The AM Forum

I was reading about different modulation schemes. Found this one in PDF form in a couple of places.  It's a audio amp pushing the grid of a 6Y6G in the cathode of a class C rf amp.   The article states that this is actuall a form of grid modulation, and that  "A useful characteristic of the system is that id does not require a fixed source fo grid bias for the amplifier."   So if I'm running a class C tetrode I don't need to grid bias this tube??  I guess I'm confused, but that sounds wrong.

I've attached a schematic for reference. 

I was looking at this and thinking how simple it would be to get something bigger on the air.  why is nothing ever as simple as it seems...

Simplest mudulator is a carbon microphonium loop coupled to the tank  ;D ;D

Yeah the tube needs to have some sort of bias, unless it naturally sits in class C.

However, the current through the tube will be controlled by the series tube, which is in this case controlling the current via the cathode. So, the bias on the last modulator tube will in effect set the bias of the transmitting tube, since all current must pass through it. But not the full voltage of the transmitting tube - just the current.

Notice that R9 in this schematic is 50 ohms?

Note how the text talks about how many tubes are required for what size transmitting tube? Probably you'd do nicely with a 6080, or 6333 series pass tube - which can handle a ton of current!  ;) R9 would be reduced in size for larger tubes, I'd expect...

Steve QIX - I am told - did a bunch with series modulation, but on the other side, the B+ side. There are/were a number of articles on that method out there too.

And, his latest PDM experiment was in series with the plate of a (gasp!) tube rig, iirc.

 ;D

                     _-_-bear

Yes, but...................keepa you lipz way clear ovva the mike!!

rig here is loop modulated with a single button carbon mike.....yep....drives slopbuckets insane... heh-HEH!

Ed,
      that is your most basic "old buzzard" handbook series cathode modulator circuit. Believe it or not they will actually work fairly well. However there is a lot of room for improvement to that circuit. Running a seperate low B+ feed to the first 2 amp stages will help out immensly. Like Bear said a tube like a 6080 (or several in parallel) could be capable of modding a very large class "C" final. The trick is to find a tube with the lowest Mu and lowest plate resistance you can for the modder. This gives the most linear response.

Bear,
         You gotta keep in mind that most of the real old class "C" phone transmitters, especially the ones with triode finals, only ran grid leak biass! Because of this you actually had to drive them into class "C" operation. That is why the reference to not needing fixed biass. This would get to be an issue if there is too high of resistance in the cathode circuit.

                                                                               The Slab Bacon

many a bottle got cooked when he excitation was lost and there was no other bias.

I built that exact circuit when I was a young ham.   Used it to modulate a DX-20.   It had good audio quality and was simple to build.   It needed no B+ supply and the DX-20 supplied the filaments.

Hi i wonder if someone already used this system before?

John FG5FC

I have heard this 6Y6 modulator on the air and it sounds very good.

One of our members of the Colorado Morning AM group built it. Nice and simple and it does sound great.

A quick and easy way to get a rig on AM.

73

Marty

Edit: Found it - http://www.mines.uidaho.edu/~glowbugs/Simple%20Grid%20Modulator.pdf

Ed, where was this published?  QST?  I do see the date of Sept. 1953.  As a newbie tube builder, I'd be interested in reading the whole article.
 
Thanks

- JT

JT,

It is in my 1955 ARRL Handbook.

I photoed it and posted it here or on Steve's site many years ago.

Here is the whole article.

(P.S. Click on the 6Y6 modulator. pdf below)

Take a look at the "Golden Gate" modulator too, uses 1/2 of a 12AT7 to modulate a 6146:

http://picasaweb.google.com/RANickels/GoldenGateModulator

It sounds a little fussy but is intriguing enough I'd like to give it a try sometime.

73, Bob W9RAN

Yep,
Straight forward cathode series modulation.
This assumes that a triode Class-C stage cathode already
has an rf bypass cap to ground (and for tetrodes a screen resistor
exists between screen and B+ and the screen is bypassed to the cathode).
The transmitter output will be less than 1/4 the normal CW output.
Ballpark example: an 807 running 500V at 100 mils (50W) would be biased down by the modulator to 220V at 44 mils (9.8W).
That is, the 807 has 220V across it when the modulator plate
is set at 280V. This allows for a bit more than 100% modulation.
The class-c stage grid leak bias should still be OK since the
rf cycles are re-biasing the grid at a rate much, much faster
than the audio rate.

I once used a high voltage fet to modulate a Viking Adventurer
with this method. Kind of a KISS circuit. Some distortion, yes,
but it was fun.
Regards,
BobbyT

Derb,

My shop teacher in High School had a breadboard Hartley with a 6C5 on the AM band as a demo unit. He would loop modulate the thing with a carbon element from a phone, 2 turns over the tank and a 3V flashlight batteries. The class was amazed by his broadcaster!

Mike WU2D

4 year Leukemia check up - doc says do not come back for a year -looks like it is whipped.

That is great news Mike! Good LUCK!

Sweet!  I have a 6080 in the box... hmm I've got some 6sn7s too..  I think I'll lash up this circuit and play around.  I like the idea of a separate B+ for the audio. Though it adds some complexity.

What I was thinking was a simple way to modulate a single large Class C final, like a 4-250 or similar.  This could be just the ticket!!

the 6080 has the lowest plate resistance of any common tube that I have ever seen.
And........er...........furthermore a 6080 with both sections in parallel can carry a tremendous amount of current.

Gee I wonder how a pair of 6080s would work in my v2-cdc ??

Tube Sheet says a 6080 can carry 125 mA per triode.  So 250mA parallel. That's smokin'.   

N1ESE -  the glowbugs reflector web sight has a lot of tube stuff on it. The glowbugs email reflector used to have a large number of very savy guys on it.  I haven't been a subscriber in some time, but if you are getting into the
building of things using Electron Tubes, their a great resource.

If you really want simple - check out my Zinc Negative Resistance Oscillator:  http://www.youtube.com/watch?v=cDBpBYkRw0Q

Schematic:  http://picasaweb.google.com/RANickels/ZincNegativeResistanceOscillator

Taking pissweak to a whole new level - but hey, it's AM!

73, Bob W9RAN

I've done that -- just dont let your lip get too close to the mike.  It can hurt

It's out of the ARRL Handbook, 1954 (31'st edition). I have a copy of the book.   I was a kid in high school then -- those were the days.

Al W1VTP

In the April 1955 issue of Radio and Television News, the "Reference Shift Modulator" is presented which the author claims is superior to both the Class K and Bias shift modulators.  It is a pretty simple circuit using a 12AX7, 12AU7, and an 807 in a Heising circuit.  The author identifies himself as being affiliated with Collins but I am pretty sure this circuit never showed up in any Collins gear!

Rodger WQ9E

John,
       RCA, and Paragon (Adams-Morgan) actually had a device called a "magnetic moidulator" they inserted between the tank coil and the antenna ground to take the place of a single button carbonium microphonium connected to a few turns loop inside of the tank coil. It was supposed to work better, even though it's doubtful it did, at least it was safer than the mike connected to the loop. People would get RF burns on their lips from such an arrangement.

If you ever see a magnetic modulator in your travels, please let me know. I'd like to pick one up to experiment with. Yes, I could wind a few thousand turns of small diameter wire and make my own transformer, but without a decent coil winding machine it would be a pain in the arse!

For anyone really interested in this scheme, or early radiotelephony techniques; look for a book that came out in 1922 by "Ballantine" called: "Radio Telephony for Amateurs". It's a terrific reference on early transmitters, CW and 'fone alike. If you really want this book and can't find it, send me an email and I'll let you borrow my copy.

Mod-U-Later,
                 Joe N3IBX

Bob,
      I looked at your skizmatic, but don't quite understand it. How does it work? Could I replace the xtal with a audio transformer of the correct impeadance to match the plate of the output tube?

Best Regards,
                 Joe N3IBX

I must be missing something.
The 3.58 mc osc. I understand, similar to a 2 pin tunnel diode in neg. resistance range. 
I believe early literature shows two pin active devices similar to the metal strip with hot spots; they just didn't understand quantum theory. The transistor was invented after understanding quantum theory, tunneling in quantum states, electron leakage across higher discrete potential barrier states, statistically some electrons having greater energies than the barrier, etc.
 
But in this circuit the note clearly states for "audio range" substitute input transformer for the crystal.  Well, if it oscillates in the audio range it will be a single,  audio range,tone howler, not a hf oscillator anymore.   So did we suddenly go from an osc. to an amplifier attempt?   If so why not just simply hook up the audio souce directly to the CD player (symbol looks like a cd player.)
Don't see AM here unless the crystal is left in circuti and the input transformer placed somewhere else affecting the CW waveform.