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3rd Jul 2022, 9:39 pm | #1 |
Tetrode
Join Date: Mar 2022
Location: Chippenham, Wiltshire, UK.
Posts: 69
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Distortion in Regenerative Receivers
I hope this is the correct place to post this question.......
I have built a number of valve based regens using the likes of DF91, DL91/93, 6J7, 6K7, EF36/9, 6SN7, 6SL7, 12AT7, 6J5 etc. and on the whole I have found that triode based receivers do not offer quite such good results in terms of sensitivity. Many of these sets have used grid leak detection and whilst most have provided good results given the design limitations, none seem to cope well with music due to distortion. I attach two examples, both made recently the first using EF41s, and the second 6K7/6J7 these are from RC, January 1954 and November 1947 they both offer reasonable sensitivity, pretty good selectivity and are OK for speech, but defeated by music. Having done a good deal of reading on the subject I "sort of grasp" why distortion is inevitable in this type of receiver. However, my limited understanding of receiver design principles (not to mention mathematics) naively prompts me to ask if some designs are better than others and is there any way of improving results or am I simply wasting my time and should stick to superhets ? |
3rd Jul 2022, 10:19 pm | #2 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
Hi.
First of all, I'm not an expert on this, however, as you are probably aware, a regenerative detector is by its nature very non linear, also, they are easily overloaded on strong signals. If you interest is broadcast listening, a well designed straight TRF, with a nice linear detector is a revelation in quality, if you have a good off air signal. Not built by me, but I did experience a home brew TRF tuner with an infinite impedance detector that was based on a design from here: https://www.cool386.com/4valvetrf/4vtrf.html |
4th Jul 2022, 3:52 am | #3 |
Pentode
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
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Re: Distortion in Regenerative Receivers
You have answered your own question. Leaky grid detection and regeneration (reaction) dates back to the 1920s when they we're much more concerned with amplification than distortion (not surprising when the horn loudspeaker was the output device).
If you don't want distortion then you would not use those designs, because distortion is built in. Superhet designs (starting from the early 1930s) paid much more attention to minimising distortion. No leaky grid detection or regeneration was ever employed to my knowledge in a superhet circuit. I restore and listen to 1920s TRF sets, and I expect distortion to be present especially on music. To me it is part of their charm and part of their vintage design, especially coupled with a horn loudspeaker. |
4th Jul 2022, 8:08 am | #4 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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Re: Distortion in Regenerative Receivers
I did some work last year building a superhet radio which used both a leaky-grid detector AND an infinite impedance detector!
The LGD was used to derive AVC control voltage, but I did evaluate its linearity, using a ramped RF generator. See first photo, where the output trace is superimposed on the input envelope. It's clear that it isn't very linear, but as has been pointed out, it works quite well! On the other hand, the IID is far better (second photo) Quote:
Photos are from notes. Pentodes were used for both - actually directly-heated types, which required a bit of morphing of the circuit (plus a high negative supply rail) to realise the IID. |
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4th Jul 2022, 3:32 pm | #5 | |||
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
Quote:
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Not sure of the valve type used, knowing the YL who constructed it, it was probably the designers first choice of EF80's: there's half a tea chest full of them in my cellar at home even now that belong to her, I think they breed! I expect the IID wasn't favored by set makers for commercial superhet radios because they saw it as an uneconomic design. |
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4th Jul 2022, 4:25 pm | #6 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
Quote:
I used it for SW listening, AM, CW and SSB. Lawrence. |
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4th Jul 2022, 5:05 pm | #7 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
So, using the mixer as band set, and the tuning of the IF as band spread?
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4th Jul 2022, 5:15 pm | #8 |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
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Re: Distortion in Regenerative Receivers
I’ve come across one or two examples of the ‘All American Five’ superhet where the compact design has resulted in some regeneration in the IF stage caused, I guess by stray capacitance coupling. This of course can be beneficial to the set’s sensitivity, helping it to function on just a short wire aerial. I often wonder whether this regeneration is a deliberate design feature incorporated in the tight component layout, or is it merely a happy accident?
Martin
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4th Jul 2022, 6:14 pm | #9 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
Quote:
To set the RF sig. gen. spot on I used the harmonics from a Xtal marker that radiated out from a Murphy B40D receiver that I had close by. The receiver had an RF amp, it's grid and the mixer signal grid was tuned with a twin gang tuning capacitor, so basically set the oscillator then peak the RF and tune the detector. The plan was to use a SLF tuning capacitor for the detector tuning and a suitable scale with equal graduations but I never got round to it. Have to say it was the best performing "regen" that I ever built. Lawrence. |
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4th Jul 2022, 9:13 pm | #10 |
Tetrode
Join Date: Mar 2022
Location: Chippenham, Wiltshire, UK.
Posts: 69
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Re: Distortion in Regenerative Receivers
Thanks everyone for your comments and for confirming my understanding.
My question was not quite as dumb as it might seem, my father (born 1893) was a radio engineer by profession and as a boy in the 1950s I had access to a number of 1920s/1930s commercial regens which he acquired when they became obsolete and unwanted after WW2. My recollection is that music from these sets sounded just fine, if "warmer" in tone than we would expect these days. But I have recently come to suspect that this is because I had no reference against which to judge fidelity. Clearly I am not "missing a trick" and so my query has been answered. G Castle - thank you for your kind recommendation, I have visited this site a couple of times and its interesting, may give this one a go I think as I have plenty of EF41s that would sub quite well for EF80s I'm hoping. Hartley 118 - I recently restored my mothers Roberts RP4, which had laid unused since the 1980s. It was always super sensitive on SW, the reason I discovered when I restored it is that it is on the edge of regeneration over much of the band. I feel certain this is not a design feature but it does make for a great little receiver . |
4th Jul 2022, 9:26 pm | #11 | ||
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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Re: Distortion in Regenerative Receivers
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4th Jul 2022, 10:00 pm | #12 | ||||
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
Quote:
Quote:
I'm custodian of several sets that wouldn't be quite as good receivers as they are, were it not for a bit of unofficial regeneration as E93AFAN says of the RP4... |
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5th Jul 2022, 10:42 am | #13 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
Quote:
I picked up on the idea from Gary Johanson. Lawrence. |
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5th Jul 2022, 12:04 pm | #14 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
Thanks Lawrence!
I'll download it and have a look when I'm next at the PC. It's unusual to find a regen circuit that the tunning is steady at threshold of oscillation for CW particularly. Cheers! Greg. |
5th Jul 2022, 12:22 pm | #15 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
Attached is a link to another schematic, in this one I was experimenting with a harmonic generator to set the band at 1MHz intervals, again I was limited to the components I had to hand so the component values etc might not be ideal but the concept did work, unfortunately due to a change in personal circumstances I wasn't able to develop it further and put it all on a nice neat chassis and put it in a case.
Lawrence. |
5th Jul 2022, 5:36 pm | #16 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Distortion in Regenerative Receivers
Thanks again Lawrence. I've now had a look at the schematics, the penny has now dropped regarding the harmonic generator, (circuit "4").
Also noted that you're fond of designing with the metal envelope valves! The audio stage, (my comfort zone of understanding), seems a very sensitive design, with the 6F6 preceded by the triode voltage amp. ISTR that the 6F6 was often used in commercial designs straight after the detector as it had sufficient gain. Thanks again, most interesting. |
5th Jul 2022, 5:59 pm | #17 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
No problem.
Lawrence. |
19th Jul 2022, 11:09 am | #18 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Distortion in Regenerative Receivers
This was also known as the "Super-gainer" design; basically a frequency-changer followed by a fixed-frequency regenerative detector; I built one of these for 80 and 40 Metres using an EF39 RF amp, ECH35 frequency-changer, EF39 regen detector and 6V6 output.
IF was 1.6MHz, and the front-end used Denco coils. A low-value [50pF] tuning-capacitor padded out allowed a good tuning-rate on 3.5 and 7MHz; The big thing about it compared to traditional regenrative-detectors was that I could optimise the regeneration so it slipped gently into oscillation [letting me resolve CW/SSB] and there was much reduced 'swaying-antenna-causing-the-tyuning-to-vary' or hand-capacitance effects compared to a traditional regen. By their nature, regenerative detectors are never going to be low-distortion devices; detection is after all a 'kind' of distortion; when combined with amplification in the same valve all sorts of odd interactions occur. [ Thought: has anyone ever tried separating the detection and regenerative-amplification functions? I'm thinking here of a regenerative RF amplifier feeding into a traditional diode-type detector? ]
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19th Jul 2022, 12:54 pm | #19 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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Re: Distortion in Regenerative Receivers
I seem to remember John Scott-Taggart's overly-complex circuits where he wrapped regeneration around RF amplifiers, and also around leaky-grid detectors, in the same circuit, each controlled by its own knob... So you could, if you wanted, set detector regen to zero and RF regen to quite a lot, to do what you ask (except it wouldn't be a diode detector).
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19th Jul 2022, 1:58 pm | #20 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Distortion in Regenerative Receivers
My last Regenerodyne did that by controlling the g2 voltage on the RF Amp, sharpened up the RF response nicely.
Lawrence. |