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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th May 2020, 6:23 pm   #1
G6Tanuki
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Default Thoughts on Regenerative Receivers.

Thoughts on Regenerative Receivers.

The idea that a handful of components could bring in stations from all over the world always fascinated me - I built my first one of these, using a 1S5 battery-valve - when I was about ten; my father wouldn't let me use homebrew mains-powered stuff but figured I probably couldn't kill myself with a 67.5V HT-battery.

Over time I played around with different designs - some valve-based using the likes of EF91s, 12AT7s and the like, later progressing to JFETs (2N3819) and dual-gate MOSFETs (40673) - with varying degrees of success.

All the designs were basically a "grid-detector" livened-up by positive-feedback - generally
dating back to the 1920s/early-30s. Back then there were always exhortations about 'not
oscillating' - indeed Mr. Eckersley, chief engineer of the BBC, regularly wrote on the matter.

That was fine back in the broadcast-radio 20s and 30s, but I was interested in hearing ham-radio operators on 3.5/7/14MHz, who were using CW and SSB, for which you _need_ your regenerative receiver to oscillate - in a controlled way - so there would be something to replace the carrier that was not transmitted. So getting your regenerative radio to slide smoothly into oscillation, and stay on-frequency, was paramount.

Regen-receiver users were invariably frustrated by issues like hand-capacitance, the regeneration being 'ploppy', frequency-wandering, or hum-modulation.

I too suffered these troubles. Then, in the mid-70s I read an article in a US ham-radio magazine which led to a major thought-shift. This article said that - since a SSB/CW regeneratve receiver would be expected to oscillate all the time, it was no longer a 'grid-detector-with-positive-feedback'.

It was now a self-oscillating mixer, as used in superhets, but converting to a 'zero IF'!

And you needed to design the thing as if you were designing a high-stability VFO!

This was a revelation: and it worked! I started doing things like using loads-of-capacitance-across-the-tuned-circuit (to swamp variations in valve/transistor internal capacitance), regulating supply-voltages and doing all I could to minimise DC resistance in the [voltage-variable] supply. Scrapping the 'grid resistor' [or gate-resistor] and using source/cathode-bias helped too; with proper design I could get the oscillator to stay within a few hundred Hertz of its design frequency for hours on end.


[more - including some circuits -to follow].
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Old 28th May 2020, 8:27 pm   #2
G6Tanuki
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Default Re: Thoughts on Regenerative Receivers.

Part 2: aSide-note on "Super-Gainers".

Some of the issues associated with short-wave regenerative receivers - antenna-sway/hand-capacitance detuning, on-frequency radiation when oscillating, difficulty getting consistent regeneration when tuning across wide frequency-ranges - were to some extent addressed by the idea of the 'super-gainer'.

This was an arrangement using a local-oscillator-and-mixer front-end, superhet-style, but rather than feeding an IF-amp/detector it fed a regenerative detector. The regenerative stage worked at a fixed frequency (so could be optimised for L:C ratio in its tuned-circuit, and grid-leak time-constant) and because it was no longer directly connected to the antenna or oscillated at the signal-frequency, issues of antenna-sway causing frequency-change, or annoying people with on-frequency radiation, went away.

But you were now presented with two oscillators: the free-running front-end local-oscillator still needed to be designed for minimal drift (difficult if you want it to sit within 100Hz for hours when listening to 14MHz SSB) and there was still no practical gain-control (a big problem in the 60s and 70s on 7MHz when you might be trying to listen to a 50-Watt amateur station when there was Radio Moscow blasting you with Megawatts only 10KHz away).

The "Super-gainer" was a cute but ultimately-doomed approach when built using 1950s/1960s designs. I once had a crazy idea to build a "narrow band" tunable-IF super-gainer, based on an optimised regenerative receiver tuning 3.6-3.8MHz and then sticking crystal-controlled converters for 7 and 14MHz in front.

A nice idea, but thankfully before I built it I realised that the [oscillating, remember we're receiving SSB here] tunable-IF's second harmonic would tune across the frequencies of the crystal-controlled converter!

Another side-note: during WWII there were numerous British RADAR receivers that used the "Superregenerative Superhet" approach up to UHF: the idea being to utilise the high gain of a superregenerative receiver but *not* to have it running on the actual RADAR frequency, rather downconverting your pulses from hundreds-of-MHz by way of a crystal-diode mixer to something like 30MHz where your super-regen could do its stuff without radiating too much 'hash' that the Luftwaffe could use to home-in on you!
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Old 28th May 2020, 8:46 pm   #3
ms660
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Default Re: Thoughts on Regenerative Receivers.

I used a standard bench sig gen for band setting in my lash up Regenerodyne receiver.

Must say it performed very well and was very stable considering.

Excuse the scrawl and any errors.

Lawrence.
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File Type: pdf regenerodyne3.pdf (447.7 KB, 79 views)
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Old 30th May 2020, 4:50 pm   #4
Bazz4CQJ
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Default Re: Thoughts on Regenerative Receivers.

The first receiver I built was a two-valve TRF design published in PW around '62. I'd been given the "Blueprint", but without the corresponding magazine and the article that explained how it worked. It worked extremely well (a pair of 1T4's, DF91's).

It was only many years later that I found the full magazine article on the internet, and on reading that, it occurred to me that although it had worked well, if I'd had the full article at the time, I might well have got it to work even better.. I stripped it down long ago... but all the parts are in a box . I had started with it on the 31m broadcast band, but then got it on to 80m where it pulled in amateurs using modestly powered AM rigs from all over the UK. No electronic noise back then!

So, Mr Tanuki, what would a 2020 receiver look like?

B
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Old 30th May 2020, 5:35 pm   #5
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Default Re: Thoughts on Regenerative Receivers.

I used to really enjoy building one valve radios. The first one was an EF91 and a Repanco DRR2 coil. The heater was powered from a 6.3v transformer salvaged from a jumble sale box of junk. [Those lovely boxes of electrical junk for 3d old money]H.T. was a B126 90v battery. It really worked well on around 50ft of aerial.

The best one ever employed a 4 volt ML4 pre-war indirectly heated triode and a home made frame aerial. It was a real belter and you could follow a whole programme with the headphones laying on the table.

The most consistent circuit returned the cathode to a couple of turns above earth on the aerial coil and reaction was controlled by a pot on the screen grid. [Electron coupled?]

I think the secret of success is to run the detector from no more than around 50V. This gives smooth reaction and a good overall results.

Not worth bothering today. Nothing on MW & LW worth listening to [in my opinion of course] Very Happy Days and I might just build that ML4 version again sometime..John.
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