Chappy Happy Builds a 25 meter Direct Conversion Receiver using a PAL VFO -- And There Is Distortion of AM SW Broadcast

First, it is a good day when I find a new Chappy Happy project on YouTube.  Thank you Chappy Happy! You make us happy! 

Second, even before I watched the full video, I could recognize the PAL VFO.  This is an old CB device.  You will find them at hamfests.  I picked one up several years ago, knowing that I will someday use it as a VFO or for VFO parts.  It is, after all, an LC VFO, and it is to be celebrated and used.   

Chappy Happy turns his PAL VFO into a simple direct conversion receiver for the 25 meter SW broadcast band.  He is on the west coast of North America and uses it to listen to Radio Japan. He uses a J310 RF amp and an NE602 Gilbert Cell mixer. 

That is great, but there is a big problem in using a direct conversion receiver to inhale AM signals: Unless you can get the frequency AND the phase of the VFO to be exactly the same as the carrier, you will have distortion. You will not have this problem if the signal is SSB, but you will have it with AM or DSB.  

I think I can hear the distortion on the Radio Japan signal.  Can you hear the flutter?  That is what happens with an AM signal being received on a DC receiver.  This is why Doug DeMaw frequently warned that you could not listen to a DSB signal on a DC receiver without significant distortion.  

What do you guys think?  Can you hear the distortion?  Should Chappy Happy turn this receiver into a simple superhet, perhaps something like the Q-31? 

Here is a video of my Q-31 super het for the 31 meter SW band: 

Again, thanks Chappy Happy!  Please keep those projects coming! 

SolderSmoke Podcast 263: DR-PR, UM, DCRX, SKN, Design, PSSST, W7ZOI, FT-101, HW32A, VK, HST, AMP, MAILBAG

Pete Juliano N6QW

February 23, 2026

SolderSmoke Podcast #263 is available for download: 

Audio:  http://soldersmoke.com/soldersmoke263.mp3

Video:  https://www.youtube.com/watch?v=Fn164glxQ6s

Opening 

May 26, 2014, coming up on 12 years! That was the 1^st podcast where Pete N6QW was interviewed by Bill in Podcast #161,  which means 102 podcasts ago. 12 Years of Julian-ismo.  Thanks Pete!  

What to build?  Question faced by VWS Makers Group and by Charlie of RedSummit RF.   Regen?  Test Gear?  Simple oscillator?  

But did you DESIGN it yourself?  What does that even really mean? Barrie Gilbert in Jim Williams' book:  https://soldersmoke.blogspot.com/2025/12/homebrew-you-say-but-did-you-design-it.html

Grayson KJ7UM on a podcast -- Make it your own way.  Copper Clad and Glue!  Manhattan style! "BUILD SOMETHING!:  Tube testers not necessary.  OK to call thermatrons "vacuum tubes."  No ruling on calling them "valves" yet. 

Pete:  

-- When hams were hams -- Turning a Heath monobander into a tribander

-- Simple SSB

-- W7ZOI rigs

-- The Yaesu FT-101

SHAMELESS COMMERCE DIVISION:   Mostly DIY RF!   Patreon!   But no more Amazon.  We do not want to help Bezos make more money. Even if this will cost us.   So please,  consider a donation or a Patreon sponsorship instead.  But no more Amazon through SolderSmoke.  

Bill:   

-- Dominican Republic to Puerto Rico on 2 meters -- status report. 

-- Working Australia on 20 meter SSB in the morning. 

-- The SpiderWeb net. 

-- The Hubble Space Telescope -- an old friend

-- One contact on Straight Key Night (Dean reminded me).  I worked NB1U on 20 meters with QCX from KD4EBM. 

Dean: 

-- The University of Michigan ant the Direct Conversion receiver project.  

-- Boxing up the amplifier.  A tale of woe.  Identifying oscillations.  A QSO with the Dominican Republic: https://soldersmoke.blogspot.com/2026/02/a-qso-with-dean-kk4das-northern.html 

 Travails with other homebrew gear... Trying to find a working transceiver.

AI assisted repair of Bitx40 PA in my Mythbuster 

VWS club station update, tendency of Flex and other SDR makers to not release even basic technical information.

MAILBAG: 

Ian VK3MO  Huge antennas, big signal, friendship with WA3O https://soldersmoke.blogspot.com/2025/11/vk3mo-and-wa3o-brotherhood-of-ham-radio.html

Bob KD4EBM  2 meter propagation info

Todd K7TFC  -- Mostly DIY RF

Mike K6STR  Worked Pete on 40, building for CW and SSB on 2 meters

Grayson KJ7UM    German Avionics,  MMM Origin.  Old Steampunk Homebrew rig: https://soldersmoke.blogspot.com/2025/12/a-very-interesting-old-steam-punk.html   

Phil  W1PJE   Forrest Mims III     Mims's sad denial on climate change. 

Ciprian  YO2DXE         Heard the SAQ Alternator  see: https://soldersmoke.blogspot.com/2025/12/alexanderson-alternator-on-172-khz.html

Tom NS6T:  Very useful azimuth map with grey lines: https://ns6t.net/AzShadowMap/experimental.html

Walter KA4KXX -- 20 meter Direct Conversion Receiver.  FB Walter! 

WN2A Mike   Dos Equis man is BACK! https://soldersmoke.blogspot.com/2026/01/hes-back-dos-equis-man-most-interesting.html

KB4HG Rhett:  On USB on the Old Military Radio Net with a PRC-74.  Want one! 

"Homebrew you say? But did you DESIGN it yourself?"

Raymond A. Heising  (1922)

That is a question we get fairly regularly when we tell the other fellow that our rig is homebrew.  I often get the feeling that the question stems from a certain insecurity -- the guy who asks it may feel a bit insecure because the "rig" he is running is completely commercial and his only role in its production was to flash a credit card number.  

But lately I have been reading through Jim Williams' wonderful book "Analog Circuit Design -- Art, Science, and Personalities" and I can see that there may be something to this question. 

It was the chapter by Barrie Gilbert that made me think more about this. Barrie is the legendary designer for whom the Gilbert Cell is named.  This circuit is at the heart of the NE602 chip that many of us used to build our first "Neophyte" receivers and other homebrew rigs.  Barrie's chapter is entitled "Where do Little Circuits Come From."  Uh oh. 

Barrie grew up in the post-war United Kingdom.  He father had been killed in a German bombing raid. As a kid, he built crystal radios and, with his brother, "shortwave sets" on softwood bases.  He used a TRF receiver that employed Manhattan-style construction.  Barrie, it seemed, was one of us.  

But then, he suddenly seemed more advanced.   He wrote:  

"Later, I began to build some receivers of my own but stubbornly refused to use circuits published in the top magazines of the day, Practical Wireless and Wireless World.  Whether they worked as well or not they had to be "originals" otherwise, where was the satisfaction?  I learned by my mistakes but grew to trust what I acquiered in this way:  it was 100% mine, not a replication or mere validation of someone else's inventiveness."  

Wow, that is certainly hardcore.  I will note,  however,  that in getting back to the the question about whether I have "designed" the rig myself, I have NEVER had the questioner come back to say that HIS rig was homebrewed from HIS OWN original design.  Never.  Not once. 

And I will note that building a rig from the schematic is an enormous challenge.  It is not easy.  It is not the mere replication of someone else's inventiveness.  Anyone who thinks it is easy should try to homebrew a simple direct conversion receiver.  They will discover that it is NOT easy. 

I guess this comes down to what we mean by "homebrew."  I prefer to stick to the old ham radio meaning of the term:  It is homebrew if it was built at home, even if it is built from a schematic done by someone else. When Jean Shepherd built his Heising Modulator, was he working off a schematic from a ham radio magazine?  He almost certainly was.  But he gathered the parts, laid out the chassis, and put the circuit together.  Most importantly, when trouble cropped up, he was able to step in and make the needed corrections.  Was his modulator "homebrew?" Of course it was.  Did he design it himself?   No, his name was not Heising! 

More than 100 people built our SolderSmoke Direct Conversion Receiver.  We resisted pressure to turn this project into a kit.  The folks who built it worked off schematics that we had prepared.  They gathered the parts and built their own circuit boards, Manhattan style. They struggled to get the whole thing to work, to make sure the VFO was on the right frequency and at the right level, that the AF amplifier was not oscillating.  Were these receivers "homebrew?"  Of course they were. 

Jim Williams warned that Analog Circuit Design was "A wierd book."  He strongly discouraged collaboration between the authors, and noted that this would probably result in "a somewhat discordant book."  We see that discord in the hardcore position taken by Barrie Gilbert.  Many of the other designers seem to take a more flexible, less austere position.  Some even seem to downplay the role of mathematics.  

I think Barrie had a right to be proud of his fundamentalism.  But not all of us are capable of that.   Writing in Jim Williams' book,  Samuel Wilensky sums it up nicely: 

"I classify analog designers into one of two categories.  There are those who do truly original work, and these I consider the artists of our profession.  These individuals, as in most fields, are very rare.  Then there are the rest of us, who are indeed creative,  but do it by building on the present base of knowledge."   

Jim Williams -- Analog Man -- Book Review: "Analog Circuit Design -- Art, Science, and Personalities"

 

Jim Williams at his bench.  Note the mess. 

The Bob Pease book that KD4EBM gave me led me to the Jim Willams book entitled Analog Circuit Design -- Art, Science, and Personalities.  I'm only about a quarter of the way through it, but I can already tell that it is great.  Get this book.   Make room on your shelf.   

Jim was the editor, and it is a collection of contibutions made by a many different analog luminaries.  Curiously, none of the bios show that there are any hams among them (but the articles of many of them seem to hint at ham radio backgrounds).  MIT shows up a lot in the bios.  Jim notes in the very first line of the preface that "This is a weird book."   He talks about how it came together -- he met with the contributors and each of them pledged NOT to consult with the others about what they would write.  Jim notes that the result is "a somewhat discordant book," that "Hopefully would lend courage to someone seeking to do analog work." " The single greatest asset a designer has is self knowledge."  "Take what you like, cook it any way you want to, and leave the rest."  Indeed. 

I found that Jim's own contributions were among those that I liked the most. He writes about "analoggery" and "digital fakery"  but then acknowledges that this is a "good natured" controversy.  He notes that "no true home is complete without a lab" (a shack?) and that "no lab is complete without an HP series 200 oscillator."  His bio reveals that he lived in California with his family and "14 Tektronix oscilloscopes."   In a chapter entitled "Should Ohm's Law be Repealed?" Jim describes the very early influence of a neighbor, Dr. Stearn,  who owned a Tek 535.  It allowed them to see into circuits:  "You knew the excitement Leeuwenhoek felt when he looked in his microscope."  But that was not always enough:  Jim tells how Stearn once successfully troubleshot one of Jim circuits simply by running moistened fingers over circuit while watching the scope. 

Tom Hornak also really struck a chord with me. He writes of things that happened in the year "10 BT" (Before Transistors).  He talks about how he and a childhood friend had trouble understanding the differences between voltage and current.  "We found someone who knew the right answer, but he did not help us too much. Instead of using a simple analog such as a phasor diagram, he started to talk sine and cosine. We accused him of not knowing the answer either, and covering up his ignorance my muttering mumbo-jumbo."  Tom explaines:  "I know that trying to 'understand electricity' early in life had a lasting benefit to me. I got used to 'seeing electricity' in analogs and I am still seeing it that way. I believe every electronic circuit designer could benefit from thinking in analogs, and it is never too late to start. This belief made me write this chapter."  

Barrie Gilbert -- the man credited with inventing the Gilbert Cell (the heart of the NE602) -- has a chapter in the book.  He writes of circuits "laid out Manhattan-style" and "built on softwood bases." He hombrewed a very early TV receiver. He tested AF amplifiers "by placing a finger on the grid of the first tube."  (We recommended something similar with the SolderSmoke Direct Conversion Receiver, but some builders seemed not to believe that this would work!)    

It is undoubtedly a tragedy that we lost both Jim Willams and Bob Pease in June 2011. Jim died of a stroke at age 63; Bob died of a possible heart attack or stroke while driving home from Jim's memorial service. But here we are in 2025 still talking about their work and their books. In a certain sense they live on through their writing.  This is a lesson and an inspiration for those of us who sometimes get a bit down by the vagaries of AI and the algorithms:  We never know when -- perhaps long after we are gone -- someone might come across something we have written and find inspiration there.  

Three cheers for Jim and Bob.   

https://soldersmoke.blogspot.com/2025/10/troubleshooting-analog-circuits-by-bob.html

https://www.jameco.com/Jameco/workshop/inthenews/pease.html?srsltid=AfmBOoqCC_QLhohtE3--OhGRLuQ0kQDyAq0rDsRhkdQDeIeZmGZs7nvN

https://www.edn.com/analog-engineering-legend-bob-pease-killed-in-car-crash/

Notes from an Australian SA612 Enthusiast

One of the first articles that I put on this blog was an NA5N post about the SA-612 -- this was originally posted on December 2, 2006: https://soldersmoke.blogspot.com/2009/06/na5n-on-ne602.html  Today our friend Paul VK3HN today noted the end of production.  The SA-612 is no longer being produced, but it is not forgotten, and there are a lot of them still around.  Thanks Paul.   

One thing I would add:  It took me a while to get my head around the Gilbert Cell mixer that is the heart of the SA-612.  Here is how I came to understand it: 

https://soldersmoke.blogspot.com/2021/11/how-to-understand-ne-602-and-gilbert.html

Paul writes: 

The end of a four decade life, the NXP SA612 (in SOIC8) ceased production and the final delivery date for orders placed by the last-time-buy deadline was August 18, 2023.

The SA612 truly is a remarkable chip. It's as if The Radio Gods got together back in the early 80s and said, 'what integrated circuit can we bring into the radio world that will supercharge homebrew radio and help enthusiasts to build their own rigs?'. And after the wise council settled on a combined oscillator-mixer in a single DIP-8 package, the rest is history
.
The 602/612 launched dozens of DIY radios and even businesses, and the careers of the QRP pioneers, from Doug DeMaw to Wayne Burdick to Dave Benson. I'll bet even Wes Hayward built a few.

SA612 discussions on Groups.io lists such as Qrp-Tech ran for decades and continue to attract attention and raise new insights today.

Even Elecraft with all of their digital radio know-how and resources launched the KH1 hand-held CW rig into production with SA612s in its receiver as late as 2023.

It really is an almost ideal part for simple homebrew receivers and transmitters. A 200MHz mixer with balanced inputs and outputs, an on-chip stable LO that can be used in three ways - a crystal BFO, an LC (or varactor) VFO, or a buffer for an external oscillator or clock such as the si5351 digital PLL/multisynth. Up to 15dB conversion gain which mostly eliminates the need for IF gain. Able to be gain-controlled with a DC AGC applied to its input. And did I mention low current?

Its only weakness is in its mixer dynamic range and strong signal performance but if you put a basic BPF in front, for simple homebrew rigs at home or portable/field rigs, this hasn't really been a problem.

Of course there are other mixers that outperform the 612 but some consume much more current, and none have the built-in oscillator.

Vale the SA612. You served, and continue to support the homebrew community like no other chip. You will not be forgotten.

Fortuitously, eBay is full of $5 a piece NOS parts on tape, which pretty much guarantees they are not fake. And at this price, for all you get, they're still great value!


Paul VK3HN.

Confirmed card-carrying SA612 fan-boy.

SolderSmoke FDIM Interview with Keith W. Whites -- Teaching Electronic Design to EE students using a QRP Transceiver designed by Wayne Burdick

When I first listened to Bob Crane's interview at FDIM with Keith Whites, I thought of the book "The Electronics of Radio" out of CalTech by David Rutledge.  Keith Whites told me that the difference between his effort at University of Kentucky was that Rutledge's course was designed for freshmen at Cal Tech, while White's course was aimed at Juniors and Seniors.  

I told Keith Whites that I had struggled to understand the Gilbert Cell and the NE602, the device that lies at the heart of the rig used in both courses:  The NE-602 Gilbert Cell Mixer used in Wayne Burdick's NORCAL 40A. Here is how I came to understand the device:  https://soldersmoke.blogspot.com/2021/11/how-to-understand-ne-602-and-gilbert.html

Here is Bob Crane's interview:  http://soldersmoke.com/2022 Whites.mp3

Here the slides that Keith used at FDIM: http://soldersmoke.com/2022 Teaching NorCal40A.pdf

Keith's students obviously got a lot out of this course.   Keith has kindly offered to make his course notes available to those who need them. 

Thanks to Bob Crane, Keith Whites, David Rutledge and Wayne Burdick. 

Barrie Gilbert and Tinkering, Surplus, and the Visceral Experience of Electronics

 

Barrie Gilbert, 1951

From https://www.edn.com/analog-back-to-the-future-part-two/  :

"Gilbert believes that childhood hardships—including at age three losing his father in World War II, leaving his mother and three other children penniless—force one to be resourceful. Before and during his teenage years, he had access to a plethora of inexpensive military surplus gear which greatly helped to make him inventive. Gilbert laments that today's aspiring engineers are lacking the visceral experience of handling and hefting large coils and tuning capacitors, transformers and vacuum tubes, and such. Today’s surplus circuit boards are all but useless as a source of inspiration, or even “spare parts” to tinker with."

-----------------------------

A really wonderful autobiography by Barrie Gilbert starts on page 7 of this page: https://hephaestusaudio.com/media/2009/06/the-gears-of-genius.pdf 

Putting a Barebones Superhet on 17 Meters with an NE602 Converter (Video)

Armed now with a NanoVNA, I took a look at the passband of the 5 MHz filter in my Barebones Superhet (BBRX)  W4OP built it on a Circuit Board Specialist Board.  He put a 5 MHz CW filter in there;  I broadened the passband for phone by changing the values of the capacitors. Here is what the passband now looks like in the NanoVNA: 

This is what DeMaw would call an "LSB filter."  You would get much better opposite sideband rejection by using it with an LSB signal, placing the BFO/Carrier Oscillator slightly above the passband, in this case near 5.002 MHz. 

When I first built the down converter to get the 18.150 MHz signal down to the 7 MHz range (where I had the receiver running) I used an 11 MHz crystal for the NE602's local oscillator.  But this created a big problem:  18.150 - 11 =   7.150 MHz.  That is in the 40 meter band, but note:  NO SIDEBAND INVERSION.   Then in the BBRX  7.150 MHz - 2.150 MHz = 5 MHz  (the filter frequency) but again:  NO SIDEBAND INVERSION.   The signal started as a USB signal and remained a USB signal. 

I briefly tried shifting the BFO frequency to the other side of the filter passband.  If I could get it to around 4.985 MHz, it might work, but because the filter passband was so large, and because the crystal frequency was so low, I was unable to shift the crystal frequency that far.  In any case the results would have been less than ideal because of the "LSB" shape of the filter.  Back to the drawing board. 

I decided to cause one sideband inversion. 

At first I put a 25.175 MHz crystal module in my down converter.  This shifted the 17 meter phone band down to the 40 meter CW band.  It worked, but I cold hear strong 40 meter CW  signals being picked up by the wiring of the receiver (the box is plastic!).  I went back to the module jar in search of frequency that would move 17 meter phone to the 40 meter area (so I would not have to re-build the BBRX front end) but outside the actual 40 meter band.  

I ended up using a 25 MHz crystal in the down converter. 25 MHz - 18.150 MHz = 6.85 MHz WITH SIDEBAND INVERSION.  After checking on the NA5B Web SDR to see that there are no strong signals in the 6.835 to 6.89 MHz range, I retuned the output circuit on the converter and tweaked the input capacitor on the Barebones.  I shifted the VFO frequency down to 1.835 to 1.89 MHz and put the BFO at 5.002 MHz.   The receiver was inhaling on 17 meter SSB.  

One more change to the BBRX:  in his June 1982 QST article, DeMaw warned that trying to get speaker level audio out of the 741 op amp that he used would result in audio distortion.  And it did.  So I put one of those little LM386 boards I have been using into the BBRX box.  I just ran audio in from the wiper of the AF gain pot.  It sounds good.  

In effect this is my first double-conversion receiver.  I usually prefer single conversion, but this project has highlighted for me one of the advantages of double conversion for someone like me who eschews digital VFOs:   Starting with a crystal filter at 5 MHz,  with double conversion I could keep the frequency of the LC VFO low enough to ensure frequency stability.  That would have been impossible with a 5 MHz IF in a single conversion 17 meter rig.  But if I were starting from scratch for a 17 meter rig, I could stick with single conversion by building the filter at 20 MHz,  keeping the VFO in the manageable 2 MHz range. 

Now, on to the SSB transmitter.   The Swan 240 dual crystal lattice filter from the early 1960s needs some impedance matching. 

How To Understand the NE-602 and the Gilbert Cell Mixer

 

I think the key to understanding the Gilbert Cell Double Balanced mixer is to separate out the three tasks that this device completes, and consider them one at a time, using different diagrams: 

1) It mixes two signals to produce sum and difference outputs. 

2) It balances out the RF input. 

3) It balances out the LO input. 

                                                                  Task 1 -- Mixing

The Gilbert cell is like the diode ring mixer in that it switches the polarity of the input signal at a rate set by the Local Oscillator. Another way of saying this is that the mixer multiplies the input signal by 1 and by -1. 

Steve Long of the University of California described the essence of this mixing this way (using the diagram above): 

 

An ideal double balanced mixer simply consists of a switch driven by the local oscillator that reverses the polarity of the RF input at the LO frequency.  http://literature.cdn.keysight.com/litweb/pdf/5989-9103EN.pdf

In an effort to see this for myself, I drew (noodled!) this diagram: 

There are four transistors -- two differential pairs with RF coming into the bases of the pairs. 

The LO is a square wave.  The LO alternately turns on transistors 1 and 4, then 2 and 3.  When 1 and 4 are on, we are in period 1 -- here there is no switching of polarity.  Portions of the RF waveform are passed to the outputs.  But when the LO turns on transistors 2 and 3, portions of the RF wave form are "crossed over" to the opposite output.  Polarity is reversed.  We see this in period number 2. 

Take a look at the resulting output waveforms.  This is the same waveform we see coming out of a diode ring mixer.  I really like this drawing because in that complex waveform you can actually see the sum and difference frequencies: 

I could see this diode ring waveform myself on my oscilloscope: 

https://soldersmoke.blogspot.com/2020/11/diode-ring-magic.html 

TASK 2 -- Balancing Out the RF Input 

In a diode ring, and in other diode mixers, the balancing out of the input signals really takes place in the trifilar toroidal coils that are part of the circuit.  Barrie Gilbert needed an integrated circuit mixer that did not use coils.  

Again referring to the above diagram, Steve Long of the University of California put it this way: 

The ideal balanced structure above cancels any output at the RF input frequency since it will average to zero.

To fully understand this I find it helps to look at the Gilbert cell circuit drawn in a different way.  Here is a drawing from Alan Wolke W2AEW that I found very helpful. It comes from his excellent YouTube video: https://www.youtube.com/watch?v=7nmmb0pqTU0

Suppose the RF waveform at I1 is causing the current through R1 and R2 to increase.  At the same time, the opposite phase current through I2 will be causing the current through R1 and R2 to DECREASE.  So there is no net effect of the RF signal at the output.  The RF is balanced out. 

TASK 3 - Balancing Out the Local Oscillator Signal 

Here too I used my own drawing, and was guided by the words of Steve Long: 

It also cancels out any LO frequency component since we are taking the IF output as a differential signal and the LO shows up as common mode.  

The important thing to realize here is which transistors are being turned on and off by the local oscillator signal.  On one half cycle of the LO, transistors 1 and 4 are on.  So  the LO signal at the LO frequency are both pulling the same amount of LO frequency current through the resistors. So you have the same change in voltage at the output terminals.  And the output terminals are differential.  The LO signal results in no voltage difference between the terminals.  So the LO frequency is balanced out. 

The same thing happens on the following half of the LO cycle.  Here, transistors 2 and 3 are turned on. Again, both transistors pull the same amount of LO frequency current through the resistors. There is no differential voltage.  So no LO frequency energy passes to the output.  LO frequency is balanced out. 

--------------------------------

I am surrounded by Gilbert Cell Mixers and I have been using them in my homebrew rigs for many years. I use them in up-converters for my RTL-SDR receivers.  I have one in the downconverter for my 17 meter receiver and had one as the mixer in my first SSB transmitter. I built a 40 meter SSB transceiver with NE602s on either end of the crystal filter. Years ago, I built a DSB transceiver with several NE602s.  My SST QRP CW transceiver is made with NE602s. I have on my bookshelf Rutledge's book "The Electronics of Radio" that is all about the NORCAL 40 transceiver, built using NE602 chips.  But until now I really didn't know how these chips worked.  Truth be told, for me they were mysterious little black boxes, and that bothered me.  Now I feel a lot better about using these clever devices.  I plan on stocking up on the old style (non-SMD) NE602s.  

Apparently Barrie Gilbert rejected the idea that he invented the circuit that bears his name.  It seems that Howard Jones first used this circuit in 1963, with Gilbert developing it independently (in an improved form) in 1967. 

 Barrie Gilbert was quite a guy, with electronic roots in the world of tinkering: 

https://www.microwavejournal.com/articles/33425-living-and-learning-with-barrie-gilbert 

https://vintagetek.org/barrie-gilbert/