Experiments in so called cathode modulation.

[VE3LYX]

This week I built  cathode modulator. Actually it is called a simple grid modulator since anytime you wiggle the cathode it affects the grid anyway. It plugs in the key jack of a CW transmitter. It passed the smoke test and will put the tx on the air when the tube warms up. I had all the parts in stock. Even the tubes. I am still pinching myself.
In the same vien I had thought of trying to modulate a 45 tube TNT with a similar but vintage circuit. So after finishing this one I built a single tube cathode modulator from a 4 pin tube socket (circa 1922) a phone plug for the key jack and wired it up. I only had 4 volts instead of 5 on the tube  as I was short batteries but plugging it in did turn the tnt osc on If the tube was warm and shut it off when the heater was disconected so the principle anyway works. It is at reduced power level (ARRL book suggests 50% at rest. )but is easly found on the shack monitor receiver.  I tried modulating it with an old pair of earphones (one actually) but it wasnt enuf. Tomorrow I will build a carbon mike electro mechanical circuit and try it again. I will also get the modulator tube glowing which should help. Larry NE1S has been playing with his vintage rigs on AM and I got some hints from him already.
You might ask why this type of modulator? Because if I can get it to work and I see no reason why not since it came directly out of the 56 ARRL book I can use any of my several CW rigs that cathode key on AM. Also my ARC5 which is cathode keyed (now. ) Modulator doesnt need a power supply. Just heater voltage.
Don Ve3LYX  

[k7mdo]

Boy am I interested in how this works out for you.  About two years ago I bought a Heathkit AT-1 with that modulator from 1956 built into the case.  It was a shambles so I totally rewired it and set the parameters of the plate current per 1956 article to "half", in this case 20 MA, and put it on my oscilloscope... with a D-104 on it I could only achieve low level modulation and as I increased the gain for the microphone I soon achieved a clear "flat topping" of the RF envelope. 

I have a friend about a mile away and so in testing (by ear by him) on 80 meters it seemed to sound fine even with the truncated waveform but that is "human ear" interpretation of fine.

I don't know what the limits of the power level one might achieve for that modulator but it clearly overdoes it on an 8 watt output transmitter.

Please post your results as I think the idea has merit but I haven't had time to experiment past the AT-1.

Tom

[VE3LYX]

I tried it on my BareEssentials 50L6 CW rig but discovered the key jack is wired backwards. (Dangers of wooden construction) so polarity is wrong. Will fix that after breakfast. On the other one, the 45 TNT I ran accross the same thing but since its cathode modultor is a "bug build" it was a moment with the soldering iron to reverse the connection and off she went to transmit.
On this one (and on all my home brew rigs) I cheat. Often a low value resistor is needed in the cathode circuit  (IE R9 in this design) 50 ohms in this case. I do not have any resistors under 1000ohms so I install a screw in bulb base and use a 52 bulb which is about 50 ohms.  Because bulbs can be had in many ratings I can vary the plate current by changing the bulb in the cathode circuit. I also have one on my 6146B Linear. Works very well. If I change power supplies to push it harder it is a simple matter to remove the 52 bulb and put in a 40 for instance to keep the thing from running away with itself. In this case the article even talks about changing the value of the cathode resistor on the 6y6G if current is too high or low in the TX. With this set up I can do it externally.
I have built the speech amp circuit of this before for my mixer tube transmitter. (6sa7) I found there it was much stronger then needed and if I turned the audio in the speech amp up anywhere over 3/4 postion Immediately popped the 6sa7. I suspect the speech amp is much more then needed  for a low power tx. The original article said this modulator was designed for CW transmitters of 100W input or more. I decided to try it for my ARC 5 T18 which I dragged into the 80M band. A pair of 1625s (12v 807s) can certainly run with a 100W input or more so it was to me a good solution. No need to butcher the radio . Just plug this into the key jack and talk away.After your post I will be sure to hook up my HB osc scope as well to make sure I am not doing something wicked. The fact that you could use it on an 8 watter at all gives me hope for the 45 tubed rig. Could be it was a bit of overkill powerwise that was giving you trouble. Frank Jones wrote a book on Cathode modulation but I cannot find an available copy however I have his 40 handbook and there is a fair amount in there about it except for the concept this modulator uses of stealing B power from the parent TX by default. Anyway we shall see what today's experiments bring.
Don

[K3ZS]

I built the same modulator 50 years ago, used it with a DX-20.    It sounded as good as any AM transmitter although with reduced power.
 

[VE3LYX]

Thanks for the experienced comments.
I had it on the TNT today. Played with the cathode resistors . Interesting how much it affects carrier power. Turns tx on and off as 6V6G tube heats and cools. Need to match up a decent mike . Went to work on the ARC 5 getting ready to use it.
Don 

[wa3dsp]

You may already know this but a search turned up the following Electric Radios that referenced cathode modulation

July 95, March 05,  Apr 05, May 06, March 09, Apr 09

The July 95, March and April 09 were construction articles.

[VE3LYX]

Thanks I would love to see them but how?
Don

[wa3dsp]

Gosh, I thought everyone on here subscribed to (ER) Electric Radio Magazine. It is rather expensive though if you are out of country. It is $34 for a periodical rate in the US. Canada is $54 and elsewhere is $70. I would have to say if I had to chose between QST and ER I would pick ER but since I am a ARRL life member since 72 I don't have to. QST is $39 in the US and $49 in Canada but includes membership.

It is a shame ER is not available via the web at a reduced rate. As far as I know there has not been a CD or DVD made of back issues. That would be another good thing. Many people just don't have the room to store all this stuff.

So I see two choices for you. You could see if the issues in question are available from the editor - Ray Osterwald - ray@ermag.com  and buy them.

[Pete, WA2CWA]

Ray is a member of amfone.net.
Back issue availability list is here:
http://www.ermag.com/index.cfm?v_link=back_issues&v_product_type_name=Back%20Issues

[VE3LYX]

Thanks I found his site. I tried the modulator on a qrp vintage rig but it has only 110 B+ and the modulator cuts that about 50%. So the test was a wash. Today I am working on a proper power supply for the ARC 5. My light duty bench supply did not like the 1625s. "POP" I repaired it first thing this morning since it is a very handy supply for vintage tube rigs. The one I am working on now is a moderately high supply for testing and other rigs. I have a 800v no load supply but it makes me nervous. I know, some of you run 2to 3 KV. My hat is off to you. I'm chicken.
Don

[VE3LYX]

Fun day in the shack! From the ARc 5 site I got a cathode modulator circuit diagram from Ben (militaryman) It was a solid state modulator done with one hi voltage transistor and very vew parts. It is a cathode modulator circuit but much simpler then what I had built from the ARRL book.  Somehow the circuit made sense to me. Even though I didnt have any hi-voltage transistors it gave me a good idea what to try. (Thanks Ben )
I dug back out my 45 TNT and tuned it up on 7160 using the morse key. I took a chunk of 2x4 and screwed a vintage 4 pin bayonet socket to it. I found an interstage transformer from a old 50s hifi set and screwed it near the socket. One end of the highest R winding  I connected to the designated cathode connection (Direct heated filaments) On the other end I connected a .02 Capcitor and soldered the other end to the tube Grid conection. I used a 6.8 K resistor to bias the grid (something near 5k I figured would be a good place to start. ) I fastened a 1/4 inch phone plug centre conductor to the socket Plate termnal (goes to the transmitters cathode) and barrel or shield connector to the designated cathode terminal on the tube socket. (pS B- as plugged into the key jack.) This puts the triode in the modulator in series with the PS and in the proper polarity. I hooked a old telphone carbon mic element taped to a pencil to the one side of the audio transformer (Low R side ) and the other mic connection to a 9 volt battery clip the other end of the 9 volt battery clip to the other side of the same winding. installed a good 9 volt. I used a D132 triode. (Tis a Canadian tube , a 171A would be comparable) I didnt have 5 volts so I used a fully charged 4.5 volt to try it. I turned on the PS for the TNT and when it was warm clipped the power to the D132 filments. With in seconds the TNT came to life but at about 1/2 power. I had it pumping into a dummy load) I had a monitoring rx set also downstairs at a bit of a distance but I could hear it and could hear the TNT carriercome back to life. Gingerly I leaned forward and spoke into the carbon mic. The silly thing actually worked, first try!
I could hear myself . Tyical carbon mic sound but I like that. Over the next few days I will try the following.
A full 5 volts on the D132 heater.
A better audio transformer more suited to this circuit.( I knew this one would work but was shy turns ratio from my screen modulation Carbon mic transmitter. In fact this was a reject from that project)
I will cut back to 3 volts I think on the carbon mic battery. If the tube wasnt directly heated I would simply steal the filament voltage for the carbon mic circuit. Anyway the battery works and was easily done.  Doesnt seem like a lot of parts to modulate a transmitter but intitial trials are encouraging. Like it actually works!
Did I have fun today? Absolutley! This is why I love amateur radio.
Don VE3LYx 

[VE3LYX]

 
As some already know I completed the cathode modulator for the TNT transmitter. Quite frankly I am shocked at how well it works. Deep rich sound that I dont get from my DX60 or my Hb screen modulated rig. JD Sumner could sing bass into the mic and it would come out good. No not broadcast quality and I am sure the "AM AUDIO PoLice " would find fault (but they always do anyway)but when you consider this a 1920s design transmitter with a single 45 tube modulated by a single audio output triode of similar 1920s vintage and has less then a dozen parts it to me anyway is quite exciting.
Exciting enuf to make me seriously consider cathode modulating my DX60. Something about that rich full audio as compared to almost have to shout audio the Dx60 has.
As time permits I will run the TNT today from studio B (B is for basement)just under 7200 (around 7195) and will call CQ as time permits between projects. It wont be super strong but if you are in the right place and prop is decent you might hear this old peanut whistle speak.
Don VE3LYX
A few more pics after I added a standby switch to standby the TX (opens the key jack circuit) and a press to talk so I dont run down the carbon mic battery.
Batteries will be hidden in the wood base. Holes are drilled but I ran out of patience.

[VE3AJM]

As some already know I completed the cathode modulator for the TNT transmitter. Quite frankly I am shocked at how well it works. Deep rich sound that I dont get from my DX60 or my Hb screen modulated rig. JD Sumner could sing bass into the mic and it would come out good. No not broadcast quality and I am sure the "AM AUDIO PoLice " would find fault (but they always do anyway)but when you consider this a 1920s design transmitter with a single 45 tube modulated by a single audio output triode of similar 1920s vintage and has less then a dozen parts it to me anyway is quite exciting.
Exciting enuf to make me seriously consider cathode modulating my DX60. Something about that rich full audio as compared to almost have to shout audio the Dx60 has.
As time permits I will run the TNT today from studio B (B is for basement)just under 7200 (around 7195) and will call CQ as time permits between projects. It wont be super strong but if you are in the right place and prop is decent you might hear this old peanut whistle speak.
Don VE3LYX
A few more pics after I added a standby switch to standby the TX (opens the key jack circuit) and a press to talk so I dont run down the carbon mic battery.
Batteries will be hidden in the wood base. Holes are drilled but I ran out of patience.

Who are these "AM audio police" that you refer to Don? I've read various veiled references to this sort of thing from you here before.

I've never come across them myself, having been on AM since the early 80s.

If you ask for an honest audio report, be prepared to accept the reports you are given? Or, don't ask for one. And then again they can be given without asking for one too.

Al VE3AJM

[VE3LYX]

I am not going to go there. If you "read the mail" on 40m much you couldnt not have missed them.
"If you ask for an honest audio report, be prepared to accept the reports you are given"?
I have never, not once ever, in my 31 years as a ham ,asked for an audio report. Or the half doz other reports I have been given nor do I give any out unless they are compliments. Tis how I was raised.
Don VE3LYX

[VE3AJM]

I am not going to go there. If you "read the mail" on 40m much you couldnt not have missed them.
"If you ask for an honest audio report, be prepared to accept the reports you are given"?
I have never, not once ever, in my 31 years as a ham ,asked for an audio report. Or the half doz other reports I have been given nor do I give any out unless they are compliments. Tis how I was raised.
Don VE3LYX

Ok Don, well you brought the audio police thing up yourself, but you only want to go so far with it. I gather then that these audio police types want to put your equipment in jail or something?

Seems a bit strange to me that with all the homebrewing and experimentation that you do there with transmitters etc, that you wouldn't want to know how you're being received at the other end of a AM contact/QSO. whether you've asked for a report or not.
But to each their own I suppose.

The whole issue of signal/audio quality reports given or received, can be a very subjective/touchy subject, based on our own preferences etc., but not always. If I'm in a QSO and I hear an obvious problem..ie.VFO drift..low audio..no audio..severe flat topping of the audio..etc.. with the other stations signal or audio, I will let them know, especially if they're trying out/testing something new etc. That would be an honest/helpful report. But thats just what I would do rather than saying nothing, because I couldn't be complimentary. I would hope/think that my report would be taken in the spirit that it was given, and they are for the most part.

Al VE3AJM

[VE3LYX]

Whatever. I dont say anything unless asked.  Except to compliment someone. When one is homebrewing or experimenting  especially exploring areas not normally investigated one tends to be a bit insular. When involved in Race Engines which I did for much of my working life I used to say it helps to be a bit deaf.
Critics are a dime a dozen. Encouragers are few and far between.  Much advice and critiques is offered in both areanas by those who have never built diddly squat in their entire life.
Same goes for building fiddles. I get all the time  comments from "experts" on how I build my homemade fiddles. Now I reply, "I see, so how did you do yours and could I see it" knowing full well that in 99.9 % of the conversations they have never built one nor will they ever. I know if my audio is good or not. I can hear it on the monitor  and test it before I go on air. I do not strive for nor do I wish to have broadcast audio. As you know I dont run huge power so getting through and being understood is what I strive for. I enjoy the challenge of it . In those conditions the fancy audio is lost anyway and only the main body of the freq response is heard. Like you never hear "you're S3 here with wonderful BROADCAST quality audio". That is just who I am and what interests me. Same reason I race a six cylinder dragster. The challenge, the low cost and the thrill of success with less is what makes me tick. Forget the Audio police. It doesnt matter.  Those folks are in every venu in some form not just ham radio. It is just a comment, not an obsession. I am in ham radio because it interests me and is fun.  Anyway this is about cathode modulating.
Speaking of which I did some further work on the single tube cathode modulator. I added a press to talk switch (homemade of course) Since it is a carbon mic circuit, that saves the battery big time. I also added a standby switch in the key plug circuit so I can drop out the transmitter between speaking easily . THEN I noticed that when the transmitter is switched with that switch from the modulator which is now controlling all the B- supplied to the set the characterisic chirp so many of the tnts and hartley power osc type rigs have is completley gone SO I got up early and hooked a key to the same switch leaving the modulator in control and sure nuf no chirp. So not only does it work as a modulator but also as a keyer. I also changed the cap from the audio transformer in the modulator from .04 to .005 to see what difference that would make. It made the audio more normal  as it was "bassy" before. I think I will leave it as is now. So far this has been a most intersting project. I never thought I would have a two tube, pre 1929 AM rig  with 90 year old tubes up and running.
Don VE3LYX

[Steve - K4HX]

If you don't want comments or critiques, don't get on the air.

If you are so upset by listening on 40 meters, why do you listen?

Seems to me you are trying to conduct your QSOs here rather than on the air.

[K5UJ]

If you fool around with vacuum tube circuits or vintage gear you should subscribe to ER.  Some of the best material so far, has been in the back issues and unfortunately they are only available in print.  To get the whole run you have to pony up some fairly serious change, over 400 bucks now I think, but it is really worth it if you can afford it.  Unfortunately around 20 issues have gone out of print and are hard to get now.  And, they have some great articles, which is probably why they are out of print--popularity exhausted existing supply.  I wish Ray would digitize ER and sell it on a CD or something but I guess editing and publishing the magazine and mailing it probably consume too much time already.  I don't think he has any assistance, or maybe one person part time, something like that.

[VE3AJM]

If you don't want comments or critiques, don't get on the air.

If you are so upset by listening on 40 meters, why do you listen?

Seems to me you are trying to conduct your QSOs here rather than on the air.

Thank you Steve. Straight to the point.

Good luck with the ARC 5 Don. I'll remember not to give you a signal or audio report, if I ever hear you on the air with it. That was a very eye opening reply in more ways than one.

Al VE3AJM

[VE3LYX]

If you don't want comments or critiques, don't get on the air.

If you are so upset by listening on 40 meters, why do you listen?

Seems to me you are trying to conduct your QSOs here rather than on the air.

???What are you talking about??
 This is supposed to be tech. Why are we discussing this crap and my personal preferences? I dont get upset. Who said I get upset? I listen where I want  and qso when I wish between builds. (or during) It was just a comment. In hindsite I probably wish wish I hadnt said it. Not because it was untrue, Just because of this nonsense.
Frankly Scalet----------
Yes, Thanks,  I checked out the ER site. looks like a good idea to subscribe.
Don Ve3LYX
. 

[Steve - K4HX]

Yes, this is the tech section - not a personal blog section. You've been treating it as such.

You brought up the audio police. When asked to explain it, you run and hide behind the "This is the Tech section" defense. That is nonsense. If it's true as you claim, then you should very easily be able to explain it or identify said "police."

Sorry Don, you cannot have it both ways.

[VE3LYX]

Whatever.
Cathode modulator is done hooked to the T18 ARC5 and on the air.
Don VE3LYX

[wa3dsp]

 In reviewing WLW documents I found the following IEEE document that might be of interest to this topic.

http://www.arizona-am.net/WLW/Docs/cathanode%20modulation%20system%20Rockwell%20R%20J.pdf

[W7TFO]

In reviewing WLW documents I found the following IEEE document that might be of interest to this topic.

http://www.arizona-am.net/WLW/Docs/cathanode%20modulation%20system%20Rockwell%20R%20J.pdf

\

Now that is one interesting circuit.  In my files for later!

73DG

[AB5S]

Jeminy Jumping Crickets!  Ignore the people trying to pee in
your Post Toasties over an innocent little comment!  GEEZ.. 
Some people got nothing better to do than raise Hell over trivialities.

THANK YOU for sharing some cool fun with your TNT and
45 Modulator.  I am really enjoying your adventures.

73 Dave S.