The AM Forum

I'm trying to understand Cathode Modulation -

I don't have the 10th edition of the Wm Orr Radio Handbook (you called it the "West Coast Handbook"?), but I have the 15th edition - which still contains information about using a transformer for modulation in the cathode of the Final Tube of a Transmitter.

I also read some of your comments and information from back in the 2011 topic discussion.  (I got lost on the stats - in the give-and-take of the series.)

Would you be willing to summarize the stats required in using a transformer to do Cathode Modulation? 

As I understand, a modulating power of about 20% of the power of the Final Amp would be required.  What was meant by a secondary with a "low-Z"?  I know it would have to match the Cathode impedance - would that imply a variant of one of the filament transformers?  One statement was that a negative voltage would be necessary? (grid bias?)  Comments were also made about the sourcing of the grid bias.

Low-Z - - for a 6146 - total current would be about 121 mils (class C - plate/screen/grid).  The bias on the grid is to be a neg 87v -that would mean the impedance would be 87/0.121 or about 720 ohms (an interstage transformer?) - do I figure right?  But then the bias would vary according to the power output!
At 20% power - this would mean about 13.5w - (transformer efficiency about 65% - or a 20 watt amplifier (a pair of 6v6 s in AB2).

Do I understand that this is a better approach than the screen-grid modulation of my DX-60, et al, without needing the modulation transformer of standard Plate Modulation? (DX-100)

I do understand that this approach would essentially prohibit PDM - since square waves would never make it accurately though the transformer.

 I can see possibilities with series tube Cathode Modulation - including the higher voltage for the final-modulator tubes being in series.

**********
Yes, I'm stuck on tubes (I've designed, maintenanced and built advanced solid state) -but as a retired Physics Prof, with an Scouting interest in emergency and disaster - I get the scientific warnings about a not too future possible-(even likely) EMP (solar or Iranian)(right now they are talking of "less than a year") - and its disaster to transistor/IC equipments.  I'd like my station to be operable in such a case (and I have these tube equipments).  The DX-60 will work fine as is, but for a dynamotor/battery power source - I am looking at the DX-40, possibly with a better modulator.  I've basically discarded SSB (SB300/400) - and the newer digital modes.

And I don't know yet, if silicon diode rectifiers would survive such!

so - back to Cathode transformer modulation - -

Some paper on cathode modulatio   https://www.montagar.com/~patj/cathmod01.htm (https://www.montagar.com/~patj/cathmod01.htm)

EMP is going to wipe out all the electronics in under a year is it?

I know they have been expecting the end of days for about the last 2000 years, and the asteroid strike is any day now, global warming is too slow for us.

Low Z like maybe 600 ohms?
It should be an audio transformer designed to have DC flowing through the cathode side, a filament transformer might not sound very good since its not an audio transformer.

You would need something like a push pull 6000 ohm center tapped to 600 ohm transformer designed to have DC flowing through the 600 ohm side.
You CAN find a 8 ohm to low impedance transformer, a single ended output transformer, Hammond makes them.


A single 6146 can do about 50 watts out with plate modulation, about 15 watts with screen modulation, and maybe 35 watts with cathode modulation?

Both the plate and screen modulation can sound very good, the plate modulation depends on the mod iron and the modulator, the screen modulation on the circuit design.
I have no idea about cathode modulation, but it would depend on the transformer if one is used for frequency response.
Screen modulation is nice because there is no phase shift or frequency response issues in transformers.

Screen modulation can be used without parts changes on most things as the normal plate voltage is as high as the voltage goes. With plate modulation, its at least 2X the plate voltage, and often has peaks up to 3X the plate voltage.
Cathode modulation would seem to be in between those?

When the giant EMP comes, there wil be no electricity, no internet, no banking, no transport, no gasoline and no one on the radio.
 

Read the docs attached.

One more.

W5FRS and I cathode modulated a 250TH stage.
A 36V-to-120V, 400VA power transformer was used as a modulation transformer on the 32 Ohm tap of the audio power amp.
Response as swept was pretty flat from around 50Hz to 6KHz.
A 175W tube audio amp and a 100W solid state amp, both PA types, were alternately used as modulators with good results.
These were quite larger than really necessary.
The carrier was around 80W, as the 250TH was run at 1800V and not being pushed.
Later more power was wanted, the plate voltage was increased, and we found it useful to increase the grid bias to prevent grid current peaking from the audio signal during modulation peaks, and along with that increase the grid drive to compensate, in the same manner that more grid drive and sometimes more bias is needed in a plate modulated stage than in a simple CW stage.

It is described in the link in the previously quoted quote;
http://www.montagar.com/~patj/cathmod01.htm
but that is an account of a successful experiment (in use to this day), but is not a guide for all purposes. It was fashioned after the handbook stuff  and magazine articles in the links above, and those are the best way to go for your own project.

Thanks - the Frank Jones paper is very practical.  I had been seeing reference to probably this very one - and had not found it.

It sounds like my search for an amp with a pair of 6V6s may not be strong enough - I had looked for 6L6s - but it seems everyone else is looking for a good mono tube amp, with 6L6s.  The pawn shops seem to echo each other - "We had one - but it is gone now!"

I thot I had a tube amp, brought home after my dad's death - but in moving recently, I now can't find it.  I think it had a pair of 6V6s.

Have been playing with CM for the last couple of years. It can also be done without a transformer and I have two of those built and working. I like their sound better. I can't give you the 30 formulas  and equations but I do have 3 that operate. Interesting form of modulation to me. 6v6s would fine and 6l6s an alternative. Number paired depends on power out. Voltage drop across the tubes needs to be compensated for in the PS in the no transformer design IF max power is important. Usable power though can be done with the simple set up. 1940s Radio Handbook has a lot. Highest I used or use it on is a ARC5 T18 and a T19. Lowest power when I was learning about it was a single 01A tube / carbon mic deal I built and used on my 45 tube TNT. Worked very well and I even tried it on the ARC5 and it worked. For me its biggest advantage is being able to put any cathode keyed rig on AM in a instant and only power supply needed is heater. I have spent many hours trying things with this. Some were good , some bad and some so so. I kept the three best in operation.
don

You do realize that a true EMP event of the effective type posited during the Cold War, (3 ea equally spaced over the U.S. etc. ) would not only knock out your solid state devices but would equally crash the very power hungry electrical grid necessary for your vacuum tubes?

Better find a couple of stand alone QRP or semi QRP boxes, necessary pre-soldered antenna, alligator clips, tape, keyer, carefully wrapped NiMoh batts, etc. and bury them in a well sealed box metal box.   ;D.  Yourself too.  Throw on a mag anode on that box and be sure the interior is electrically blanket insulated and dehydrated.

Don't come out for a couple of weeks or so, because the first trials from third world are likely to be quite dirty, even the duds. You won't be going far  anyway in any auto that post dates electronic ignition, let alone the later computer-everything loaded rides. (I gotta quit editing this,  too compulsive.)

OTH, great that interest is taken in a technology that is still maturing, e.g. If only a in great big daddy to the very critically spaced 8877 architecture.  Think weaponized and now operational laser.

On topic, good articles guys.  Don, K4KYV, might weigh in with cathode mod ideas., form of really grid modulation as outlined in the RCA article and other good stuff. He has great perspective.

Re:  EMP - Iran - - I almost refused to respond -

Present possibilities are based on a ship standing offshore and firing a rocket near straight up - to about 80 miles height.  The EMP would have a 400-600 mile blanketing radius

If the event were single - the probabilities are that it would be off the Atlantic seaboard - somewhere between Richmond and Baltimore - which would have a destructive pattern that covered the Eastern US, inland to about the Mississippi River

A multiple pattern would include a second shot off the gulf coast and a third on the pacific coast line.  The gulf would take out Texas and up to near US I-70 - the pacific shot could be anywhere (possibly California - or near Portland) , but would go into the Rockies.  It is considered that Iran will only have one bomb ready to use.  (Anything more would be aimed at the "Little Satan" - Israel.)

Shots on the Atlantic and Gulf would not have serious radiation effect on most of continental US, except as the gulf shot might blow into Florida - since prevailing winds are from the west/southwest.

The biggest problem would be the loss of the power generators and lines (essentially those active at the time - thats all of them) and of most vehicular traffic (modern cars and trucks -having solid state controls) - which would mean no electricity - and no food.  It is anticipated there would be rioting after the grocery stores are emptied (a couple days) - and starvation would set in - yes, the loss of over 90% of the population.  Then - what if it is winter - and no heat! - what about water - sewage?

The noise of a generator running would definitely attract destructive attention.  A grounded faraday shield would protect certain pieces of equipment (redundancy - even a solar panel) - especially if they were inactive at the moment of the strike - not hooked up to any line (which is now in the government's approach on protecting itself - the government is refusing to talk of this - it might get people upset).

This would be followed by the formation of surviving gangs, gaining area control - and near total loss of personal freedom and action.

A recent fiction that my daughter had me read was based on an eastern strike.  The capital of the US then being reestablished at Seattle, and that the eastern US was basically "unknown territory".  The story continued with one thread being a Moslem Terrorist followup invasion of New York City - and the actions of survivors there - with reports (radio?) getting to the US government in the west.

If it were a solar flare EMP - this would be world wide - with the worst effects being to that side of the earth facing the sun, at the moment the flare struck.  The problem is the sophistication of American and European society - we are not a localized rural society any more, like we were in 1880.

When I get dwelling on this, my wife reminds me (a preacher), that I had better depend on prayer! - that I really have nothing else!

Well that is certainly like cathode modulation. ALL Neg.
don

All right guys! This isn't a forum for end of the world scenarios. The topic is CATHODE TRANSFORMER MODULATION. If you have nothing to contribute to this topic, CATHODE TRANSFORMER MODULATION, please go to your underground bunker and ponder.

I was on an airbus taking off on a trip home from Brussels and we flew into a storm and got hit with a direct lightning strike at about 8000 ft. Blinding big blue flash, shotgun loud. The lights went out in the plane. in a few seconds they flickered back on, the pilot with a thick French accent says " pardon madams monsieurs, we have experienced ze electrical disturbance which is completely normal" and then we flew across the ocean.

Cathode modulation essentially being simultaneous in-phase grid and plate modulation requires less power than a plate modulation system. A 6146 can be modulated with a pair of 6AQ5s in push pull at say 15 Watts of audio power and by getting the balance right, the efficiency of the final will not be bad, perhaps 50%.

Several years ago around 2002 I have used cathode/grid modulation in my PA with 6 parallel tubes PL519.
Instead that I use an modulation transformer, I used an big NPN high voltage transistor for the series modulation with the output tubes.
The transistor was the type BU508, you can find them as output deflection transistors in televisions.
The collector voltage was around 150V, the collector/kathode current was set to half its maxium current e.g. for full power.
The modulation was very hi-fi.

Martin

Here is the Cunningham CM-30 Cathode Modulated transmitter.

I'll try and recreate it in LTSpice since the copy is light.

Click here for a  downloadably copy of the  7th edition of Orrs' Radio Handbook.......

http://www.tubebooks.org/technical_books_online.htm#Radio

I've  used this website to kill time and to fog over my remaining brain cells.

KLC

Frank Jones has some of the best material I have seen on the subject. 

Joe GMS

Here is the Cathode Modulated Cunningham CM-30 schematics capable of 36 Watts output.

A very simple circuit actually.

We used to use these in BC work for night time operation:

Phil - AC0OB

Pete, regarding staying on topic, the dear professor stated in his opening remarks why he was interested in vacuum tubes or the rest of us wouldn't have even thought about straying to the topic's EMP basis.  ;D

Now also compared to the 4x PEP AM with full plate modulation of the final class C amp, you'll get less PEP in cathode modulation of Cunningham's 6dQ5.

Here is the whole manual, with schematic, for the Cunningham...

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

I have a copy of that same pdf and attempted to upload it but when I attempted to upload it (three attempts), I kept receiving a "time-out"  message, which is why I redrew the schematics.

Phil - AC0OB

Probably the problem was that the pdf is about 8.25 meg and the attachment limit is 5 meg. If you post the link to the pdf the file doesn't get hosted on AMfone.

Neat circuit. Seems like an easy way to add AM to CW only rigs like the Eico 720, Drake 2NT, etc.

This is a general comment to all. To try cathode modulation all one needs to do if your lucky enough to have an old or new public address amplifier hanging around is attach the cathode of the final to the 70 volt tap of the amplifier. Modern transistor PA amplifiers have an internal output transformer in series with the 8 ohm primary tap to ground and therefor allowing outputs of 4.8.16 ohms plus the 70 volt tap. One of my amplifiers allows for direct connection to the transistor circuitry to the speakers or by changing a tap, the output is sent through the transformer. Still others have a higher voltage tap that I think is around 125 volts? Maybe in the spring I'll hook up my PA amp to the BC 610 and see what it will do? If anyone would like to try this approach, I sure would like to hear from you! Thinking out loud, perhaps the PA amplifiers transformer may become saturated so a choke in series with the cathode to ground may be better and the audio could be coupled to the choke with a large value capacitor?

The 1940 Radio Handbook spoke of Cathode Modulation as being with a transformer in the ground circuit of the Final Amplifier Cathode(s) and called the direct tube placed there as "Series Modulation' of the Cathode.

My question - In Cathode Modulation, what is an advantage of using the transformer in the Cathode circuit, in stead of the tube?  (of course, there can be no PWM through a transformer)

A possibility that I am considering is that with the transformer it works with less power loss thana with the tube (power coming from the modulator - but only something like 20%).  That the additional tube placed in series requires consideration of the dissipation lost in the tube so a much high voltage is required (based on the peak voltage limits of the tube(s) used - a type of Series Resistance form).

[Another paper on "Series Modulation" used the tube(s) as a Plate Modulation approach, not using a transformer (nor Heising), but simply having the tube control the flow of current through it to the Plate of the Final Amplifier (being placed between the Final Plate and the HV Power source - cathode of the Modulating tube going to the Plate of the Final Amplifier) - again - a higher voltage is required to compensate for the extra tube.]

or - am I missing something? - vital? -

I've got a 100 watt Bogan PA amp around here (PP 6146s or Parallel PP 6146s) that has the 70 volt tap that should work; sounds like a good use for it. Of course it'll have to get in line with getting the radio room built first.

No you actually understand it. The advantage of using the transformer is you don't suffer the voltage loss or drop across the tube as the transformer winding makes that connection and then you wiggle that from the primary winding with your modulator/speech amp deal. The disadvantage is it takes more umph to do. With the tube completing the circuit you have the voltage drop across the tube which is why you dont want a big hd tube as some would think but a small one with low internal resistance and if you need more current carrying ability you add more little tubes in parallel IE 2 to4 6v6s or 6l6s if absolutely necessary. It is very effective and the voltage drop across the mod tube can easily be totally compensated for in PS design. IE if you want 600volts on the finals and you are dropping 200 volts across the mod tube then an 800 volt supply would get you back on track. I have tried a lot of stuff with this over the last couple years. You understanding is very good from what I see having actually done a lot of work experimenting with this. I use the direct tube method series cathode mod on both my ARC5s and on my bare essentials Cw rig when I want it on AM. I have also several times used it on my 45 tube TNT before I got into loop mod. It was harder on the TNT because of the direct heated cathodes in both the TX and the modulator but providing them each with their own independent heater supply cured that. It is a very practical way to take any rig with kathode keying and put it on AM. All the palaber about mod %s and this and that become irrelevant because plain and simple it is straight forward and it works well. After a couple of years of trying both I have retired the transformer one and now only use my direct tube ones of which I have built three and have a fourth under construction.
don
Inspired by the thread I fired up the 80M ARC5 and went on air twice today. Still happy. klappt gut

Series and cathode modulation (as defined by Jones) are two different things. One is the equivalent of plate modulation and the other is a combination of grid and plate modulation.

In one of my sources (not recorded [horrible] - from AMFone?) - I seemed to see a difference on cathode modulation, based on where the grid receives its bias. 

If I understand the statement - - If a bias voltage is obtained from a separate source (ground? - PS? - at least - tapped down on a cathode transformer), then the grid receives some modulation through the cathode - but if the bias was "floating" - bias obtained from the cathode above the cathode modulator(?) - so that no change occurs in the voltage relationship between cathode and grid - then the modulation was essentially "Plate Modulation" - just achieved in the cathode. 

From this, do I understand that the Cathode "Plate Modulation" would give a larger Final Amplifier Power level (54% to maybe 75%?), because of the differences between grid dissipation and total tube dissipation?

Had I better forget about my favorite 807 (or even the 6146) and go to triodes (811? - 572b?) - just to forget about the "confusing" screen? or doesn't cathode modulation have problems with the screen? (because it includes the screen?)