to make like a Dual Gate MOSFET or to add a 40673. A DGM has an allure of sophistication not found with a single JFET. Ultimately your journey will get you on the path to the Si5351 and a large size color TFT -- but you did good by taking the 1st step.
ReplyDeletePete
I feel somewhat ambivalent contributing Bill, but there are (or were) 7 pin plugs available- I think originally for the NB socket in the R-4 line. Might make experimenting easier. Have fun.
ReplyDeleteDale W4OP
I would be afraid that a ghostly 6CB6 would torment me in my dreams forever. 😥 Paul VK3HN.
ReplyDeleteOh I wish there was a "like" option ... both Grayson & N6QW comments are brilliant! :)
ReplyDeleteI had to chuckle with this post because I had been down just about the same path not long ago. It was not to actually build anything at the moment, but more just a check of could I later use this in a build. I had found a Collins 70E-8A PTO at the Dayton Hamvention fleamarket for only a few $. “I can do something with this” was the grand thought. It did come with the tube, an actual RED 6SJ7, the high reliability version. The challenge was, can I run this mechanically elegant (and sealed) PTO with a FET in an octal tube plug instead of it’s pretty tube. I used a MPF102 with a circuit that was nearly identical to yours. It worked great. Output was taken through a 10 pf in the source side of the FET. Power was also applied externally to the FET. The rest of the tube circuit and the internal output stuff was just ignored. The FET was basically just connecting to the L&C of the Hartley oscillator. Nothing inside had to be touched. Sorry to say no project has yet developed, but some day.... And yes, a buffer, a level adjusting amp and great power stability are all healthy things to do.
ReplyDeleteHa! So VE3EAC shows that I am not the only one to do this. I am not alone! Look, I understand the anxiety that this causes. But like I said, this is not all my fault. As happened with OM EAC, we were presented with just the VFO portions of these venerable rigs, and we felt the natural ham urge to get them going with newer, simpler technology. 73 Bill N2CQR
ReplyDeleteDid anybody else wonder if that socket is big enough to fit in a esp32 and an si5351? that way the vfo could contain it's original look, but Bill could control it from his smart phone.
ReplyDeleteNow I'm done triggering Bill into a stroke, It does look like you could get all of that vfo into the old socket without even resorting to surface mount components. stick a small metal cap on it and you've got a nice grounded can surrounding your vfo.
I had a similar idea (and this one is likely to cause even more consternation among tube aficionados). I thought about ripping the thermatron guts out of the tube. Replace the guts with the FET and a few resistors. Seal the whole thing up, putting it back in the envelope. Then just pop it into the tube socket! This would be like what Hayseed Hamfest does for electrolytic capacitors. We could even come up with NEW NAMES for these new "tubes." It would be FANTASTIC.
ReplyDeleteExcuse me now. I have to go put on my Kevlar vest. I sense a wave of incoming death threats from the Boatanchor gang. 73 Bill N2CQR
Seems like I remember something called "tubestors" from 20 or more years ago.. direct replacement for a lot of thermatrons and whole rigs (my buddy did a collins s line) converted to all solid state, but has nice mechanical stability and all the advantages of cool operation.
ReplyDeleteI also like the way another guy you referenced did the wall anchors adaption. Gives some mechanical stiffness and resulting reliability.
Keep goin Bill .. all very nice adventure.
73 de wa5wcp paul
Wow, check this out. This guy built a solid-state replacement for an old "magic eye" tube. And he fit it into the old tube's glass envelope. Tube fans will see this as an indication that we are in the "end times."
ReplyDeletehttps://www.youtube.com/watch?v=T6HPaSh0j1o
I have two and a half HT37s I'd love to find a new home for. The half was Murph's part donor box he unceremoniously dumped on me, the other two are whole, clean and functional. (they're still HT-37s, of course.. so your mileage may vary.)
ReplyDelete-W8SDF