The AM Forum

Thanks to Lou VE3AWA who purchased an ancient REL175 rig I have been experimenting with loop modulating a power oscillator. In this case my 45 tnt 1929 style. The rig he bought had a loop and a mic jack. Just for fun he tried it. It works too!
I am astounded at how well it works. Tis a completely passive system from the very early days of Ham radio and AM.
Don VE3LYx

Nothing found on 'Loop Modulation', everything refers to PLL.  

The only reference to a "Loop Modulator" I can find is from QST Sept. 1961, Hints and Kinks. The voltage seems rather low (I would expect more like 6v or higher).

Next is a common carbon mic circuit using a transformer.  

In essence they are identical.  Both use a transformer, however simple, and are really Grid Modulation methods.   I'm sure we could find a way to capacitively couple them instead of using a loop/transformer/inductor.  bill

I did something similar when I was a young Novice. I put several turns of wire around the output coil of my Ameco AC 1 (wish I still had it) and and connected it to the carbon element out of our house phone  ...no battery . I yelled into the mic with the rig tuned up and ZAPPED myself..rf burn too !! I should have taped the back of the element ..I didnt know what I was doing  but I read it in a book. I didnt realize how much voltage a few turns of wire around the tank coil of a 10 watt tx could produce !!

Rick

I had exactly the same experience.  Burned my lip - went to 100% modulation when I yelped  ;D

We must have read the same article!! Glad I wasn t alone.

Rick

Lou's radio is a commercially made or so it appears unit from the 1920s . Mine is a 45 TNT. the single turn, not even closed at the end of the osc coil (only coil in these rigs) the carbon mic is connected as a load across this single turn coil and absorbs power. As you speak the power varies accordingly and very good modulation results. I use a bakelite mic which eliminates the lip burn. I have been testing now for two days. Lou has made a Contact with a nearby Ham and has challenged me sort of to see who can get the LOOPIE Dx record. Doesn't matter to me because if Lou hadn't mentioned it I would never have known about it. I had to search back to 1924 to find any reference to it EXCEPT it works very well. It has zip to to with cathode grids and plates and there is no transformer. It is merely a passive absorbtion device . I am currently using my tune lamp loop to run it although I started experimenting with a 3 turn loop of hookup wire stuffed in the ant link coil. I was concerned about varying freq so I tried the antenna but Lou's was built with it at the main coil so I tried that too. (dad said try everything you think of so I do) It took a matter of minutes to hook up. XYL was unable to distinguish between me standing behind her or a rx in the TV room with me 70 feet away and downstairs at the mic.
In fact she didn't look around and was still talking to me when I came upstairs unaware it was the radio she heard and not me.
With the BK thing on this weekend and I have a few old style rigs although not all legal for the Bruce Kelly thing (some of my tubes are 1930s)I am fascinated with this seemingly lost technology. Probably because it works.
I have now explored screen mod with and without carrier control , parallel tube mod (works but not good) mixer tube mod, cathode series mod with and without a transformer so loop modulation caught my eye immediately. Cost is zero and appears to work as well as any of the above.
Will be testing the next few days around 7120 (7116 to 7120) should any wish to hear it.

Don

Here is a picture of the loop I am currently using to modulate. It is that bent up thing close to the front panel. Tis a bit the worse for wear having been used with several different coils I have had to twist it a few times over the years. If the loop will light a flashlight bulb then it will also work with a carbon mic to modulate. Don't use a metal mic. That of course seems obvious to me but some do and the  wonder why they got a little sting when getting too close with their lips.
The loop and a carbon mic is all that it is . NOTHING else! No grid bias discombobulation or any such gobblygook. A loop and a mic. How well does it work? Astoundingly well. Unbelievable really. TX power output is much better then with cathode modulation (unless modulator tube voltage is added to the B circuit)
Don

AM , PM & FM all for the price of one.  ;D

I wouldn't be all too hasty to say that. Re read what I said so far. Modulating does not change the freq to any noticeable amount. It just changes the fact does the power go up the spout to the antenna or is it absorbed in the carbon mic. I use a narrow digital rx to monitor beside my normal boat anchor RX just for that reason. As I mentioned above , I first tried it in the antenna link because I had such concerns about FMing. But when I tried it using this loop I discovered my fears were unfounded. It changes the freq no more then does the tuning lamp.  I am sure there is a measurable movement if one has the super duper equipment and I don't want to get into a discussion of how much but as one who works a lot of AM with various boat anchors I can say it is pretty good. Keep you hands away from the radio is the secret to power oscillator vintage stability when built in the old traditional way. A spiral or pancake coil helps big time but this TNT doesn't have it. It is my Bruce Kelly rig (This weekend BTW)
Listen on or around 7120 as I will be testing on air over the next few days. 3.5 watts out!
Don
And I will answer CW transmissions in CW as it still has its key for those who don't have AM privilages at 7120 and down

Just kidding a little Don.
Very nice little rig and I see your #12 wire loop.  

Have you tried a little battery bias on the carbon mike, with appropriate cap. bypass of course?  Possible reduction in distortion and a little more drive.

Or does the coupled carrier self rectify through the carbon granules to provide the bias?  I think it does.  More reason to adjust the fractional number of turns of the coupling link.

Superposition theorem, etc.

The carbon mic acts as a variable resistor, the varying resistance causes the amount of RF absorbed to vary, which is how the modulation is created. Fessenden used a carbon mic (that was water-cooled) in series with the antenna to make AM.  I wonder if there's a way that you could make a tube do the same thing, act as a variable resistor to absorb RF? I know the variable resistor part would work with a tube (that's how a series cathode modulator works by acting as a variable cathode resistor which causes a variable gird bias) but I don't know if it would work by absorbing the RF. Definitely would be neat if it could work that way, since then the tube would provide isolation between the mic and the tank circuit.

EDIT: I just thought of something, the loop could also be coupled to a buffer stage after an oscillator and still modulate the carrier, but without having much (or any) effect on the frequency of the carrier (if you didn't have a stable oscillator). Then feed all of that into a linear. Definitely would be something interesting to hear on the air.

Ah you thinking right along the same lines. I have a Mopa here and may try it on the dummy load to see. It may not be suitable as it is a transceiver I built from a regen I had designed. Has a 56 osc and a 42PA(I think. I have had a couple of Pa tubes so if I remembered wrong PSE forgive.) Anyway for the time it would take I will try it. But your right it should work. IF a modulated signal can be amplified it shouldn't matter how it was modulated. I tried a lot of small thing so far. By passing the mike with a cap just to see. (WRONG!) And the other things I already mentioned.
Anyway will be testing as time and band permits
Don

I had to play tonite but when I got home I did the Bruce Kelly thing for a bit then gave up and turned my attention back to this interesting deal. I dug out the mopa . It is pictured below. Just so you know what we are dealing with I give a quick 50 cent tour. Based on the fact that a regen and a osc share the same circuit except for the regen control I realized some time ago that a regen is just an oscillator before it reaches osc voltage. So I did a lot of experiments and found Hartley Osc also makes very good regen. This is the final outcome of that and is a two tube transceiver that works pretty good. The OSC tube is a first generation 56 with the early globe style glass. On receive it functions as a regen with a pot and a plate resitor switched into the plate circuit supply line. On transmit they are bypassed and the plate gets proper voltage for good osc operation. The key is always used PTT&PTL(press to listen) The left hand toggle switch controls that function(osc or regen). The Pa tube is a 42(I checked tonite just now to be sure so I wouldn't be misinforming) On receive it is an audio amplifier taking a signal through a coupling cap from the plate of the 56 tube when it is in regen mode. It outputs to the earphones. On transmit I leave the biasing as it is for audio but switch in a Tank circuit with a link for antenna output. The right hand toggle switch looks after that. So tonite I fired it up on 7118 and dumped it through my tuner into my dummy. (yes, it is a 100 watt light bulb) I tried first looping the tank for the PA and could get pretty decent audio. Then I tried using the carbon mic on the 3 turn antenna link normal used on receive. OSC would not start. So I took a length of hook up wire and made one turn then twisted them at the opposite end of the coil. Not much of a coupling but it had worked on the TNT so I had to try it I even if I felt it was likely to be a disaster. I had the monitor rx cut way down (XYL was asleep, didnt want to wake her)even though this is a very low power rig. Probably barely 2 watts if that. So now I have it hooked essentially to a mopa and I am modulating the audio at the osc with a single turn link and a carbon mic for its load. WOW!!!!!!!!!!. That is all I am going to say for now .
Tomorrow I will try it with my Bare Essentials CW rig. I have the loop already there. If that works then on to the ARC 5s. Hard to believe this technology was lost more or less for 90 years. Now remember these were CW rigs but they are now AMers. Exciting times these last few days.
Don  

I think there is more to this then I initially thought. I have now signal reproduction as good as "in the room" but it appears to be a very narrow signal. Going to take it to a much higher power level and see. (My monitor scope does not have an amplifier but samples live in the feedline) Tis possible since nothing is added to the signal but a portion is removed.
Don Dulmage
VE3LYX
Dec 8 2013

If I'm right, a carbon microphone has a very narrow frequency response, so audio will be very narrow, but at the same time should be a fairly clean signal (other than maybe a little frequency modulation and carbon microphone noise) since the modulation is being generated passively. There's really nothing there to generate too much distorion, other than the microphone itself, and there's no way to get it to modulate over 100% negative, since 100% negative modulation means the microphone is absorbing all of the RF that the tube is putting out. Very crude way of generating AM, but very effective too. I'd be curious to know around what the percent of modulation is. If you boosted the power up some I bet it would make a very effective transmitter for some fairly long distance contacts, due to the narrow response, it should be heard very well.

Could be. I am going to try it on my ARC 5 80M set up. If you want I will give you a heads up first if your within signal distance. I get about 50 to 60 watts out on the arc 5 (Pair of 1625s at 770 volts) I am going to loop the osc coil exactly as I just did this one. Right now I am going to try it on my bare essentials 80M rig . I have it all set up on 3561 (crystal controlled) Just have to go down to studio B (as in basement) to get the carbon mic.
Friday I was playing with it on the TNT and took a grundig crank upstairs and set it in the TV room on freq . The XYL was at the computer with her back to me in the entry room and didn't notice) I went back down and called CQ, asked what kinda soup was for supper and IDed. When I came back up she was talking to me asking why I was talking to her like that. She didn't know I wasn't standing behind her in the next room as she hadn't turned around. That is pretty faithful audio I would say. Instruments are one thing XYLs are a whole different deal!:>)
That fueled my interest to persue this further. It sounds different, but you'll see. Sort of like SSB without the squawk. Not a good description but in that vein anyway. Audio quality is now very good.
Don

I'm about 280 miles away to the south, which is a little far for me to hear low power stuff on 80, even at night.

Well the low power days are over (relatively speaking low power) I just tested it on my one ARC5 which is on 160M (1880kcs) Took a bit different loop but is now working very well. Since I had the VFO open I made the mod permanent and tomorrow will sort out a good permanent wiring system to bring it out of the VFO shield to a 1/4 phone plug. I was going to do the ARC5 for 80M tx but I already had the covers off this one as I just recently got it and modded it for 160M. 280Miles? Last week on the lil Transceiver pictured above I got a sig report from Sault Ste Marie 367 miles as the crow flies. 60 watts from the Arc5 should do 280 with ease on a good day. Anyway I just offered in case it interested you since you seem to have a grasp on this.
Don

The problem isn't with how much power you are running on your end, it's with how much noise I have here, even the stations with 300 to 400 watts of carrier are affected by it.

Oh sorry to hear that. Using a beverage antenna on rec will kill the noise completely BTW. Even a short one
Don

I removed it from the ARC5 While after some fooling about I was able to make it work on the ARC5 which is kinda surprising when I put it all back under the VFO cover  and put the lid on it became unmanageable. I removed it as I don't need it that bad so the redundant cathode modulators have been given a reprieve.
Don

All experiments now completed successfully with on air test and contacts. Even the ARc5s have it now on their PAs.
Never had so much fun! No mod tube, no mod iron. It just works! Now to rub on it a bit making it a bit better each day. Hope to establish a loop mod net.
Don

I tried it on my TPTG and smoke came out of the T1 element!

You guys need teflon lipstick before somebody gets bit.

Gee Dave.  Where were you when I needed you.  Then again, I would have had a hard time explaining the lipstick to the guys in high school machine shop class.

I hate to show my ignorance but just what is the purpose of the battery in previous examples of loop modulation?  The carbon mike is just a variable resistor.  I assume you adjust the power out for 1/2 power (or is 1/4?) with the loop and let fly with the CQs

Al

You don't need the battery. The resistance of the carbon mic provides a constant absorptive load on the oscillator, which is the carrier's output level (the oscillator is running at full power into the mic, the mic absorbs part of the power). The changing resistance causes the power output to vary, which makes the modulation. A very crude way of making AM, but rather effective.

"I hate to show my ignorance but just what is the purpose of the battery in previous examples of loop modulation?"
I am sorry I don't understand. I don't use a battery. Where did that come from?" I couldn't find it.
You assumption is correct. The mic functions as a variable resistor (absorbing a portion of the RF into the loop  that would normally have gone to the antenna or at least the next stage.)
Re the TPTG comment above,  No, you didn't try it on your TPTG  because if you did you would already know it works.
It is not an iffy thing.
Don

I wouldn't have choosen crude , interesting perhaps or unusual but your assessment is correct.
It also works on other stages besides the oscillator. I have it on the buffer stage of a crystal controlled solid state QRP rig for 80M , on the PA stage of a two tube mopa, and on both my ARC5 transmitters. Plus my TNT which I have been using on air the last week or so. The TNT was probably a bad choice as they are not the most stable rig however I spent a day reworking its vari cap positioning and installing a vernier drive to get the capacitor free of hand capacity effects and make tuning easier so I'm not swishing around. Seems like a new radio now. Did an recorded on air test today. 
Don.  

Hi everybody.
Interesting subject, and indeed the first AM modulation method, using carbon mics. But it started putting the mic in the ground of the antenna system, not in the coil of the transmitter. That came a little later. The first AM transmitter was a spark transmitter with a very high quenching resulting in a hundred thousand sparks a second, giving CW. The best there was was the Moretti spark gap, a spark gap with water jet quenching. This amazing gap could produce 1 kW RF with an efficiency of approx 65% max!! A figure not too bad many transistor amplifiers. All CW transmitters, Spark, arc, small machine transmitters etc. were modulated with a carbon mic in the antenna system. The antenna current however was half to several amps, so the carbon mics ran hot. There were carbon mics with oil cooling and systems with very many microphones in series and parallel. But Marzi made one of the best one's, a vessel with carbon powder constantly flowing out of a small hole wich was partly blocked by a tonge connected to the membrane of a microphone, but more often to a relay system driven by a normal carbon microphone, a mechanical amplifier.
In the years 1918, the system of connecting the carbon mic to the coil of the transmitter was used in the Netherlands by the first broadcast transmitter in Holland. Indeed was the modulation mixed AM and FM.
Please look at the attaceched photos, a diagram of a 1912 transmitter with AM modulation, a Lorenz microphone and the Marzi microphone.
I have more info about this old technology, so when you like to see e few pic's, mail me. All my books are already stored because I like to move out of Spain, but I have nice pic's in my computer.

It really isnt a lot of FM. That is a kinda unfair assumption. Certainly little if any more then would be found in normal 1940s transmitters but I am tired of arguing about it , suffice to say I have used it and even recently. Some have talked with me probably not knowing that is what I am using. The secret to the deal is to make the mic in a fixed position and speak at it through a tube rather then holding it your hand. The Fm, if it occurs, doesnt come from the method but from hand capacity problems associated with most single tube Power oscillators. Once you mount the mic solidly and dont touch anything it become very stable You are NOT modulating the oscillator. You are absorbing output that otherwise would have gone to the antenna. The RF is already established. It either goes up the wire to the antenna or into the carbon mic. Anyway I have several and have had some decent longer distance QSOs with them. It has been a very interesting journey and often , almost always misunderstood. Actually doing it is a real education. It as also been used in the microwave industry BTW. Thanks for the post and the links. The modulation loop or absorption loop is typically one turn or less. Only enough to absorb . Too much like any type of modulator is too much. It doesnt take much to work well. Same as you would use for an RF sniffer lamp loop for those who have ever made and used one. I have spent a lot of time experimenting with this method. Very interesting to me anyway. 
donVe3LYX