poor modulation- Cunningham cathode modulated TX

[N0BST]

Don't know how familiar you guys are with Cunningham transmitters, but they're typically cathode modulated.  I built one out of his Low Power Broadcasting book, the 3-12W model, but am running into trouble modulating it. It was designed for broadcast and I've currently got a rock in it for 1460 with an eye on moving it to 160m some day.  It's modulated using an external audio amp- I'm using an old 12W Gates monitor amp, 6V6pp out.  The transmitter supposedly can be modulated with the standard 8 ohm output, but I found the 600 ohm/70v line output works best.

TX topology description:  output circuit is a 6L6 driven by a 6AQ5; 350V plate through a small 2.2mH choke; 33K to screen for about 225V and bypassed with .01 and 4.7u caps.  The output of the 6L6 is coupled by a .002 cap to a modified pi network, I think- tuning cap, coil, a second coil where the load cap is at the center tap, coils mounted at right angles to each other.  Cathode circuit- calls for a 0.3H choke (don't know what I've got in there now- I just grabbed what I had and it's probably 10x that) and a 500 ohm slide tapped resistor for power setting for the DC path, RF bypassed at pin 8 with a 0.1u cap.  Audio is applied directly to pin 8 via a big electrolytic.

The modulation problems- when I start to modulate I notice on the top of the waveform the peaks aren't the same on the top of the envelope as the bottom.  As I raise the modulation to 30% on top, I'll see 20% positive peaks on the bottom.

As I raise it further, the waveform gets fuzzy as though there's some instability.  When I reach near 100% negative peaks, the top of the waveform is only 75-80% and the bottom shows only 50% positive peaks.  Also noted on a trapezoidal display, beyond about 50% or so the narrow end becomes bullet shaped rather than staying wedge shaped.  I don't believe it's a lack of RF drive as I get something like 70v p-p on the 6L6 grid.  Ready for any assistance you can render and thanks in advance!   

Scott Todd

[WD5JKO]

Scott,

  Any chance you can post that circuit?

 Cathode modulation adds the issue of where to return the grid leak resistor, or where to reference the bias supply if there is one. Also in general if we see the modulated waveform get "fuzzy" on modulation peaks, suspect a parasitic oscillation. If this is happening the g1, g2, and plate currents should hop around too.

  You might take a neon bulb and attach both legs to pin 3 of the 6L6 (plate). This should glow orange when the RF is keyed up. You can lean it over towards the chassis for more sensitivity. If there is a VHF parasitic, the neon bulb color will turn violet. This might happen by playing with grid or plate tuning, or/and on modulation peaks. The 6L6 has a fair amount of grid-plate capacitance so if the circuit is spaced out, or if the grid drive is not well into class C, then Mr. Parasitic might come a knocking on the door.

   Some level of grid modulation is desirable to linearize the trapezoid ramp, or you could modulate the driver tube. Getting the levels, and phase correct is important. I wouldn't worry about the linearity until the modulated tube is proven to be stable.

Jim
Wd5JKO

[KX5JT]

Could the differences in peaks on each side of the sinewave be contributed to the 6L6's in push pull?  Maybe the tubes are not matched?  Just a thought.

[The Slab Bacon]

Are you sure that you are loading heavily enough?? IIRC, cathode modulated transmitters are just like scream mod rigs in that they need to be loaded very heavily.
Usually to where your plate efficiency drops to around 33-35% in order to be loaded heavy enough to make the audio peaks.

Just my $.02 worth........

[N0BST]

To answer each of your questions- I'll try to get it posted, both the original and the as-built plus a couple of shots of the actual unit.  I did make numerous mods but none to the output stage.  The original oscillator used a 12BH7, but since I only had a few of those in my collection but more than enough 6AK5s to make an analog computer, I opted to make a near copy of the oscillator RCA used in their BTA-1R1.  The driver was changed from a 6V6 to 6AQ5, which is essentially the same tube within the 'Q5's max ratings because I have tons of those as well.  I did add some extra plate dropping and bias resistors to cool it down some.  I still get some 70V p-p RF drive to the 6L6 final.

Your point about parasitics was my primary suspicion too.  One of the build instructions was to not crowd everything together which I thought I followed at least as far as the output stage was concerned.  Tried both glass and metal versions and got similar results with both.

I'll try swapping positions of the output tubes in the amp and see if that makes any difference in the mod envelope.  As to loading, it's entirely possible it's not loaded enough.  I goofed and put the taps on the output coil in the wrong places, but they can be easily moved.

Scott Todd

[DMOD]

Your grid bias may be off.

In my CM design the grid is fed off the cathode with a zener and a paralled electrolytic capacitor to bias the grid negative wrt to the cathode. In my circuit, coming off the zener toward the grid is first a 10k resistor and then a 150 uH choke, then grid.

So starting at the cathode, we have a paralled zener(68v)/cap(6.8Uf), then a 10k resistor, then a 150 uH choke, then the grid connection.

Phil - AC0OB

[N0BST]

Sorry it took longer than anticipated to get these posted.  The following are the original circuit, my as built circuit, a shot of the underside, and the top side from the back.

http://www.flickr.com/photos/77061005@N05/6911539281/sizes/m/in/photostream/

http://www.flickr.com/photos/77061005@N05/6911537087/sizes/m/in/photostream/

http://www.flickr.com/photos/77061005@N05/6911534873/sizes/m/in/photostream/

http://www.flickr.com/photos/77061005@N05/6911532653/sizes/m/in/photostream/

Sorry I don't know how to post pictures here directly yet.

Scott Todd

[WD5JKO]

Scott,

   I'm just waking up here, but that schematic in the book seems to be anything except high fidelity.

Some groggy observations:

1.0 Cathode modulation in the pure sense is the same as plate modulation. Here it takes audio power at 1/2 the DC input of the final to modulate 100%. So sticking the mod transformer in the plate or the cathode is the same thing, so long as we return our grid 1 leak resistor, and the screen bypass capacitor (bypassed for audio and RF) directly to the cathode, and drive the RF tube into class C.

2.0 Your circuit is using a combination of cathode and grid modulation, and that power adjust pot to me says this is grid modulation where we need to adjust the RF output to 1/4th the full power level.

3.0 The grid leak resistor at 150K seems way too high to get any useful class C grid current. This suggests to me that the upward modulation peaks will be limited since there is insufficient grid current. A napkin scratch says with 70v P-P you might get about 80 ua grid current when the 6L6 in class C might need 2 ma or more.

I don't think you will achieve your goal with that circuit without a major re-design. But what do I know at 6 am before my cup of coffee. 

Jim
WD5JKO

[N0BST]

Thanks for the observations.  I should point out the idea was the driver and PA were actually being run more or less class A in the original design.  I'm open to modifying it to class C, or at least class B.  Got plenty of room under the chassis to add a bias transformer.  I could even use the filament supply, since it's not grounded, and run a voltage doubler to get some semblance of DC bias to the PA.

[WD5JKO]

A good discussion on class C and grid leak resistors:

http://www.radiobanter.com/showthread.php?p=699035

[The Slab Bacon]

A class A amp doesn't modulate very well (if at all) You need to get the modulated PA biased down to class C in order to get the munky to swing.

In the original skiz, the .3 Hy choke is no where near enough henryage to get any real munky swing out of the PA, should be at least 5 Hy (or more)

Jim, one of the advantages of cathode mod is that you need a lot less audio power than plate mudulation. However you are looking into a low impedance situation so a special mod tranny is necessary to cope with that situation.

For cathode mod, you would be much better off with series mod using a tube with a very low plate resistance (and impedance) like a 6Y6, 6AS7, or 6080.

I dont care what anyone sez, cathode modulation IS both cathode and grid modulation because the cathode is the "commom" point of the tube, if you shake the cathode, you shake everything else as well.

[WD5JKO]

I dont care what anyone sez, cathode modulation IS both cathode and grid modulation because the cathode is the "commom" point of the tube, if you shake the cathode, you shake everything else as well.

   Good discussion. Years ago I bought a book that Don Chester K4KYV was selling from the AM Press Exchange. It was a Sams book called something similar to, "Understanding Amplitude Modulation". Within it was a discussion about cathode modulation. They had a representative circuit with a tapped modulation transformer. The secondary went from ground on one side, and the tube cathode on the other side. The grid leak resistor went to a tap on the transformer. As I recall they had graphs showing the drive power required, audio power required, and the RF amplifier efficiency. With the grid tap at one extreme the audio power required was the least, and the RF amplifier was least efficient like any grid modulated amplifier. With the grid tap at the other extreme things were just like plate modulation. The recommendation was somewhere in between where the audio drive power was about 1/2 of that for plate modulation, and the RF amp efficiency was about 50%.

I cannot find that book! Anyone have a copy? A copy of those graphs and other data could be posted on this thread.

Jim
WD5JKO

[The Slab Bacon]

Grid modulation at it's best:

[W8IXY]

Some years ago Cunningham offered a low power AM transmitter for AM broadcast stations to use with their new low power post sunset authorizations.   Here is the address for the user manual for one of his models.

http://home.comcast.net/~filebay1/Cunningham_Transmitters_Model_CM_30_50.pdf

If the link doesn't work,  Just cut and paste.

73
Ted  W8IXY

[N0BST]

I think I'll up the bias and make sure the PA is class C or close to it.  Fortunately the choke is probably closer to 2H than the .3 called for, though I'll also endeavor to get a bigger one of those as well.  I'll probably have to tame the amount of DC through the driver stage as well since I'm getting plenty of blue fluorescence on the inside of the envelope.  At least the plates don't glow.

Scott