My Mate for the Mighty Midget Thermatron Receiver

A few days ago I received an e-mail from Jan PA3GSV -- Jan is working on a receiver similar to my old Mate for the Might Midget and had some questions about how I'd handled the filter portion of the circuit.  Then on Sunday, while listening to Eric's interview with Farhan on the QSO Today podcast, I got so enthused that I felt compelled to work on a homebrew receiver.   So out came the old Mate for the Mighty Midget.  I built this thing in 1997-1998.  It is described here:

http://soldersmoke.blogspot.com/search?q=Mighty+Midget

In the above  video  I was listening on 75 meters to a very congenial early morning roundtable featuring W4CH, K5KBZ and others.  I know, I know, this is the third or fourth video that I've made of this thing.   This is almost as bad as 2B-mania. Or the Michigan Mighty Mite thing.   I blame Jan.  And Eric.  And Farhan.  And Grayson.  And Lew McCoy!

Here is my e-mail exchange with Jan:

Bill:

I recently build the W1TS two tube xtal controlled transmitter, and am looking for a 80/40m companion for this that has a crystal filter.

At first, I wanted to build the “Simplex Super” and finally got hold of the 1700 kc crystals, but then they got lost in the mail…

Only part of a two box shipment arrived, with 455 kc fundamental frequency FT-241 crystals, so now I am looking for a diagram using a 455 kc IF. Jan

Jan:   In this link you will find the schematic for the receiver I built. 

 http://jvgavila.com/mmrx_3.gif

 I was not able to build the filter with the two 455 kc crystals.  I could not obtain the needed crystals.  I used two 455 kc IF transformers as described in my article.   This resulted in a very broad frequency response but it was OK and quite good for AM. 

 Last year I put in a Toyo 455 kc SSB filter, but I did not match the impedances, so the results were not good.  Your e-mail makes me want to work on this again! 

 Let me know how your receiver turns out. 

 73  Bill  N2CQR

Thanks for your reply.

It is a neat looking little receiver.

And yes, I also have a cardboard box labeled “good junk” which is filled with stuff from ham fests J

Finally it will be put to good use!

I printed the article for some evening reading this week.

It will take me some time building this receiver, as there is some metalwork and mechanics involved.

I will let you know how the receiver turns out, and I am also curious what improvements you make on yours.

 Thanks again, and I will let you know.

 73  Jan PA3GSV

Wow, Jan has the Knack!   Check out his station:

That's the W1TS rig on the left.  More pictures from Jan on his QRZ.com site.  He too uses wood cabinets!  I'm not alone!  Here's Jan, PA3GSV

 

WA9WFA's Mate for the Mighty Midget 1966 QST Receiver

Scott WA9WFA and I have been exchanging e-mails about his Mate for the Mighty Midget receiver project.  This morning I finally took a look at Scott's we page on this effort.  There was a lot there that resonated.  For example: 

"I remember the moment very clearly.  June 1966 I was sitting in a lawn chair reading a stack of QST's that I had brought along to the summer cabin.  The February 1966 issue the Beginner and Novice section introduced the "Mighty Midget Transmitter", a 10 watt crystal controlled transmitter.  The April 1966 issue Beginner and Novice section introduced the "Mate for the Mighty Midget" which was a three tube super-heterodyne Novice type receiver.  Over the course of that summer I read and re-read those articles a jillion times.  Being 13 years old, I didn't have the electrical or mechanical skills to pull off such a project so I could only dream about it.  In 1970 I bought a handful of the parts.  In 1976 I bought more parts.  In 2021 I decided to build it while I still had the ability to do it.  This project is only my second homebrew radio project so I am still learning things every second of the way...

 

While I am not expecting to much in performance, the 13 year old in me is ever hopeful that this 1966 Novice receiver will be the most wonderful radio ever made.   73, Scott WA9WFA"

Scott's MMM RX page: 

https://www.qsl.net/wa9wfa/MateForTheMightyMidget.htm

Scott and I are now both updating the MMM RX by substituting 455 kHz ceramic filters for Lew McCoy's FT-241 crystal filter.   I have my filter wired in now, and it is working well.  Scott plans on soldering his in today.  I will post on this mod soon. 

On his QRZ.com page, Scott notes the need to fight the temptation to further soup-up this simple receiver:  "I did have to resist the temptation to add another audio stage, a mechanical filter, AGC, 2nd IF amplifer stage, etc..."

Exactly right Scott.  Resist the temptation.   Simplicity is a virtue.  I do use an outboard, powered computer speaker, but I justify this by telling myself that I just don't want to use headphones.  But I could use headphones, so this is OK.  OK?  

Scott's QRZ.com page: 

https://www.qrz.com/db/wa9wfa

I must add that I think the yearning of Scott's inner 13 year-old can be fulfilled by the MMM RX.  I think it is pretty wonderful.  It is -- in my view  -- not as good as a Drake 2-B, but it is FAR better than an S-38E, and it is better than a Lafayette HA-600A (wjm).  

Mike WU2D's Video on the SimpleX Super Receiver -- Part II

 Another FB video from Mike WU2D.   

But you know,  I too find myself kind of opposed to front panel on-off switches.  I power my rigs with small DC supplies.  I just turn on the supply when I want to use one of the rigs.  I don't have or need a switch on the front panel of the rig.  

I especially liked Mike's use of the gate dip meter and, of course, the Q meter.  FB OM. 

I must say I have a preference for the first version, but only because I dislike the regenerative circuit in the second version.  I do like the newer-style coils -- I have one in the BFO of the Mate for the Mighty Midget receiver.   

Thanks Mike for the sideband inversion factoid in Part 1!   The Hallas Rule -- words to live by. 

One word of caution.  I used 6U8s on my Mate for the Mighty Midget receiver.  I had good results, but WA9WFA had a lot of trouble.  We eventually concluded that the 6U8s didn't age well.  And they were quite long in the tooth.  We found (from the tube guys) that 6EA8s aged better and were a good and easy sub for the venerable (perhaps TOO venerable) 6U8s.  I switched tubes in my rig and it did seem to work better.  BTW, this is the receiver that I use to listen to the Old Military Radio Net on Saturday mornings.  

Here is the story of our switch from 6U8s to 6EA8s: 

https://soldersmoke.blogspot.com/2022/02/6ea8s-in-mate-for-mighty-midget-and.html

Jan PA3GSV's Amazing Mate for the Mighty Midget

We have featured the amazing homebrew work of PA3GSV before:

http://soldersmoke.blogspot.com/2016/04/my-mate-for-mighty-midget-thermatron.html

Jan is at it again, this time building a Dutch version of Lew McCoy's Mate for the Mighty Midget.  Look at front panel! But wait, there is more!

Here is a side view of the chassis.  Wow.  What, you may ask, is that round thing?

Holy cow!  Homebrew Vernier reduction drive made from the cap of a sewer pipe.  And a homebrew dial cord arrangement.  Jan is clearly breaking new ground in ham radio homebrew re-purposing.

Jan writes:

Bill:

Here a little update on the MMrx.

Got almost all the parts, except for the Miller 4411 300 uH coils, for which I am attempting to make my own.

While I was looking for some pictures of this coil, if it was shielded or not (found a vintage online Miller catalogue J ),

I stumbled across this article, the W2MQ “Hamster”

There is no mention of the MMrx, but the text is very in line with the latter. Nice read.

 http://www.njarc.org/broadcaster/BC2010-04.pdf

 Also while looking for parts, I saw this supplier of FT-241 xtals who sell for reasonable prices ($10) .

Maybe you already know this company:

 https://www.fairradio.com/catalog.php?mode=view&categoryid=198

 Vernier drive 1:19

The drum is made from a sewerage end cap got from the hardware store.

Still under construction J

Added a front control to switch the BFO on/off, together with another crystal or some arrangement to make AM (broadcast) reception possible.

I want to try a regen detector instead of the 2 germanium diodes, followed by a pentode for some more audio output.

Regen and AF gain control are also at the front.

In your video it looked like there was a screwdriver sticking out of the BFO oscillator coil, a trigger to put this control on the front as well J

As there is no AGC, the (S) meter will measure the plate current of the RF amplifier tube.

So, a lot of experimenting to do, and still busy with some of the mechanics.

 73  Jan

DONE! Jan's AMAZING Mate for the Mighty Midget Receiver

Hi Bill,

  

Finally the Mate for the Mighty Midget is finished, just in time for the G-QRP Valve Day 12-13^th of November.

Got the LO fixed for 40m by lowering the parallel capacitor from 150 pF to 100 pF.

Also the 68 pF series capacitor was lowered to 33 pF for some more band spread on 40m.

It now receives from about 7.0 to 7.4 MHz and from 3.45 to 4.0 MHz

Had to exchange C1 in the end, the one used initially quit every now and then.

I only had a large 3 section variable in the junk box covering 10-550 pF, which works fine now.
For the lower end of 80m I had to add additional 47 pF next to the 47 pF trimmer caps, so there it is about 600-650 pF max!

At the high end of 40, it is also just not too much.

The meter was used as a position indicator for C1.

Tried several ideas, but with no separate tube for AGC, I couldn’t get it to work as a S- meter

Read something about audio derived AGC, maybe this is worth a try.

 

The BFO can be switched off for AM reception.

Simultaneously the input on the mixer side of the crystals is disconnected but still coupled by some capacitance of the switch wires.

AM reception is possible, but not very good.

Need to find a better solution which doesn’t degrade the crystal filter properties to much.

(By the way, the detector regen. control ads about 4 dB to the AM sensitivity)

The receiver will mainly be used for CW/SSB reception, so maybe it stays this way for a while J

 

I hooked it up to the W1TS transmitter, which was very loud.

Didn’t foresee a T/R relay (learned a lot from this project ;-)  ), so added this one between the front plates next to the RF and audio gain control.

The quit down everything a little, the RF gain pot is lifted of ground as suggested by James, N2EY at QRZ.com.

It helped a lot, but was still too loud if tuned exactly in the bandpass of the crystals.

The T/R relay now also switches an adjustable potentiometer at the input of the audio pre-amplifier.

 

The dial cord has no lag, and works very well for fine tuning.

Unfortunately the reduction drive went from 1:19 to 1:9…, the tuning capacitor only has a 180 deg. span.

Something to remember for the next receiver.

 

It’s a nice little receiver and quite stable after warm-up.

The only extra luxury a next receiver will have, is AGC.

But with no AGC it’s easier to tune the antenna tuner by ear J

There’s now a complete homebrew station here, antenna, feeder, tuner, receiver, transmitter, power supply, al home made J

 

About the Mystery Hole….

If you haven’t guessed it by now, or Pete hasn’t told you, it is revealed in one of the pictures below.

 

I also made a little video:

 

https://www.youtube.com/watch?v=4-STxQI36_s

Putting the "Mate for the Mighty Midget" Back to Work -- With a DX-100 on 40 Meter AM

After working on it for a while I got so fond of my old Hammarlund HQ-100 that I moved it from the AM/Boatanchors operating position over to a more convenient spot right next to my computer.  This left a big gap on the receive side of the AM station.  

I briefly put my HRO-ish solid state receiver above the DX-100, but I'm afraid that receiver needs some work.  More on that in due course. 

I thought about putting my SOLID STATE Lafayette HA-600A atop the thermatronic DX-100, but this just didn't seem right. The Radio Gods would NOT approve. 

So I turned my attention to the Mate for the Mighty Midget that I built in 1998 and have been poking at and "improving" ever since. 

This receiver worked, but not quite right. It received SSB stations well enough, but when I turned off the BFO I could no longer hear the band noise. I wasn't sure how well the RF amp's grid and plate tuned circuits tracked.  And I had serious doubts about the detector circuit that Lew McCoy put in there when he designed this thing back in 1966. 

As I started this latest round of MMMRX poking, I realized that I now have test gear that I didn't have in 1998:  I now have a decent oscilloscope.  I have an HP-8640B signal generator (thanks Steve Silverman and Dave Bamford).  I have an AADE LC meter. And I've learned a lot about building rigs. 

FRONT END TRACKING

The MMRX has a tuned circuit in the grid of the RF amplifier, and another in the plate circuit of the RF amplifier.  There is a ganged capacitor that tunes them both.  They need to cover both 80/75 and 40 meters. And they need to "track" fairly well:  over the fairly broad range of 3.5 to 7.3 MHz they both need to be resonant at the same frequency.  

McCoy's article just called for "ten turns on a pill bottle" for the coils in these parallel LC circuits.  The link coils were 5 turns.  No data on inductance was given.  Armed now with an LC meter, I pulled these coils off the chassis and measured the inductances of the coils.  I just needed to make sure they were close in value.  They were: 

L1 was .858uH L2 was 2.709         L3 was .930uH  L4 was 2.672

Next I checked the ganged variable capacitors.  At first I found that one cap had a lot more capacitance than they other.  How could that be?  Then I remembered that I had installed trimmer caps across each of the ganged capacitors. Adjusting these trimmers (and leaving the caps connected to the grid of V1a and V2A, I adjusted the trimmers to get the caps close in value.  I think I ended up with them fairly close: 

C1: 63.77-532 pF          C2 64.81 -- 525.1 pF

I put the coils back in and checked the tracking on 40 and on 80/75.  While not perfect, it was close enough to stop messing with it.  

DETECTOR CIRCUIT

I've had my doubts about the detector circuit that Lew McCoy had in the MMMRX.  In his 1966 QST article he claimed that the circuit he used was a voltage doubler, and that this would boost signal strength.  But I built the thing in LT Spice and didn't notice any doubling.  And consider the capacitors he had at the input and output of the detector:  100 pF.  At 455 kHz 100 pF is about 3500 ohms.  At audio (1 kHz) it is 1.5 MILLION ohms. Ouch.  No wonder years ago I put a .1 uF cap across that output cap just to get the receiver working. 

Scott WA9WFA told me that by the time the MMMRX appeared in the 1969 ARRL handbook, the second "voltage doubling" diode was gone, as were the 100 pF caps.  Now it was just a diode, a .01 uF cap and a 470,000 ohm resistor.  I switched to the 1969 Handbook circuit (but I have not yet changed the 1 meg grid resister to 470k -- I don't think this will make much difference).  Foiled again by a faulty QST article, again by one of the League's luminaries. 

6U8s out, 6EA8s in 

We learned that the 6U8 tubes originally called for by Lew McCoy are getting old and not aging well.  So I switched all three to more youthful 6EA8s.  This seemed to perk the receiver up a bit. 

MUTING from the DX-100

My K2ZA DX-100 has a T/R relay mounted in a box on the back of the transmitter.  When the Plate switch goes up, it switches the antenna from receiver to transmitter.  The box also has a one pole double throw switch available for receiver muting.  I put the common connection to ground, the normally connected (receive position) connect the ground terminal of the AF output transformer to ground -- it is disconnected from ground on transmit.  The other connection (normally open) is connected to the antenna jack -- on transmit this connection ground the receiver RF input connection.  These two steps mutes the receiver very nicely. 

Replacing Reduction Drive

Over the years I have had several different reduction drives on the main tuning cap.  I had a kind of wonky Jackson brothers drive on there that needed to be replaced.  I put in a new one -- this smoothed out he tuning considerably. 

Ceramic Resonator

I never could get McCoy's 455 kc two crystal filter to work right.  So at first I made due with the two 455 kc IF cans.  This made for a very broad passband.  Then I put a CM filter in there.  This was more narrow, but with a lot of loss.  There may have been others.  But the filter spot is currently held by a 6 kHz wide ceramic filter.  This one is my favorite so far. 

Digital Readout

When I was running the DX-100 with the Hammarlund HQ-100 I built a little frequency readout box.  The box was from a Heath QF-1 Q multiplier (I am sorry about this).  The readouts are in Juliano Blue and come via e-bay from San Jian.  I now have it hooked up to the DX-100's oscillator.  I haven't tapped into the MMMRX's oscillator yet. 

Mate for the Mighty Midget with 6 kHz Ceramic Filter

I built this receiver back in 1998, but I continue to have fun tinkering with it. I wrote an article about it for "Electric Radio" magazine (Number 115).   One of the major shortcomings was the crystal filter that Lew McCoy prescribed.  It was very difficult to get 455 kHz crystals to work well as filters.  At various times I've had all kinds of replacements in there in place of Lew's filter:  a 455 kHz IF can, a Toyo CM-5 hybrid ceramic filter, a fancy Millen high Q IF transformer. None of them really worked well. 

Recently I put a little +/- 3 kHz ceramic filter in there.  This is a 6 kHz wide filter at around 455 kHz.  I think it works really well.  Above you can see the receiver in action.  I use it with a little powered computer AF amplified speaker -- I just don't like headphones.  

The latest  filter mod with the 6 kHz ceramic filter  is shown above. 

Above you can see what the whole 455 kHz filter and transformer passband looks like. The input was through a 2k resistor placed between the .001 uF cap and the filter. The output was also through a 2k resistor placed at the top of the secondary of T1. (So don't pay any attention to the insertion loss.) 

The NanoVNA is displaying 2 kHz per division.   I put the BFO at 451 kHz.  This results is excellent opposite sideband rejection.  The filter is really too wide for SSB, but it is about perfect for AM, which I listen to quite often on both 75 and 40.  SSB and AM both sound quite good.  Check out the video above. 

It is kind of amazing what can be done with just three 6U8 tubes. 

There are many previous SolderSmoke blog posts about the Mate for the Mighty Midget Receiver here:

https://soldersmoke.blogspot.com/search?q=Mighty+Midget   Be sure to keep selecting "earlier posts" so see more. 

Scott WA9WFA's Mate for the Mighty Midget Receiver is WORKING! (Video)

Wow, Scott got his Mate for the Mighty Midget receiver to work and he is obviously overjoyed with the result.  All of us who have struggled with a homebrew project know just what this feels like.  And it is very cool that Scott got some useful guidance from Charlie Morris in far-off New Zealand.   Congratulations Scott.  I'm really glad you stuck with it.  

Scott's success comes at a good time:  Pete N6QW is building W4IMP's three tube "IMP" SSB transmitter (also from the 1960s).   I accept responsibility for naming Pete's project:  It will be known as "Pete's IMP" or, memorably,  "The PIMP."  For a look at Pete's rig go here: 

http://n6qw.blogspot.com/2021/09/lets-build-something-with-vacuum-tubes_23.html

Scott had problems getting Lew McCoy's 455 kc crystal filter to work.  So did I.  It turns out that this is a very old problem, going back to World War II.  In Don Stoner's 1959 "New Sideband Handbook" on page 54 he writes of homebrew filters in the 400 to 500 kc range:  

"Inexpensive crystal filters constructed from war surplus FT-241 type low frequency crystals are very popular with the 'do it yourself' hams. These CT cut crystals have been plentiful and relatively cheap for a number of years and are in the hands of many Amateurs. The general run of war surplus crystals may or may not be good. Experience has shown that one out of four of these crystals are usually defective in one way or another." 

Stoner was writing just 14 years after the war.  Add another six decades to the age of these crystals -- often decades spent in musty basements -- and you can imagine the percentage of bad 455 kc FT-241  crystals increasing.  So I think Scott is wise to seek an alternative to McCoy's crystal filter. 

Scott's original build of the MMMRX receiver is just so nice.   In the video he says he plans to go back to it after he gets the expanded version fully functional.  He should definitely do that -- his  original version looks so good.  I think it is probably very close to working properly. 

Thanks Scott, and again, congratulations OM. 

First Signals from the PA3GSV Mighty Midget Mate

Obviously the Radio Gods (Spirits in the Sky) approve of Jan's work.  How could they not?  I can now see why he took the trouble to cut that hole in the sewer pipe cap that forms the large wheel on his homebrew reduction drive.  But what are we going to see through that center hole Jan?  What will the frequency readout be like?  
----------------------
Hi  Bill,

Just finished the last stage of the Mighty Midget MK2.

There are first signals!

The first one I heard was a broadcast station, believe it or not, the song that was on was “Spirit in the Sky” ..

All stages were built, tested and as far as possible, adjusted separately.

It was built from back to front, so the RF amplifier was last.

I added an ECL82 for more audio, the first thought of only using an EL84 didn’t bring enough.

The triode of the ECL82 as a pre-amp, the pentode as final.

Furthermore ECF82’s were used instead of the 6U8, they’re more widely available over here.

The Miller coils are hard to come by, so the 300 uH coils are homebrew.

Also used a grid detector instead of the two germanium diodes.

The triode of V1 originally intended for audio was used for this.

Made the BFO adjustable as well, still remember the screwdriver sticking out of the coil on your side... 

 

Happily there was not much troubleshooting needed.

The 80m coil was only 5 kHz off, the 40m coil 300 kHz (to low in frequency), still have to fix that.

Initially the receiver worked reasonably well without adjusting, but C1 quit at some point.

After some investigation, the problem was a dirty wiper contact on the rotor.

An ultrasonic bath fixed the problem, so no looking out for a replacement there. (hope it stays that way)

 

After adjusting, sensitivity is around -114dBm (0,4 uV) / 10 dB S/N!  (with the FT241 crystals in place, and careful tuning of the controls)

Really not bad for this small receiver, Lew McCoy was right, it really is a Mighty Midget.

I wanted to make some video’s, but over here there’s a terrible S9 rattle from 160 to 15 meters.

Every now and then it appears out of nowhere, and disappears the same way.

As soon as it is gone, I’ll make some video’s.

I made one video though, just after completing the receiver.

 

https://www.youtube.com/watch?v=CMQDYpZRkPw

 

Reception on CW and SSB sounds really well, but unfortunately didn’t record that.

The receiver is not finished yet.

Next to the 40m coil,  S-meter has to be tried, and there’s still some work on the cabinet and front panel.

 

More to follow.

   

73  Jan

PA3GSV

6EA8s in the Mate for the Mighty Midget, and WA9WFA Re-Builds a Heath HW-12

I reported to Scott and Grayson that I had finally gotten around to changing the three aging 6U8 tubes in my Mate for the Mighty Midget receiver.  I replaced them with three more youthful 6EA8s.  This switch really seemed to perk up the old receiver.   I'm listening to 75 meters on it right now. 

Speaking of 75 meters, Scott sent me this picture of his latest effort:  re-building a Heathkit HW-12.  FB.  This is a way of experiencing (or re-experiencing) the construction of a Heathkit.  I did something similar, but much less complicated)  with a Heathkit VF-1.  Scott did a wonderful job taking this old rig apart.  That PC board looks great (see photo below). 

Scott's e-mail:  

Hi Bill and Grayson, I’m glad to hear of your good results with the 6EA8’s in your MMMrx! I had similar results when I finally got rid of the 6U8’s with their iffy performance and went with the 6EA8’s.  I did put a set of 6GH8A’s and tried it out, it worked, but I don’t have any data on performance improvements.  After completing the outboard power supply and audio amplifier, I’ve taken a break from my MMMRx and it’s sitting there on the bench. I’ll get back to it in a while.  

 

In the meantime I’ve started a new project where I’m re-kitting a Heathkit HW-12 eighty meter transceiver.  I have completed the disassembly process including the pcb.  I bought a Hakko vacuum desoldering iron for taking all (ALL) parts off of the pcb, and it’s bare now.   I’m planning to start rebuilding this coming week.  73 Scott WA9WFA  

Joe Galeski's 1960 "IMP" 3 -Tube Filter SSB Transmitter, and the Spirit of SSB Homebrew

Here is another important bit of SSB history.  In  May 1960, Joe Galeski W4IMP published an article in QST describing his super-simple SSB transmitter.  While Tony Vitale's "Cheap and Easy" rig was a phasing design, Joe came up with a filter rig.  He built USB filter at 5775 kc.  With it, he ran a VXO at around 8525 kc. This put him on 20 meter USB. 

Here is the QST article: http://marc.retronik.fr/AmateurRadio/SSB/A_3_tubes_filter_rig_%28SSB%29_%5BQST_1960_5p%5D.pdf 

In discussing how to put this rig on other bands, Joe got the sideband inversion question exactly right: 

Thank you Joe!  

Joe even provides an comment that seems to capture an important element of the homebrew SSB ethos.  Joe homebrewed his filter, but he mentioned the possibility of using a store-bought filter: 

That's the spirit Joe!  

Along the same lines, Jim Musgrove wrote in Electric Radio: 

Having built Lew McCoy's Mate for the Mighty Midget receiver (which also used just three tubes), I can't help thinking that an IMP-ish transmitter would be an excellent complement to the Mate for the Mighty Midget.  

Jim Musgrove K5BZH knew Joe Galeski and wrote about him in the January 1992 issue of Electric Radio.  Jim wrote that Joe was an optometrist by profession. When OE1FF wanted to know the cost of building an IMP, Joe Galeski boxed up the original and sent it to him.  FB Joe. 

In December 1961 Joe Galeski published a QST article describing a transistorized version of the IMP -- this rig ran on 15 meters.  K5BZH wrote that Joe later published an article about a small, solid-state transceiver,  appropriately called "The Shrimp." 

Glowbug Christmas

My "Mate for the Mighty Midget" receiver (with 6U8s)

Some excellent Christmas verse from Brad, AA1IP:

The Glowbugs' Christmas
******************************

T'was the night before Christmas
And all through the shack
Not a heater was glowing
In all of the rack.

Sitting totally silent and quiet that night
The old HRO shed nary a light.
The Johnson exciter and its homebrew final
Felt cold as leftovers, or seat-cover vinyl,

I drowsed at my workbench feeling tired and weary,
The print in the Handbook looked fuzzy and smeary.
I thought, "I'll make coffee", and groaned to my feet
When I heard a loud clatter outside in the street.

What the--? I wondered and turned on the lights
And there I beheld a wondrous sight:
A battered old van heaped high with components
And a grizzled old ham with a bagful of doughnuts.

I noted his callsign-- can't recall it today--
But a patch on the side read "FMLA".
I opened the door and hollered "Come in!
The coffee pot's heating, and we'll sit down and chin!"

He spoke not a word but whistled in Morse
A "GE OM", and "By golly, I'm hoarse.
Too many contacts, and hot rosin smoke."
I nodded and poured him a mug of jamoke.

He emptied the doughnuts in a pile on a plate
And explained in a whistle, "I'm running real late.
I've new 6L6s and fine 211s, 6146s and good 'SN7s.
And 866s and 0B2s, type 45s and mil-spec 807s."

"For the regennie crowd, 201s by the score
And good ol' type 30s and 19s galore.
I haven't neglected the passive-parts run
There's lots of good iron by old Thordarson."

I nodded and smiled, suppressing a chortle
As he reached in his pack and left me a 304TL.
He whistled, "I'm leaving, the coffee was great,
But I'm overdue in the neighboring state."

"Keep everyone building the rigs of their choosing
Or we'll lose the bands that we're lazy in using.
Transistors or tubes-- any project is fine--
Just keep on constructing and sharing on line."

He leaped to his feet and waved a gloved hand
As he sprang for the door and his rusty old van.
I heard him exclaim as he drove away from me,
"Merry Christmas, you Glowbugs, and to all 73!"

****************************

(With apologies to Clement Clarke Moore, who surely would have
been a ham had radio existed in his time. Alas, all he had
to build with was words.)

For new readers: the FMLA (Five Meter Liberation Army) appeared in a
series of short stories written by Michael N. Hopkins, AB5L (sk). If you have never
read these, you're in for a fun evening!

For QRP fans: the numerical references in the poem are for classic
vacuum tubes, several of which operate at QRP levels (whether or not that was the
original builder's intention) and (mostly) within the amateur bands.

Mr. Carlson Restores a BC-348 -- But 40 Meters Sounds Very Weak. Why?

Mr. Carlson mentions that he is a ham radio AMer.  He is also on SSB, but AM is, he says, his preferred mode.  FB. 

I got a real chuckle about the MVC switch position on the front panel.  "Manual Volume Control"  like "Manual Gain Control."  I thought I had invented this term.  You know, "real hams control their gain manually."  This is why Farhan never put AGC in the BITX20. But it seems the Army Air Corps was way ahead of me with the BC-348.    

Mr. Carlson makes it sound (!) like the Dynamotors are a thing of the past.  Not true.  Every Saturday morning I listen to the Old Miltary Radio Net and hear a number of Dynamoters spinning in the background.  One belongs to Buzz W3EMB who uses a BC-348.  Buzz has an SDR attached to the IF of his BC-348, which I think is an admirable mix of the old and the new.  Those BC-348s are, after all, quite old.  WWII old -- like 85 years old.  And still working.  

Paul has a good discussion of the importance of short lead length, and of mounting shielded capacitors properly, and of the usefullness of a good groundplane under the capacitor.  Go Manhattan boards!  Paul's presentation on how to identify the outside foil of a capacitor was very good, but I was wondering if you could also find out by using a file to remove some of the yellow insulation, then test with a DMM to see which terminal is connected to the foil. 

"Lots of times you have to add solder to remove solder."  Indeed. 

The importance of testing for BOTH capacitance and leakage.  Yes. 

But why bother with "period correct" internal wiring harnesses, when you have already put a bunch of modern caps in there?   I mean, I'm in favor of the re-capping, but this seems inconsistent with the need for "period correct" internal wiring harnesses. 

When Paul first fired up the receiver, I was hoping he would disconnect the antenna to see how much band noise was getting through. 

When Paul got to the IF alignment, he spoke of the dangers of working on energized high-voltage gear.  I had been thinking about getting a BC-348 myelf, but Paul's comments reminded me of why this is probably not a good idea for me.  Paul's comments about "knowing where your hands are" is on target.  "One hand behind the back, " is a good rule for this kind of energized testing -- this will help prevent current from a mishap from flowing through your chest.  

I may have more comments later.  Off to the beach now.  

Back from the beach:   

"Contrary to popular belief, the simpler the receiver, the better they hear."  Amen Mr. C. 

Paul's heroic cleaning of the 915 kc crystal made me feel like a wimp for not having tried to do this with some "bad" 455 kc rocks I was given while trying to build the Mate for the Mighty Midget receiver.  I may go back to those crystals and try to clean them as Mr. C did. 

The grand finale of this 2.5 hour video was, as expected, a demonstration of how good it sounds.  But unfortunately it did not sound good.  Paul tuned through the 40 meter band and I heard NO CW signals.  I didn't even hear FT8.   There were a few anemic SSB signals and, as he reached the upper portion of the band some very weak AM broadcast signals.  I didn't even hear CHU Canada's time signal.  Huh?  Why?   Our very simple homebrew Direct Conversion receiver sounds a LOT better than that.  I mean look at the sweep of 40 meters that I did using only a simple dipole:  https://soldersmoke.blogspot.com/2024/12/an-evening-bandscan-on-40-meters-using.html  Why is there such a big difference in performance? 

Could it be the antenna that Paul was using?   He was on what he calls the 369 antenna.   Could there be a problem in the receiver?  Could it be band conditions?   

Radio History Question: Why 455 kHz as the IF frequency?

My work on the S-38Es, on the HRO-dial receiver, on the Mate for the Mighty Midget,  and on various mechanical filters has caused me to think (once again) about why we ended up with 455 kHz as the  IF frequency for so many radios.  I've heard many explanations for this, but unfortunately I've forgotten the explanations and lost the sources.  I started digging into this again today.  I found the below e-mail from Al N3FRQ on the Boatanchors mailing list (2008). 

I contacted Al to find out if he had learned anything else on this topic.  He has not.  So if anyone out there has answers to Al's questions, or anyother info that would shed light on why they went with 455, please let us know. 

------------------------------- 

Every so often the question comes up: Why are all the IF’s 455 KHz? I’d 
like to get an article together that solves this riddle while the people 
who know are still with us. I know parts of the story, but I need help 
with a couple of issues.

There are two major consideration is the choice of the intermediate 
frequency used in a superheterodyne receiver. The lower the frequency, 
the easier it is to attain high selectivity. Also, in the early days, 
before tetrode and pentode tubes, it was easier to achieve a high degree 
of amplification at lower frequencies. Conversely, a higher IF frequency 
results in better image rejection.

Early superhets had the IF at 100KHz or lower in order to get adequate 
gain from the available triode tubes. They suffer severely from 
“two-spot tuning” (images). By the early 1930’s, broadcast set had 
settled in at 175KHz, and automobile receivers would later adopt 262KHz 
as a standard.

The advent of the short-wave craze, and multi-band broadcast receivers 
dictated a higher IF frequency to achieve adequate image suppression on 
the short-wave bands. The broadcast band occupied 550-1500KHz at this 
time, and the designer encounters sever problems if his radio tunes 
across it’s own IF. Some shortwave sets used 1600-1700KHz for better 
image rejection, but one couldn’t go higher if the 160-meter ham band 
(1800-2000KHZ) was to be covered. Most multi-band receiver settled in 
near 450KHz, a comfortable distance from the first broadcast channel at 
550KHz.

Questions:

Odd multiples of 5KHz, 455, 465, etc., were usually chosen so that the 
image of the carrier of a broadcast-band station could be zero-beat with 
the carrier of the station being tuned to achieve minimal interference. 
(This assumes 10KHz channel spacing. Did the Europeans (9KHz) do 
something else?)

The Radiotron Designers Handbook, Third Edition, p. 159, states “A 
frequency of 455 Kc/s is receiving universal acceptance as a standard 
frequency, and efforts are being made to maintain this frequency free 
from radio interference.”

(1) Do FCC and international frequency allocations reflect this?

(2) I’ve heard the term “Clear-Channel IF.” Can anyone cite references?

(3) At lease one news group posting claims that broadcast frequencies in 
a particular market are assigned to prevent strong inter-modulation 
products from falling near 455KHz. Is this factual? Need reference.”

(4) Was this (3) at least part of the reason for “Radio Moving Day” in 
1941? See: http://www.dcmemories.com/RadioMovingDay/032341WINXFreqChange.jpg

(5) Many National Radio sets used a 456KHz IF’s and I think I remember a 
437 somewhere. Why? Are there different considerations for short-wave CW 
operation?

Further input, corrections, and elaborations are greatly appreciated. 
Scolarly reference will be looked upon with great favor.

Regards,
Al

-- 
Al Klase - N3FRQ 
Flemington, NJ 
http://www.skywaves.ar88.net/

The meaning of "CM" in the Toyo CM-455 Filter

Photo by ZS1KE

A while back I picked up (from e-bay?) a 455 kc crystal filter for use in my Lew McCoy "Mate for the Mighty Midget" receiver.  I did a quick and dirty installation.  It kind of worked, but I had it in the back of my mind that I had to work on the impedance matching to ensure minimum passband ripple.  But when I learned what the P, B, E, and G pinout designations meant (plate, B+, earth and grid), I realized that this device had been designed with tube impedances in mind, so I probably didn't have to mess around with input and output networks (as I've done with the BITX rigs).  Last week I installed it as the manufacturer intended -- it sounds great.

Today I started wondering about the passband characteristics of the device.  What do the skirts look like?  So I started Googling.  There is not much out there, but I did come across a really interesting Epson site that describes the origins of this filter, and what the CM means.  CM is for "Crystal Mechanical."   Wow, this little box combines the characteristics of  a crystal filter AND a Collins Mechanical filter:

http://www5.epsondevice.com/en/information/technical_info/qmems/story2_5.html

An excerpt:  

"While at the Electrical Communication Laboratory of NTTPC, Mr. Nakazawa had had a flash of inspiration: ‘We could develop a crystal unit with a high Q factor by using the wire mount technology I'm studying now. Then, if we can achieve the idea of a mechanical filter that mechanically joins multiple units using quartz material, we should be able to develop a compact filter that achieves both excellent filter characteristics and thermal characteristics.’ Without a pause, he quickly tackled the next development issue, which resulted in the creation of the ‘crystal mechanical filter (‘CM filter’)*5). This CM filter was manufactured by processing the quartz substrate into an ‘H’-shaped filter element and functioned by using the long thin sections on the left and right sides as resonators (Figure 1). The middle portion connecting the two sides fulfilled the role of the coupler. This was precisely the ‘mechanical filter achieved using crystal (quartz)’ that Mr. Nakazawa had envisioned.

This filter was released on the market as a 455kHz intermediate frequency (IF) filter for single-sideband (SSB) modulation in radio communications. The use of quartz material meant that not only were good filter characteristics achieved, but thermal characteristics were also excellent. As this was the first filter to offer properties of this caliber, it sold extremely well throughout the world. Furthermore, this technology received the honor of being granted the Notable Invention Award from the Science and Technology Agency."

Does anyone have the specs on these filters, and perhaps a passband graph?

Three cheers for Mr. Nakazawa!