N0WVA’s One-FET Regen Optimized For SSB/CW Sounds Great!

Since I began posting about regens, it has been gratifying to hear from fellow regen builders, and quite interesting and inspiring to communicate with other hams who are also taken by their charms. The Sproutie has proved to be a great little receiver and it’s a pretty straightforward circuit, but it’s not exactly a minimalist design. Some hams are so intrigued by the regen’s ability to deliver an impressive amount of gain and selectivity in just one stage, that they focus on building a station with an absolute minimum number of parts. Doug N0WVA has the goal of building a complete (and usable) station with just 2 active devices, and he’s halfway there, having come up with a regen receiver that works well on SSB and CW, contains just one active device, and has a total of just 9 parts including the coil. Better still – it works well. Doug writes, “I’ve been building regens since the 90’s and always wondered why the tube versions always ran circles around the solid state rigs that I had been building. Seems like it always took two transistors to equal what one triode could do as far as recovered audio. Well, I think I might have figured it out. You really can get all that and more from just one FET. Actually, I’d say now that a two tube Doerle will not be able to touch this one transistor rig for serviceability. My intentions are to have only two active devices for an entire CW station. It’s not difficult to get a half watt or so from one device, and even be able to slide around the band a bit via VXO. So the main problem was a decent receiver. Well, I now have that problem solved. All the SS (solid state) regens I had been building had a source resistor and cap. This is to supply self negative bias to the gate. It works, but audio suffers because of the use of a small bypass capacitance at audio frequencies. Larger electrolytics here cause howl when oscillating. By taking the source to ground you can then inject the negative bias through the 1 meg gate leak and set your optimum operating point with a 10k pot. However, this adds to the total parts count. I found that by using a green LED in the source worked almost as well, and bypassed all the audio to ground as well.  Also, since the receiver was to be optimized for CW, I wanted minimal pulling, even on strong stations. To do this we must keep the tank coupling very light. That way the capacitive changes in the junction of the FET has minimal effect on the tank. Finally, another plus for SS regens is the need for much less feedback to achieve regeneration. A properly built SS regen should need no more than 1 turn for a tickler. Much more than this and you get wild instability and a heavier loaded tank. One experiment I did was to try different coil forms and see which ones took less tickler feedback. I found that pill bottles were about the best form for low loss, as I could use just one turn spaced well away from the tank. Even better was a pill bottle with slots cut out of it for less loss, but the wire would try to crush the form. I am using a 1k to 8ohm transformer from Radio Shack to couple into Radio Shack phones, which are a bit more sensitive than other cheap headphones I have used. This results in audio that is loud enough to be highly usable, probably where you would normally run the volume on a regular receiver. The dial drive is a ball bearing vernier integrated into a gear reduction capacitor. The NPO trimmer is a band set. All capacitance is kept to a minimum. I find the less plates on a variable capacitor the better for keeping drift down. The regen control is actually a homemade tickler variometer inside the tank form.  This eliminates the need for a choke and throttle capacitor. I will attach some photos and schematic. Hopefully you will find them interesting. I still need to get the transmitter together and get this on the air. Having just two transistors for a complete CW station will be a blast I think.” Doug made the shaft of his variometer from the plastic barrel of a syringe. I have a number of syringe/pipettes here that I use for giving medicines to my cats and am thinking one of them would be useful for this purpose. Of course, anything cylindrical could be used for a shaft. Doug’s schematic was a fairly low-res scan, so I redrew it. Here’s my re-draw of the schematic of Doug’s one-FET regen –

Schematic of N0WVA’s single-FET regen. It is a monobander on 75/80M. Note: Barry W6YE looked closely at the bandset trimcap in the photos below (the one marked in the schematic above as being 10pF) and discovered, using the part number visible in the picture, that it is a 7-45pF part. Thank you for this information Barry!

I re-drew Doug’s schematic, as the one he sent me was a little small. I figured my re-draw might be a bit easier to read. However, for the sake of completeness, here is what Doug sent me –

I don’t think Doug mentioned the type of FET he was using, so I assume the usual suspects (J310, MPF102) would be fine here. He also doesn’t include coil winding details on the schematic but you should be able to get a rough idea from the photos and besides – if you’re hardy enough to give this receiver a go, you’ll want to figure out the exact values of inductance and capacitance for yourself. It’s good for the soul  🙂 These two online calculators should help – Resonant Frequency Calculator Coil Inductance Calculator (NOTE – Doug provides more details in the comments section at the end of this post) It occurred to me that a good pair of balanced armature headphones (the type referred to as sound-powered headphones and liked by crystal set enthusiasts) could work well. He replied that he had tried a pair of 2000 ohm headphones and they were rather loud, so he would probably need to incorporate a volume control if he used them. Just imagine that – a pair of headphones being too loud when the only thing separating them from the antenna is a single transistor – and with ham signals too, as opposed to big broadcast signals! Here are some pictures of what Doug’s single-FET regen looks like –

N0WVA’s single-FET regen. View from above. Photo courtesy of N0WVA.

In this side-view, you can see Doug’s home-made variometer tickler, with the syringe body that he used for the control shaft –

Side-view of N0WVA’s one-FET regen, showing the home-made variometer tickler, and control shaft. Photo courtesy of N0WVA.

N0WVA’s one-FET regen. Photo courtesy of N0WVA.

N0WVA’s one-FET regen. Photo courtesy of N0WVA.

View from the other side of N0WVA’s one-FET regen. Photo courtesy of N0WVA.

The front of N0WVA’s one-FET regen, along with the RS headphones that give good sensitivity. Photo courtesy of N0WVA.

Doug made a video of his receiver in action. He says it was a little difficult, as he had to use one hand to hold the headphone to the microphone for recording, while tuning with the other hand. If you can, while watching this, have another window open so you can see the schematic and remind yourself that the stable and well-resolved SSB signals you are hearing are coming from that simple circuit. Great stuff –

I can’t wait to see the 1-transistor transmitter he pairs this great little receiver up with. Thank you Doug, for allowing me to share this info on your regen receiver. This is what ham radio is all about.