[Boatanchors] Want to buy some Parts For Receiver Projects

Tue Oct 5 11:59:56 EDT 2021

Scott,
Here is a compilation of some emails on the McCoy 3-tuber that you may find interesting. These are from several years ago when i posted a question about the diode detector. I guess most people have forgotten about this controversy, but for some reason, I save this stuff. Mike, W6MAB

----- Original Message ----- From: "Michael Bittner" <mmab at cox.net>

Re. the "Mate For The Mighty Midget". QST, April 1966, by W1ICP, once again we see the floating diode detector about which there has been some controversy. Apparently these things work using the back resistance of the 1N34 diodes. However, the coupling capacitor to the audio amp grid seems small. Shouldn't it be more like 1000 pf rather than 100 pF? Mike, W6MAB

It seems everyone is so dazzled by Lew McCoy's 3-tuber that they forgot about my original question which was:

The coupling capacitor to the audio amp grid seems small. Shouldn't it be more like 1000 pf rather than 100 pF?

Regarding the voltage doubler detector, I first saw this in an article titled "Rejuvenation of the AM Detector" by Leonard Geisler in the October 1957 issue of Radio-Electronics. Below are two other magazine articles where it appears. In all cases, there is always a load resistor, no floating diodes.

Crystal Diode Field Strength Meter, Radio & Television News, Feb 1950, pg 35
High Gain Crystal Set, Radio-TV Experimenter, Oct-Nov, 1967, pg 40

I built two crystal sets based on the latter article and found that it works well as long as it looks into a high impedance load. With a low Z load, it's no better than a single diode. BTW, major thanks for attaching the circuit! This should actually be quite a sensitive little thing, I think, with the pentode and not pentagrid mixer. I just noticed the 'floating diode' detector is also a voltage doubler, so that's a new one on me. It instantly occurred to me that a modern version of this circuit might use two diodes instead wired as a product detector. Pat Hawker's 'Amateur Radio Techniques' book shows some circuits, I am sure, where such a product detector has switching to switch back to single diode for AM reception. Now the crystals Y1, Y2 here, what are their frequencies?

Something I am VERY curious about in this kind of circuit, with the 6U8 mixer - osc circuit, is the question of 'pulling'. Maybe it only comes into play on the higher freqs, say, starting at 14 or 21 MHz ? I have wondered about this for a long time. There are MANY radios that used this kind of circuit. One thing I

really admired about the Star SR-200 receiver schematic I recently looked at was that the osc used two triode sections, one as osc, one as cathode follower buffer. That is exceptional for a Novice - priced receiver. -H

Well, while I am enthusing over this circuit, another thing I thought was cute and smart is the single tuned circuit as the mixer load, how the inductance is maintained above ground so that the circuit can be ( capacitor ) center tapped. This is a pretty smart little receiver. -H

Tanks, Jim, N2EY, for the comments. That jibes with my small experience in the past aligning single conversion receivers, how the RF amp adjustments shifted the tuning. I don't exactly remember, but I believe I have used a receiver where adjusting the antenna trim control shifted the BFO note. So a receiver with a 455 kHz IF, like the Knight R-100 would be worse in this respect than the few single conversion receivers with an IF around 1600 kHz, like the Heathkit GR-54 or HR-10. Makes sense, and also makes another reason to use double conversion in those old receivers. -H

Yes, 100 pF does look crazy, and when I have built this circuit myself in the past, voltage doubler crystal radio, with hi-Z piezo headphones of course, I have always used 1000 pF. Possibly with the very high impedance load,
in fact the highest Z load, here 100 pF suffices? It still seems small. If I was building this, I would have a couple 1000s on hand and try paralleling them just from curiosity. Here's something that will surprise you: in a crystal radio circuit, if you do use piezo earphone and absolutely not a magnetic phone, you can use a floating diode circuit with no lack of performance. -Hue

The voltage doubler crystal circuit must have appeared earlier, in a publication, maybe a P.E. annual projects compilation, because I used it in a 3-circuit crystal radio in the early 1960s. I found some lack of tracking with the 3 tuned circuits - I used a 3-gang capacitor - but it was workable. I was impressed with this project not because it looked good, which my version didn't, but it did separate classical music KXA 770, 1 kW, from KIRO 710, 50 kW, and I was pleased with that. -Hue

Some interesting experimental results there, Hue. Thanks.

The general consensus seems to be that the 100 pF is way too low for the audio coupling capacitor. Probably a drafting error. Maybe the draftsman left out the load resistor (another 1 Meg) too? Mike, W6MAB

It seems that once you give some attention to the 'floating diode' circuit, you start noticing it everywhere, or at least in a lot of places. I think the 'reverse leakage' is not explanation. I gave the example of the circuit I saw, where there were capacitors around the input and output of diode. No leakage path there. I believe I saw somewhere a vacuum tube implementation of the circuit, but I'll be darned if I can recall in which equipment's manual. I think the understanding "may" have to do with the output load of the diode. It is equivalently a resistance and capacitance in parallel. But I'm not ready to reduce my still somewhat fuzzy understanding to words. -H

It's quite possible that the value is a typo. Has anyone checked for feedback?

The "Mate For The Mighty Midget" design was an attempt to make a usable receiver which was only a little more complex that the typical regenerative, yet offered much better performance. Another objective was absolute minimum cost - hence the pill-bottle coils and home-made chassis.

How sensitive it really is, I don't know, because there are only four stages with gain from antenna to headphones, none of which are high gain. Crystal filters have loss, and so do diode detectors. Why the design uses three of the same tube type is a mystery - maybe the idea was quantity discount? Or perhaps it was inspired by the Mosley CM-1, which used five 6AW8s and various diodes.

All sorts of improvements are possible - if one uses a bigger chassis to allow room for them.

The crystals Y1 and Y2 can be any frequency that the IFTs will tune to. The bandpass is determined by how far apart the frequencies are - check the FT-241A chart for choices. According to the article, two of the same frequency gives such a razor-thin passband as to be unusable, while two crystals 400 Hz apart gives a decent CW passband and two 1.4 kHz apart is OK for SSB. Best bet is to check the FT-241A charts and come up with a combo that works. The good news is that one can try all sorts of combinations. "Pulling" is caused by many things:

1) How sensitive is the oscillator to changes in loading?
2) How sensitive is the oscillator to changes in supply voltage?
3) How sensitive is the oscillator to another tuned circuit near the oscillator frequency?

Problem 2) usually manifests itself as changes in tuning when the RF gain is changed. Problems 1) and 3) are usually the result of interaction with the mixer input tuned circuit. This interaction depends on the frequency difference as a percentage of the oscillator frequency - which is why a receiver with a 455 kc IF may exhibit no pulling on 80 (difference is more than 12%) yet be very bad on, say 15 (difference is about 2%).

Like many projects of those times, the power supply uses a single diode in a half-wave rectifier circuit. This was done because such diodes were expensive and hard to get back then. Today, 1N4007s are very low-cost, and can even be salvaged from burned-out CFLs.

So it's a good idea to use the full-wave-bridge circuit for the B+. It will make the ripple easier to filter, may make the regulation better, and most of all the power transformer has an easier life. 73 de Jim, N2EY

> On October 4, 2021 at 9:48 PM wa9wfa <whitebear1122 at comcast.net mailto:whitebear1122 at comcast.net > wrote:
> 
> 
>     Hi, I’m looking to buy some parts for some home made receiver projects that I am current working on and plan to build. I’d appreciate if you could look in your junk boxes and help me out with parts that were once easily available in the 1960’s.
> 
>     I’d like to find:
>        1. 1 or 2 Miller 2112 365 pF dual section variable caps.
>        2. Some Miller 455 Kc IF transformers, interstage like a Miller 1312-C2, and output, preferably the newer slim chassis mount version like a 12-C2
>        3. Some Miller screw adjustable coil forms or coils like the 4411 that go up to maybe 350 or 400 uH.
>        4. Miller MD-8 two speed vernier dial. I’ve never ever seen one. Has anyone ever used one? I see it on the front panel of the HB-67 in the 1967 ARRL Handbook.
>        5. A couple voice coil transformers, small for a small receiver, preferably with multiple output impedances
>        6. Compression variable capacitors
> 
> 
> 
>     I am currently working on a 3 tube 40/80 meter superhetrodyne receiver first published in QST April 1966. I built a duplicate of Lew McCoys circuit last Spring, got it working a bit, but had a lot of struggles with it. The circuitry was tightly crammed in the chassis so cross coupling of oscillators and amplifiers were a problem.I wasn’t ready to abandon it because there was much more to learn, so I am now building a breadboard version of the circuit that is spread out, separating sections, reducing cross coupling of oscillators, using coax for section interconnects, shielding, etc.. I put up a Youtube video of it last night.
> 
>     https://youtu.be/kEgELROqNtw <https://youtu.be/kEgELROqNtw>
> 
>     It’s a hot little receiver but needs more selectivity so I need to get the crystal filter working. The filter uses two military surplus 455 Kc crystals. Once I’m done with the breadboard I want to go back to my compact version and figure out what’s wrong with it.
> 
>     I was ready to be done building tube radios after struggling with the Mate but with this breadboard success I am jazzed up to build a more complicated one with Collins mechanical filter, AND AGC! I am thinking of something similar to the HB-67 from the 1967 ARRL Handbook.
> 
>     If you’re no longer building, or have spares, how about giving me a chance to use those parts. Many thanks! Let me know what you have.
> 
>     73, Scott WA9WFA
> 
> 
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