No RF gear here?

Does anybody remember the 6H6 dual diode clipper circuit in your AM broadcast/Shortwave receivers?
All of you are reminding me of my youth (if I can just remember it).
Thanks!
I have collected a good size box of metal octal tubes for a guitar amp project that I will call MetallicAmp, all tube no glass. I quick look in the box reveals no 6H6's. There is probably one inside an old console radio that I have, but it's buried under a plie of boxes right now. The more modern equivalent is the 6AL5, and two 6AL5's (4 diodes) stuffed into a single bottle is the 6JU8 I have some of both of those though.
....... So in theory it's possible to make a MW DC RX, with 1 PLL, extra product detector, a phase shifter and such a slew rate filter. Remember the NE561?
I have a few of those....somewhere. I briefly experimented with them over 20 years ago. The Signetics ceramic packaged parts were often found on surplused PC boards that seem to have originated at NASA auctions at Cape Canaveral Florida.
 
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I still have several antique PLL chips, NE560,NE561, XR-S200 etc. At work I produced several FM tuners and because in publications every RX had to be different from previous ones, I used about anything viable as product detector, even the OTA CA3080. My late best friend also built one and was quite happy with it, despite the difficulty of aligning the variometer tuner:
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I finally got around to reading the chapter on DC receivers in EMFRD and I didn't find it particularly useful. Most of it is oriented to prevention or solution of the typical DC receiver irritants - picking up AC hum ( even when on battery power ); microphonics, unwanted AM demodulation, audio oscillations / feedback, etc., and the device under construction, maybe through sheer luck, just does not suffer from any of these issues. At all.

I've added RF filtering / blocking at the output of the product detector, followed by an RC high pass filter with the corner frequency at 270 Hz.

When I left town before Christmas, I totally forgot to take my loop antenna / base to try to rig it to pick up that VLF station in Sweden, so instead I spent my soldering time on an audio AGC system built around an LM370 subsystem IC. This took a good chunk of solder time because the data sheet circuit oscillated vigorously and was really tedious and time consuming to make stable. I wound up with the circuit fairly close to, but not quite identical to the AGC circuit in Linear Application Note 51 which is required reading for use of this device. It looks like heck, because I did not budget board room for all the extra components I wound up needing to stabilize the thing.

Anyway, it actually works well - I have a working, reasonably wide range audio AGC system for a DC receiver. I took advantage of the extra control input on the LM370 to put all of the audio gain, less the LM386, into the AGC loop. Since the LM370 provides a lot of extra gain, I turned the '386 back down to 26 dB to quiet it down.

For the last week, I've had the AGC threshold set to run at max gain from the LM370, but this morning I turned it down to about what I lost by turning the LM386 down, plus 6 dB or so. The overall end to end gain is about 82 dB AF gain, and 32 dB RF gain. This seems to be completely adequate. Right now I have the 50 foot end fed wire connected to the auxillary input and the receiver is happily listening to FT8 on 10 meters. The receiver seems to have more than enough gain for comfortable loudspeaker listening on any band. In fact the rcvr is on the kitchen table right now, and I'm listening to it in front of the TV in the living room area while I type this.

One of the reasons I read the DC chapter in EMFRD was to try to get some information on just how much gain is recommended for loudspeaker use, but it seems to ignore that not everyone wants to listen to a DC receiver with headphones. I think 105 dB is a reasonable target.

Win W5JAG
 

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The year of publishing might give a clue on that EMFRD chapter. The problems mentioned arise when using a single device (bjt, fet) as mixer, a sloppy vfo that radiates (and arrives at the antenna input) plus an AC supply that isn't properly grounded. Your choice of devices prevents them. Audio AGC is not limited to hardware DC receivers but even is used in several (famous) SDR like SDRplay and HDSR. For AM it doesn't sound well because during a pause the gain goes up and so does the background noise, and the relatively high attack time (to prevent noise pulses to crank up the AGC) causes changes in volume where there shouldn't be any.
 
I would guess that it was done in the late 1990's. Purely from the non technically educated, just an ordinary ham with a soldering iron point of view, I think Solid State Design for the Radio Amateur is a more useful, easier to understand and follow publication.

I've probed around on the LM370, and can't find any pins that have a useful voltage movement under AGC to allow me to meter incoming signal strength. I think the next round of improvements will be to add another 3 KHz low pass filter after the product detector, but before the LM370, and pick off audio from the output of that to drive a signal strength meter. This should also cut down on wideband energy going into the LM370. Right now it's looking at the entire wideband output of the product detector, so that should further improve the AGC action.

I suppose I could fiddle around with the signal routing to try to put the LM370 in between the low pass filter, but before the 741 audio amp, but it was so darn difficult to make the thing stable, I'm reluctant to open it back up to instability issues.

Win W5JAG
 
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During the 1990s DBMs in IC form still were relatively expensive so that explains a lot. Diode DBMs require much more LO so no solution either.
AGC is a major issue and when done properly greatly eliminates listening fatigue. If the LM370 is allowed to follow the signal after the LPF, attack is fast enough. What matters is decay: the AGC should "hang" for the duration of a syllable, than rapidly update status. The result is that listening to QSOs much more resembles "live" conversation as the AGC has ample opportunity to adapt between sources and on each, "hangs" so within AGC range, hardly any audio level change despite large changes in RF signal.
In principle the same LPF can be used for the AGC as for the audio. An LPF after the product with BW equal to BW of RF input filter could serve for noise blanker. An ideal one is a variation of the track & hold, with the noise blanker pulse interrupting signal flow. So for the duration of the noise pulse, audio level would remain at the level just before that pulse.
 
Added another 3 Khz lowpass filter, ahead of the AGC subsystem, to cut down on the wideband noise going into the AGC.

Hooked up the RF attenuator to verify that it was in working order. I haven't yet measured the attenuation, it sounds like at least 10 dB with the present 5K control pot and 3K3 current limiting resistor. It can be configured for more attenuation, I think, but I doubt that will be necessary. The HP MMIC RF amp is sufficiently quiet - even at maximum RF attenuation, hooking up the random wire antenna to the auxiliary input clearly increases the background noise in the receiver at 29 MHz. The overall end to end gain seems to be about right.

I had an XH connector for the RF attenuator pot but managed to rip it clean off the board. I didn't feel like replacing it right away. I made a new temporary breadboard to replace the broadcast receiver reject I had been using. The VFO is under the main PCB so the ground plane can (sort of) shield the receiver from the VFO.

Win W5JAG
 

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I added metering capability, but not without some wasted time and difficulty.

I went about it in what seems like the usual way - JFET source follower, followed by an amplifier, then a rectifier and filter, and a microamp meter. I used a leaded MPF102 JFET as the source follower, a 2N3904 amp, and 1N4148 diodes in the half wave rectifier. The meter is a cheap edgewise CB radio type, probably 50 uA movement. I connected it to the output of the product detector, which is at the +32 dB gain point in the radio. With disappointing results. As in, no results whatsoever. Nothing budged the meter.

To see if it worked at all, I connected it to the top of the volume control, which is at the +100 dB+ gain point in the radio, and simple background noise pinned the meter. I had to change the meter sensitivity pot to 500K to be able to adjust the meter sensitivity to less than full scale.

This gave me the opportunity to see how well the AGC works. When the meter is adjusted for zero on the background noise, only the very strongest signals would provide any positive meter deflection, and then only three or four needle widths, because of the strong AGC action. The better method was to adjust the meter for half scale on background noise. A strong signal would still only deflect the needle a few widths above mid scale, but when the station stops transmitting, the gain reduction (and signal strength) is readily apparent by how far the meter deflects below mid scale, before the AGC recovers and brings the receiver back to maximum gain. The AGC is pretty effective - it appears to perform just as the data sheet and application notes suggest. This was interesting, but not really what I wanted.

I decided I would reconfigure that first LM741 op amp that I was using as a 3 KHz filter, into a gain stage, and use that to drive the meter circuit. After using it a bit, I didn’t care much for that filter in that place - I felt it didn’t add much to either the AGC action or selectivity, and whatever it did add was outweighed by a propensity to ring at the high volume levels I sometimes need. That worked, but after some more thought I had an epiphany that this was taking a lot more effort, and using more parts than I needed, for what should be a simple task. The epiphany was that what I really needed was a separate audio strip dedicated just to driving the meter, and - drum roll here - since an LM386 can have its gain set anywhere between 26 dB and 73 dB ( if you don’t care how it sounds at the high gain end ), and since this would only be driving a meter, who cares how it sounds, one LM386 should easily do the job.

And it can. I connected the high gain AGC audio strip to the product detector through the source follower so it would be isolated from any shenanigans going on in the meter strip, and made up a minimum circuit using an LM386N-1 set to 46 dB gain, and connected that direct to the output of the product detector, and - it worked. I reduced the meter sensitivity pot to 2K to get a better range of adjustment in my particular setup. I didn’t even use an “expensive” LM386 - the part used here is one of the 10 / $1.00 USD types from AliExpress - undoubtedly some type of reject, counterfeit, or some other dubious origin, although all of them seem to work just as you would expect.

I think diodes with a lower junction voltage would give a better response on weak signals, so I will probably tinker some with that. That could lead to perhaps making the gain of the LM386 variable to adjust meter sensitivity in place of the pot at the output of the rectifier block. A smaller output cap might be okay, or even better.​
 

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I changed out that LM386N-1 to an LM380N-8 and this was close to transformative. Less hiss, less distortion, plays a lot louder.

I changed the 1N4148 diodes in the meter rectifier to 1N60P and this made the meter more sensitive to weak audio, but it still has more peg slamming than I would like on big signals, so that needs some more work.

I was a bunch of software updates behind on the Red Pitaya, so I upgraded it to the latest firmware that can be run without upgrading the Linux version, and tested it out by tuning up the bandpass filter on the front end. Ultimately, it seemed like the best compromise for the top coupling cap was 22 pF. I started with 47 pF. the -3 dB points look to be about 3600 Khz and 4050 KHz and 3500 Khz is about - 7 dB. The receiver has plenty of gain to deal with that. Pics are before and after tuning it.​
 

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