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The Frank Jones supergainer
In Tube Radios
dayleedwards
Jun 26, 2024
Yes Larry, that is exactly it. You will find with a "good" regen receiving an AM signal, the selectivity and volume will increase as the regeneration is advanced, Doing this very slowly and tuning up or down slightly from the center frequency, the audio w ll become increasingly muffled as the sidebands are progressively attenuated with the added selectivity. As the feedback is further advanced a good regen will then start to become very noisy, There seems to be a point where the received signal is no longer able to be amplified any more, the limiting effect I have mentioned, but the noise being of lesser amplitude can be, and becomes a greater percentage of the total signal. A good regen will then slide gently into an oscillating mode. With even more careful tuning, you can hear the regen actually being "kicked" into oscillating on audio peaks, this is called ringing. Very careful tuning is needed to hear any of this. Once it begins oscillating, everything changes. Sadly for us though, the regeneration curve is is very non linear, the closer to oscillation we get, the more rapidly it approaches oscillation.... a logarithmic response curve.... making fine adjustments very difficult .. I call this the negative void, the RMBK Chernobyl curve. I also think the AGC exhibited by a regen and reported by many users is also caused by the above limiting behaviour.. There are LOTS of unexplained things going on, for example, and as you mention yourself, why some sets work better than others, when in reality they all work on identical principles, and why each one must be fiddled individually for best performance, even two identical sets will have quite significant behavioural differences, not always explained by component tolerances. I remember when very young, I made radios from magazine articles of the time and had a low success rate, one disappointment after another, until I accidentally found that they all needed to be "mothered" and cajoled into action, rather like a lazy teenager that wont mow the lawns... Another interesting find, was that the simpler the circuit, the more "squirrelly" it was likely to be. But then, thats the fascination of these things that brings us together.... we are chasing superheterodyne performance with just a few components. Thanks also for the kind words.
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The Frank Jones supergainer
In Tube Radios
dayleedwards
Jun 25, 2024
Probably not. With any oscillating regen, if you tune too close to the AM carrier, it will pull it into synchronisation. If an SSB signal, it will pull and release at the modulation frequency, giving a "wobbly" audio. So it is a common issue. The more powerful the oscillation, the less an outside signal will affect it. A similar effect was used in early TV sets, where the horizontal oscillator frequency was locked via the synch separator circuit. Early valved TV circuitry makes for good study because EVERY cost cutting method was used and some really clever ideas were used just to save a few pennies. It is quite astonishing just how easy it is to synch an oscillator, and just how hard it can be to prevent, By looking at the metronome synching videos on youtube can give a very good idea of this. Even humans and plants synchronise to daylight hours, a process that takes weeks or even months to reprogram. Frequency locking can also happen at sub multiples, higher or lower of the fundamental. I think what happens, is when the two frequencies roughly match, the phase difference becomes important. So if they are 180 degrees apart, each signal is unaware of the other, so have no effect. When the phase angle changes slightly, the external oscillator or signal will either begin to help, or hinder the oscillator, advancing phase will add, retarding will subtract from the slight bit of energy influence. ... until lock is eventually achieved. That is why harmonic locking works, but is less effective as the harmonic number increases, the added influence happens every other cycle, every third or fourth or even 50th cycle, with progressively less influence as the number increases. Fortunately when resolving AM, a regen when oscillating lightly will lock to the received carrier at 90 degrees, giving true DSB Synch reception A more heavily oscillating regen will resolve SSB without problems by virtue of the 1.5kC frequency offset between the RX center frequency and the selected sideband. The important thing here is to prevent overload into the detector, or random synching will occur, giving very poor results. Making the detector oscillate to resolve SSB is really not the best way for two reasons. Advancing into oscillation destroys the massive selectivity gain of being on the threshold of oscillation, and the detector then works as a very poor self excited product detector, prone to frequency pulling as above. This also explains why regens are "fiddly" to use, and performance varies with each design. BUT, they are simple, elegant and can work well with operator skill and patience. Just my thoughts.
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The Frank Jones supergainer
In Tube Radios
dayleedwards
Jun 13, 2024
That would work fine, but you will have difficulty with BFO leakage getting into the RF input of the high gain detector.. This problem also occurs with direct conversion receivers, but lesser so, the front end gain is much less, and sometimes non existent I have used a seperate BFO with a regen. just having the BFO close with no direct connection. Interesting things happen. The detector locks to the BFO, the BFO is always many times stronger than the RF signal, even when you try hard to shield the damned thing. The stability of the regen then is reliant on the BFO stability, strong SSB or near zero beat AM signals dont pull the regen. The noise decreases also, the dynamic headroom of the detector is all but used up with the added BFO signal, so acts more like a limiter.... any amplitude modulation is removed. A product detector is nothing but a mixer by another name, so what we have is a vry sensitive regenerative mixer/detector, exactly what you are looking for.. Of course, this concept is easily demonstrated by injecting an external signal into a regenerative detector at the signal frequency it is tuned to, and by messing with levels, a good idea of performance will be got.. The supergainer idea works well with a BFO because of the fixed IF frequency, in my case around 1700kc At this point, is the radio now a BFO injected regenerative detector, or a regenerative direct conversion receiver? The detector now has little need for even a regeneration control, this can be preset. My radio tunes a 3 to 1 frequency ratio, or 4 to 12 megs and can be set on the verge of self oscillation over the whole band without any further adjustment. With a BFO, the radio is even less "squirrelly" than without. The min reason my supergainer lacks this feature is I listen more to AM SW stations, not any great interest in amateur ssb transmissions.
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The Frank Jones supergainer
In Tube Radios
dayleedwards
Jan 22, 2024
Hello QRP, its been a while since I have heard from you....... The Supergainer uses characteristics with valves that are very hard to replicate using silicon, especially gain and dynamic range. You would have run into this very difficulty using unusually low voltages in your design. The conversion gain of the ECH84 is way above any FET or transistor by virtue of the high tank impedances and free voltage amplification with the coils. Even using IGFETs such as the old 40673 will get close, but still lacks the dynamic range inherent when using high voltages with tubes. The old fashioned pentode makes for the best regen, whether in IF amp/detector or straight RX. These are just my opinions and experience. There are also some unknowns with my supergainer that I have yet to figure out, one of which you mention. Using the ECH81, the usual goto valve for AM mixer service, and using in the supergainer , the regeneration of the IF varied considerably over the 3 to 1 tuning range, (4 to 12 megs) requiring the regen control to be adjusted over the band.  Using the ECH84 cured this. I cannot find much info on the differences between the ECH81 and ECH84, I am assuming the latter is a sharp cutoff heptode as opposed to the remote cutoff of the 81, as this one is designed to take an AGC control voltage. The anode resistance is similar for both, so why the difference here, I just dont know. The next issue is conversion gain, valves have TONS. The negative here is the potential overload of the IF amp/detector. We need the added regenerative selectivity, but not the gain. Unfortunately, we need one to get the other. Therefore, the IF amp needs lots of dynamic range, but this is destroyed the tighter the bandwidth before oscillation sets in. One thing in our favour, stong signals require less regeneration and can have wider bandwidths, hiding the issue.  Nothing is easy.  This design relies on an RF attenuation control at the aerial input rather than a volume control, to keep the signal levels in bounds. Now we have the problem with mixer noise increasing as we reduce the input signal. Solid state is MUCH quieter, that would be a big advantage . The more control grids in a valve, the noisier it will be, thats why triodes were used at VHF. The regen IF would be hampered using silicon because of the low impedances, the mixer stage would require an RF amp to get similar sensitivity, not benefitting from the mixer load Q multiplication from the reflected IF resonance..... think Q multiplier. Audio wise, silicon beats valves anytime, but by using ridiculously high turns ratio transformers, a triode can acheive very high gain in one stage. One interesting valve to fiddle with I think would be the triode section of the 6AV6. These have a anode resistance of 80000 ohms, with a 500k anode load have a voltage gain of round 70 with few hundred volts. Thats lot of gain in one stage, around 40 dB I think . So, I dont think a solid state supergainer with a similar number of parts would perform without disappointment, it would need more complexity for a similar performance to the two valve version. As always, I love being wrong, thats how I learn new stuff.
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Deltahet is alive!!
In Tube Radios
dayleedwards
May 07, 2023
Have spent even more time experimenting with this radio. It seems it has reached the limit of its performance insofar as sensitivity and stability is concerned. The biggest ongoing issue is drift in the tuneable 2.455 to 3.455 mhz oscillator in the final IF section. As the radio warms up, it drifts out of the narrow 455kc xtal IF filter passband, only stabilising after 10/15 minutes. The 2mm alloy chassis has lots of thermal inertia, ie, it heats slowly. I am wondering if a series of holes drilled through the chassis would create a thermal "island" in the ECH81 3rd mixer area to help isolate the external heating, especially from the mains transformer. I have now settled on 6BE6s pentagrids for the first and 2nd signal mixers, with low screen voltages and high value cathode resistors, the mixer noise has reduced considerably, conversion gain still remains high, the signal and injection grid isolation is a welcome bonus. A quick 12 dB SINAD test reveals -107 dBm AM at 30 percent mod, thats around 1 microvolt across 50 ohms, this is quite good, I think up from the -92dBm earlier. This sensitivity holds from 3 mHz through 30mHz... more than adequate for HF.. AGC is now applied to the RF, ist IF, and second IF amplifiers all using 6BA6 remote cutoffs, the RF amplifier now using a 6BZ6, these seem quieter in operation. Cosmic noise is now audible over all bands. Another slight issue is BCFM breakthrough in some higher bands, this would be eliminated with a LPF after the RF amplifier. Another annoying issue, I cannot quite get the new valve VFO to exactly coincide with the existing FRG7 tuning drum markings, so have opted for a digital display. This is due to arrive from China in a few days. Unfortunately it uses a six digit 7 segment display, so may be a tight fit in the front panel. These modules have a +- programmable IF offset so setting this to -3.455 should allow it to read 0 to 100000 as the VFO tunes 3.455 to 2.455 mHz, ie, the Wadley loop 2nd IF runs backwards in frequency as it tunes the RX frequency higher. Its a cheats way of doing it, I have spent far too much time on this project already. With my current fascination with Wadley loop receivers, and before I forget how they work, have picked up a realistic DX300. I am wondering why the reviews were so bad, and just how bad are they, can it be improved etc. I may do a write up if anyone is interested.
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Deltahet is alive!!
In Tube Radios
dayleedwards
Mar 03, 2023
Yes, the ECH81 is by far the best bottled mixer made, and easy to use. . I dont know why, but these valves never got used in the USA, the only triode/hexode that I am aware of that had any traction was the older 6K8. The 6BE6 was used extensively, from the AA5s through to those big Collins radios. I think Hallicrafter used 6K8s extensively, but Im not really up on American stuff. Europe was a leader in valve tech, especially Philips, they printed many books of radio designs for the hobbyist, and gave out very complete application notes for all their products. I hated repairing anything made by Phillips , most of their passive components were rubbish, remember those horrid IF cans that were held together with wax, and would break if you even looked at them. The "lollipop" capacitors that would self disintegrate over time, weirdly made potentiometers that could not be replaced by anything else, even their knobs were different. Philips TVs were the same. But, to be fair, their stuff performed really well. Your circuit would have been a fairly standard use of preferred valves, and that radio would have performed very well. Even today, valves easily outperform transistors in simple radio circuits, and are definitely easy to use. The endless fun, to build a simple radio, and then try improving the performance by tweaking, adding and modifying is an awesome hobby..... everyone smiles as they receive that very first signal on that collection of just a few random parts.
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