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Ed/KB1HYS
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« on: October 28, 2011, 10:21:53 AM »

In researching something else, I started reading about Cathode Modulation.  Simply put, inserting a transformer in the filament transformer center tap and applying modulation through it to the RF finals.  It seems simple enough, but,

I couldn't find any mention about the ratings of the modulation transformer to be used.  Since it's essentially a form of grid modulation (biasing the grid by applying a voltage to the cathode) I would figure that you'd need a relatively small transformer, and audio amp to modulate a large RF final tube, is that true?  What are the downsides to cathode modding a large RF Final?
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73 de Ed/KB1HYS
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« Reply #1 on: October 28, 2011, 10:47:23 AM »

Coincidentally, I just posted something about cathode modulation.

On http://amfone.net/Amforum/index.php?topic=12945.0 there is a link to http://www.montagar.com/~patj/cathmod01.htm. This may help answer your questions.

I'll be watching this thread, as I'd like to build such a thing myself :-)
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73 Mike 
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Steve - K4HX
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« Reply #2 on: October 28, 2011, 10:49:00 AM »

If you have a 30/40's vintage west coast handbook, you'll find extensive information therein.
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« Reply #3 on: October 28, 2011, 11:09:05 AM »

I couldn't find any mention about the ratings of the modulation transformer to be used.  

Per Steve's suggestion, I got my old West Coast handbooks out. The 10th edition (1946), p. 200, implies that we need an 'audio modulating power of approximately 20% of the DC input to the cathode-modulated stage'. If I find anything different, I'll post it here.
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73 Mike 
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Steve - K4HX
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« Reply #4 on: October 28, 2011, 11:13:06 AM »

It's a tradeoff between final efficiency and modulation power - more mod power, more efficiency. Another way to look at it is the amount of grid modulation versus plate modulation. IIRC, the book recommends something around 55-60 % final efficiency and 20% DC input for the mod power.



I couldn't find any mention about the ratings of the modulation transformer to be used. 

Per Steve's suggestion, I got my old West Coast handbooks out. The 10th edition (1946), p. 200, implies that we need an audio power of approximately 20% of the DC input to the cathode-modulated stage. If I find anything different, I'll post it here.
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KC2ZFA
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« Reply #5 on: October 28, 2011, 11:17:20 AM »

Details:

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0301.pdf
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The Slab Bacon
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« Reply #6 on: October 28, 2011, 11:19:03 AM »

Ed,
      If you are going to do it with a transfoma, you need a somewhat special transfoma to do it. It has to have a low-Z secondary. They are getting a little tough to find these days.

A better way to do it is series modulating the cathode with another tube.
The tube should have a very low plate resistance. Multiple 6Y6s in parallel were often the choice. I think a couple of 6080s with everything in parallel would do very nicely. They could easily mod a 4-400 (or maybe even a pair). You set the biass on the modders for your resting carrier level and the audio would provide the munky swing.

Some kind of big FET might do it as well.

Ed in joisey (KB2NSP) used to run a very nice sounding cathode mod 4-400 rig. IIRC, he had a pair of them modded by 5 or 6Y6s in parallel.
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« Reply #7 on: October 28, 2011, 11:33:52 AM »

Ed,
      If you are going to do it with a transfoma, you need a somewhat special transfoma to do it. It has to have a low-Z secondary.

One article I recently read about a ham who tried cathode modulation using a transformer (perhaps in one of the links I posted above) showed amazing results (including superb frequency response) from a 60 Hz power transformer salvaged from a solid-state audio amp.

EDIT: Here's the excerpt from that article: "The modulation frequency response from 100Hz to 6KHz was found to be flat within 1dB. This was attributed to both the large size of the modulation transformer and the capability of the Altec amplifier to deliver power far in excess of that required to modulate the stage."

Quote
A better way to do it is series modulating the cathode with another tube.
The tube should have a very low plate resistance. Multiple 6Y6s in parallel were often the choice.

A circuit like that is in the Editors and Engineers West Coast handbooks I was just looking at. Looks quite interesting! :-)
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73 Mike 
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« Reply #8 on: October 28, 2011, 12:13:18 PM »

I dont know of anyone who has used them, but the specs on 6080s look like they were designed for it!  They are now cheap as dirt cause no one wants them anymore. They have the lowest plate resistance of any tube that I know of.

Also keep in mind that no mod transfoma = unlimited frequency response.
(at least to a degree)
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« Reply #9 on: October 28, 2011, 12:40:19 PM »

Dan W8NWF in Whitehall MI runs a HB cathode modulated 833 rig and has a strapping sig.with good audio.
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« Reply #10 on: October 28, 2011, 12:49:50 PM »

Dan W8NWF in Whitehall MI runs a HB cathode modulated 833 rig and has a strapping sig.with good audio.

Any idea what his circuit looks like?

I looked him up on qrz.com. Nice looking rig!
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73 Mike 
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steve_qix
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« Reply #11 on: October 28, 2011, 01:01:29 PM »

It depends on what you mean by cathode modulation, and the term is not well defined (in my opinion!).

"Real" cathode modulation (as defined in radio amateur handbooks, etc) implies grid modulation because the grid to cathode voltage is varied by some amount, as well as the cathode to plate voltage.  I would not use this - it is more complex than straight grid modulation.

Now, if you are simply applying high level modulation to the cathode side of the RF amplifier - AND - the grid to cathode voltage does NOT change under modulation (I.E., the grid-cathode circuit floats along with the cathode), then the requirements for the modulator, etc. are the same as if you were applying modulation to the plate side, and the RF amplifier efficiency will be the same as if the modulation were applied to the plate, etc.

I have used this method many, many times in numerous transmitters.  Notably, because I used series modulation (with tube transmitters), it is easier to float the RF amplifier cathode circuit than it is to float the modulator cathode circuit.  I have done this with both analog (standard series) and with pulse width modulators.

The key here is to know whether your intention is to modulate the grid-cathode voltage (AKA some amount of grid modulation), or simply apply modulation to the negative (cathode) side of an RF amplifier, with the no change in the grid-cathode voltage.

Regards,

Steve
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« Reply #12 on: October 28, 2011, 01:03:30 PM »

Dan W8NWF in Whitehall MI runs a HB cathode modulated 833 rig and has a strapping sig.with good audio.

Any idea what his circuit looks like?

I looked him up on qrz.com. Nice looking rig!

Don't hold me to this but I think he wrote about it once in Electric Radio.  I'd have to go dig around to be sure.  

Update:  Found it in my index:  ER issue number 239 page 11 "W8NWF Homebrew Carhode Modulated AM Transmitter."
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« Reply #13 on: October 28, 2011, 01:26:06 PM »

It depends on what you mean by cathode modulation, and the term is not well defined (in my opinion!).

"Real" cathode modulation (as defined in radio amateur handbooks, etc) implies grid modulation because the grid to cathode voltage is varied by some amount, as well as the cathode to plate voltage.  I would not use this - it is more complex than straight grid modulation.

Now, if you are simply applying high level modulation to the cathode side of the RF amplifier - AND - the grid to cathode voltage does NOT change under modulation (I.E., the grid-cathode circuit floats along with the cathode), then the requirements for the modulator, etc. are the same as if you were applying modulation to the plate side, and the RF amplifier efficiency will be the same as if the modulation were applied to the plate, etc.

I have used this method many, many times in numerous transmitters.  Notably, because I used series modulation (with tube transmitters), it is easier to float the RF amplifier cathode circuit than it is to float the modulator cathode circuit.  I have done this with both analog (standard series) and with pulse width modulators.

The key here is to know whether your intention is to modulate the grid-cathode voltage (AKA some amount of grid modulation), or simply apply modulation to the negative (cathode) side of an RF amplifier, with the no change in the grid-cathode voltage.

Thank you. This is real food for thought.

I thought cathode modulation was a combination of grid and cathode modulation. I see I have some studying to do. :-)
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73 Mike 
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« Reply #14 on: October 28, 2011, 01:33:25 PM »

Found it in my index:  ER issue number 239 page 11 "W8NWF Homebrew Cathode Modulated AM Transmitter."

Thanks. I couldn't find that issue in a Google search just now, but it looked like he used three 6146s in parallel as the cathode modulator for a single 833?

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73 Mike 
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« Reply #15 on: October 28, 2011, 01:39:32 PM »


Thank you. This is real food for thought.

I thought cathode modulation was a combination of grid and cathode modulation. I see I have some studying to do. :-)

You've got it right!  It's a combination of high level modulation and grid modulation - mostly, from what I've seen - grid modulation.
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« Reply #16 on: October 28, 2011, 01:54:19 PM »

Quote
Dan W8NWF in Whitehall MI runs a HB cathode modulated 833 rig and has a strapping sig.with good audio


I heard him check into a net and he sounded great Grin Grin Grin


* dan.jpg (36.35 KB, 480x640 - viewed 123 times.)
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« Reply #17 on: October 28, 2011, 03:04:42 PM »

If you want to use a transformer coupled cathode modulation configuration, couldn't you use a smallish multi-match plate modulation transformer as a cathode mod trany?

You can match almost any impedance with one of those.

Perhaps something like a CVM-3 would work to cathode modulate an 833.

You would probably want to select one of the parallel configurations on the secondary side to handle the current.

But voltage and power levels would be OK.

Dave
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« Reply #18 on: October 28, 2011, 04:44:57 PM »

If you use a modulation transformer you could swing the cathode negative and get higher output. Straight series modulation you are just changing the resistance in the cathode. A FET with a source resistance will do a great job.
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Opcom
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« Reply #19 on: October 28, 2011, 07:56:24 PM »

Here is a successful cathode modulation of a 250TH. The modulation was from the CT of the filament transformer to ground. It was excellent and we made 90W carrier at 2000V and max current at 100% was about 200mA, very nearly 100% modulation, maybe 95% to 98%, very cleanly and very symmetrically using a sine and a trapezoid test signal. It was possible to overmodulate. The drive had to be high and so did the bias, and the plate had to be tuned and loaded carefully. Very important to fidelity for many reasons was the oversized transformer (60VCT 8A rating), and big modulating amplifier. He has also used a TOA 100W solid state public address amp to feed it. The peak voltage on the transformer 120V winding was higher than for 60Hz 120V RMS but there have been no issues for many years. We found the load impedance to be more like 600-1000 Ohms, but I guess it varies with the tube. We did at first follow the Frank Jones impedances but found that in our case it was different. Unknown why. I have heard both claims of high and low impedance.

http://bunkerofdoom.com/w5frs/index.html

"The 250TH amp is modulated by an Altec 1570B tube amplifier running a pair of 811A's. The audio is coupled from the 32 ohm output of the Altec through a 30V to 120V power transformer, to the cathode circuit ofthe 250TH amplifier. Fine fidelity is obtained since the Altec has a low output impedance and more than enough power in reserve to compensate for any impedance mismatch in this service. "

Just scale this up for your needs.

(I forgot about us writing that 250TH article till after I posted this.)
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« Reply #20 on: October 29, 2011, 02:32:31 AM »

This subject was discussed a short time ago with reference to a book by  
Frank Jones.

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


Ross, VK3ZZ, built a series modulated transmitter using 4 x 6L6's driving a pair of 100TH's.

Bob
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« Reply #21 on: October 29, 2011, 11:31:26 AM »

I built a simple two tube modulator that plugged into the key jack (assuming cathode keying).   I used in to modulate a DX-20.  Only power required was for the tube filaments.    It was in September 1953, page 15 of QST.   I suppose you can get it from the ARRL site. It used a 6Y6 as the series modulator and two halves of a 6SL7 for the audio preamp.   It was good to modulate up to 200W final power, you parallel 6Y6's for more power.

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