The AM Forum

If Bob Pease were alive, and reading these recent posts, perhaps he would say, "What is all this Cathode Modulation Stuff?"

Here is a good AMFONE thread on the topic that occurred right after Bob passed away.

http://amfone.net/Amforum/index.php?topic=28282.0

My interest here is I just obtained a rare UTC CM-15 Cathode Modulation Transformer.

I found a schematic using it, but no data sheet or terminal connection diagram.

Looking through many of the UTC on line catalogs, I find nada. Time period is most likely mid to late 1930's.

Anyone have info on this little Jewell?

I attach the schematic, and two photos of my transformer.

Jim,
Wd5JKO

Nice find.

The schematic shows PP 6F6's driving the transformer. The 6F6 data sheet recommends 10K P-P load so that may be close to the input impedance of the transformer. The UTC catalog states 300 to 2000 ohm secondary. It shouldn't be too hard to drop some AC on the primary and make measurements to obtain the turns ratios and then calculate the impedances. Knowing the secondary maximum is 2000 ohms, you can work backwards from there.

But, even having determined this, you'll probably want to try different taps when setting it up to see what works best with a given tube/load.

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This makes me imagine the guy, back in the day, who had to save-up the $2.40 from his pay to buy that transformer to modulate his CW rig so he could "join the guys on 'phone". That little piece of iron must have given him something new to write in his logbook —  like no more dittos under CW in the mode column!

Yeah, Don's got the "plan"... looks to me like multiple taps for the secondary.
The side with the two lugs together? Likely the primary...

Wonder what, if anything, is "special" about this transformer, other than the winding
ratios...

The larger of the series shown says "100 watts" audio in, modulating a 1kW transmitter?
Hmmm... not bad. Rather obviates the need for big mod iron right there. Of course, ur not
going to run >100% modulation, I don't think??

There are some modern transformers with similar ratios.
They're made for driving "70volt" lines!

The Altec 1570BT amplifier (not easy to find) had just this: 2 x 811A to 70volt line.
There are other high power P-P to 70v line transformers out there to be had!

Then too, one might consider the heresy of a SS amp driving a matching transformer
to the cathode!

Oh yeah, almost forgot: "Easy Peasey!  :)

There were some early articles on CM.

W6AJF had a number of articles published on cathode modulation in Radio Oct. 1939 page 14 and Radio Apr. 1940 page 35.

RCA Ham tips also had an article in the Jan-Feb. 1940 issue on Cathode modulation.


Phil - AC0OB

So as I wait for the coffee pot, I drew this up. What do you All think?



For me, pros are: I have every part in the sketch. 

I’ve been debating myself and asking questions here trying to decide what to build next, based on what I have — I’m limited by lack of bigger iron for modulation and lack of budget to change that. Maybe cathode modulation is my answer??

Ed

Build it and they will come?  ;D

Ok, the Christmas guests have mostly gone....

I hooked up an AC signal generator set to AC 1Khz, 600 ohms source impedance, 2 vac RMS output unloaded the the transformer in my first post.

First off the DC resistance of the windings are as follows:

Pins 8-13 adds up to 37 ohms

Pins 2-7 adds up to 600 ohms

I presume that 2-7 is the primary, and 8-13 is the secondary. The Low DC Resistance side is for the RF cathode return circuit.

Exciting AC 1khz to 2-7, no droop in the 2vac RMS on DVM

With DVM hooked to secondary at 8-13, I get 1vac RMS.

So that means the overall turns ratio is 2:1 and 4:1 impedance ratio.

When driving 3, 5 with 2vac RMS, I get 1.38 vac RMS at pins 8-13, turns ratio 1.45:1

When driving 2, 6 with 2vac RMS, I get 1.23 vac RMS at pins 8-13, turns ratio 1.63:1

I also find that pin 4 is the CT when driving pins 3, 5, or 2, 6. Therefore the 6F6 tubes in the schematic likely had the plates to pins 2, 6 and B+ to pin 4.

Need to look up the Taylor TZ-20 Spec's to see how they tapped the secondary winding.

More to come, back to getting new tires on the car.

Jim
Wd5JKO

Using a filament transformer for the 813's with a primary of 110 vac will put the 813 filaments way up there.
Regards,
Gary

Here is the link to that article:

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0301.pdf

As to the 813 circuit by Ed, K8Di, look at the ratio of grid to plate modulation as described in the RCA Ham Tips.

If the bias supply is fixed, and ground referenced, you can Para-Feed with a choke and a cap. The choke must be high in Henries and capable of handling the grid current.

The use of a 70v line transformer may work, but realize that the cathode current through that transformer 70v winding will put a big DC magnetic bias on the core. Para-Feed is the answer again, but that choke will be big!  Also the 100v peak capability means you need to increase the grid modulation to a high value. See curves at RCA Ham Tips.

Jim
Wd5JKO

Pre-coffee sketch, the 10v 13a transformer I have is modern, with 120v primary.  Testing with it plugged in a shack AC outlet and a single 813 and clip leads, it was 10.3 volts.

It’s a new part, because vintage 10v transformers seem to be fairly uncommon...and that happens to work out to keep the filament voltage inline too. I just have to mount it inside the chassis so I don’t have to look at it...


Ed

Thanks for the link. I’d found one of the Radio magazine articles but not that one yet.

The 70v transformers I have are big. Rated for 350 watts, that is several amps of ac/audio.. anybody know an easy way to test for saturation? I’m kinda guessing that the cathode current, being only 400mA or so, won’t saturate them; I could even run two in parallel, or run them backwards (turns ratio is 1:1.85, step up, normally, so backwards, the ampere-turns thus flux would be less). My main concern/thing I do not know is what voltage swing do I need at the cathode. I have about a dozen pa amps in the recycle pile at work, ranging from 200 to 1800 watts, along with these big transformers.  I’m just hoping there’s a proper combination...

Ed

If it's possible to remove the transformer laminations, you could re-stack them and make a gapped core. Then it could handle unbalanced DC without getting magnetized.

If one studies the various early descriptions of cathode modulation methods as found in the References you will see circuits that simultaneously and directly modulate the cathode and control grid using a modulation transformer with secondary taps.

Depending on where the higher tap was placed, the transmitter was either majority-grid-modulated or majority-cathode-modulated. Simultaneously modulating both cathode and grid theoretically improved transmitter efficiency.

Phil - AC0OB

Here is a link to a Frank C Jones article back in 1940:

Go to page 35...

https://www.americanradiohistory.com/Archive-Radio/40s/Radio-1940-04.pdf

Some good info.... Gets into the cathode impedance, and how to approach situations based upon audio power available versus RF amp DC input.

Here is another idea from VK7DR, Cathode and Screen modulator combined using a Line transformer:

http://www.robert-heyward.com/html/am_mod_notes.html
http://www.robert-heyward.com/html/80m_am_tx__new_.html

The above has carrier control as well....


Jim
Wd5JKO

Attaching an RCA Add from that same 1940 Radio magazine linked in my last post. The imp ratio of 11600/2800 reveals the same 2:1 turns ratio as my CM-15.

That add seems similar to my transformer (CM-15), but the big brother up (CM-16). The text about using just enough grid modulation is most telling.

Jim
Wd5JKO

I saw that ad on the back cover, quite timely for the current discussion.

I ended up reading the entire issue, and I find these old magazines to be filled with lots of good technical information that is still of value today.  There was a very good article discussing the asymmetric quality of the male voice with relation to optimal modulation techniques, a good read.

Jim, thank you for sharing the link!

A bit of "bootstrapping"??

I'm still thinking about cathode, grid, and plate modulation, and the parts I have, and the variations they lend themselves to...

So, Friday I went to the shop for the first time of the new year (the 'day job', that is...) and looked at the junk audio amp shelf, and found a couple Bogen HT-250A amps.  These are old style, heavy (50-ish lbs), commercial duty, mosfet, mono, 250w, 70v (and other) outputs. They work fine, they are pulls from a high school football stadium I redid the sound system in last summer. Schematic attached.  So, it looks like I have taps, I can play with percentages grid and cathode modulated.

I think I'm building a dual purpose RF deck, with the tube sockets on a sub-plate with some terminals to make that part interchangeable. Then I can put in a plate with one or two 813's, and cathode/grid modulate them one way or another.  Then a second plate with a pair of 814's (uncommonly used tube that seems like it's the 813's baby brother) that get plate modulated with the 250w Thordarson modulator I have.  Power supply changes from the 1250v for the 814s full wave capacitor input, to a capacitor-choke-capacitor filtered bridge rectifier for 2200v for the 813(s). I ran the two PSU configurations in the Duncan PSU designer program to get these numbers, so I should be close on the power.  Separate transformer and regulator for screen supply, or a big resistor from the modulated plates of the 814s, I'll try both. I have a transformer for that, and some big mosfets, and John W9JSW is sending me some regulator boards. I'll do a regulated bias supply as well, using a TI TL783 regulator; maybe floating it up on a zener if needed. 

I just grabbed a used 18 space steel rack from a different stadium install to stick most of it in, and a six space rack cabinet to sit on top for the exciter or speech amp or VFO or whatever.

Now, all I have to do is carve out some time to start cutting, drilling, screwing, and soldering....  I think I'll start with the 814 setup and plate modulation.  Seems like I've made a plan that will allow for a bunch of experimenting and playing around for the next few months...

Ed

Ed, sounds like some big plans, and a lot of fun. When I was WB8PEP, I really loved them wintertime projects in the basement. Here in Texas they don't know what a basement is!

I found another resource that you might find useful.

The article was written by Pat KD5OEI and Dennis W5FRS.

https://www.montagar.com/~patj/cathmod01.htm

Jim
Wd5JKO

Looks like some images are missing from that article, it's pretty old. It worked very well though the main thing being we needed more bias than was being used, so that was increased. The unusual thing is that the transformer was a E-I laminated power transformer 120V/60VCT I think, anyway it was on-hand and cost nothing. An oversized tube amp drove the transformer. The frequency response was quite good and a triangle wave was used to set up for best linearity. That amp is now plate modulated.