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Negative audio feedback in Heising circuit




 
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Author Topic: Negative audio feedback in Heising circuit  (Read 861 times)
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ka1tdq
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« on: October 06, 2020, 09:42:01 AM »

I'm going to drive the grids of the 4-1000 rig I'm building with a backwards connected audio push-pull transformer. I'll be using a modified Heising circuit connected to the secondary of the modulation transformer. I'm not aware of any plans out there to develop negative audio feedback with this arrangement.

I thought of possibly using a toroidal power transformer is series with the secondary of the modulation transformer. These transformers behave satisfactorily at audio frequencies, and the Heising circuit keeps the HV off of it. There's no need to worry about breakdown voltage. The primary of the power transformer would be linked out of phase with the primary of the audio transformer. The audio transformer I'm using has a 20 ohm primary, but I have taps for larger impedances.

There wouldn't be a concern for core saturation since the DC is kept off of the toroidal transformer.

I've attached a schematic of what I'm thinking. The negative output of the audio PA amplifier would be attached to ground and the positive lead coupled to the 20 ohm taps. A small value resistor is in parallel with the audio output to allow for the coupling capacitor to charge to some level without affecting the amp.

I wanted to see what everyone thought of the idea. If it seems feasible, it would then just be a game of impedance ratios and guesswork to make it all work.

Jon


* Negative feedback circuit.jpg (1973.52 KB, 3816x2858 - viewed 100 times.)
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« Reply #1 on: October 06, 2020, 12:10:58 PM »

Hi John,

Going thru three transformers with audio NFB is not a good idea.  If you want to do it correctly from the start, use a tap off one of the modulator tube plates before the mod xfmr.  Use a string of (non-inductive) resistors and tap off the bottom to ground.  Then go into the GFZ MOSFET audio driver NFB input.  This way there are NO transformers in the NFB loop at all. The mod xfmr is the only audio transformer in the rig and is not used in the NFB loop.

NFB can be tricky and unstable and there is phase shift using transformers. Keep it simple and without transformers and it will work right from the start.

I use NFB with all my plate modulated rigs using the above described method. I can add up to -12 to -15 dB if I desire and the rig is stable. When I played with transformer loops it was constant problems.  These days I keep the NFB around -4 to -5 dB area and everything is happy.  The rigs can pass triangle waveforms easily. This includes my 4X1 plate modulated rig.  The bottleneck becomes the mod xfmr (and Heising components) only -  and a good quality BC xfmr will do wonders from 30 Hz to 12 KHz if set up properly.

T
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ka1tdq
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« Reply #2 on: October 06, 2020, 12:46:18 PM »

Hey Tom,

I understand the audio restrictions of going through transformers. My goal is simplicity while still being able to use NFB. Yes, using the GFZ design will clearly accomplish the task better but I'm set on driving the grids with an audio amp.

Apparently you've experimented with transformer style NFB options and I guess you've found instability. My question is have you tried this method or something similar? If not, what's your take on the concept?

Jon
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« Reply #3 on: October 06, 2020, 04:14:35 PM »

another point of interest is that audio power will tend to pass both ways thru the toroidal xfmr the way your circuit is drawn .... this will make this modulator very hard to drive .... not knowing just how big your audio pa is ....

there were many discussions in the past about toroids and their potential and they have good potential...

consider a different topology for the toroid that does not have it passing power in the forward direction
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« Reply #4 on: October 06, 2020, 05:59:57 PM »

Jon,

Not sure how good this would work, but maybe you could find an Antek that you could use as the INPUT xformer, with another, smaller winding on it?  Apply feedback there???


I love those Antek xformers.  Built a 3.5Kv supply out of one.  I was able to get about 1.2Kw pep from it, and runs cool cool cool.


--Shane
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K8DI
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« Reply #5 on: October 07, 2020, 07:00:56 AM »

Don't forget that the mod tubes are phase inverting. As drawn, you have PFB.....

Ed
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ka1tdq
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« Reply #6 on: October 07, 2020, 10:01:26 AM »

Right, I see that the secondary of the mod transformer arrangement that I've shown wouldn't work.

When I was having an eyeball with Dennis in Arizona he showed me how to split the primary tap and use feedback with an external transformer there. Unfortunately full there's full B+ and you need to worry about winding insulation breakdown.

So, changing gears, I'm not going to worry about implementing NFB. I'll run the grids from the audio transformer as planned. Yes, I've used Antek power transformers for audio in a few class E transmitters. They sound very good!

This circuit will notify me visually and audibly if I hit a negative peak. I'd just need to cut back the audio a little bit. It's not fancy but I've used something similar before.

As for the added high fidelity benefit of NFB, eh... I don't talk in triangle waveforms anyway.

Jon

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** Also, I got a spike in traffic similar to the size the day after I applied for my Secret security clearance on my website yesterday. You guys are the best!


* Negative peak notification.jpg (775.68 KB, 4032x3024 - viewed 64 times.)
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« Reply #7 on: October 07, 2020, 08:47:26 PM »

The "10ufd" in series with the driver transformer will be insufficient.
More like 10x that would be a start...

As far as a "passive" NFB?
Not sure about this.
I've never seen it done.
Therefore I suspect there is a very good reason.

Not the least of which is that the low Z drive on the primary of the
drive transformer is wanting to drive the winding on the so-called
feedback transformer. It wants to see it as another modulation transformer,
not as feedback.

Typically feedback needs to be applied to a summing node, usually not low-Z,
put certainly electronically isolated from the input signal...

                     _-_-
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ka1tdq
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« Reply #8 on: October 08, 2020, 12:21:02 AM »

Iíll just stick with what works. I will continue this thread on my website.

Jon
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« Reply #9 on: October 11, 2020, 09:17:36 PM »

Done carefully, NFB can fix a lot - even through 2 transformers. It is a great help to military iron that needs ironing out.
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ka1tdq
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« Reply #10 on: October 16, 2020, 09:32:47 AM »

Negative feedback for amateur applications really isn't necessary, honestly. Yes, there are bragging rights to be able to pass a difficult-to-modulate triangle waveform, but the end user won't notice. I've spent many years in New England. I do know what quality AM sounds like.

My tend to reduce my project level of complexity to a bare minimum. Look at my class E projects. I'm doing the same with my hybrid 4-1000 transmitter. I'll drive the grids with a PA amp and have my negative peak notification circuit in there. Pretty simple.

And, at 1000 watts carrier, who cares?

(I know, insert pearl clutching here)

Jon
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« Reply #11 on: October 17, 2020, 09:46:50 AM »

I'm hoping someday we can end the use of "Heising" where it neither belongs nor makes sense. Unfortunately, the incorrect use is widespread so it's worth addressing.

Heising is a form of constant current modulation where single RF final and single AF modulator tubes alternately draw current from a power supply at the rate of the modulating waveform. https://www.w8ji.com/Heising%20modulation.htm

From the 1976 ARRL handbook:



Disqualifying components are a modulation transformer, a push pull modulator, and a D.C. blocking capacitor. The presence of any of these eliminates the term "Heising". As for "modified Heising", that opens a can of worms that would include terms like "end fed dipole" or "6146 collector current".

Modulation choke or reactor would be a finer, and for accuracy, proper term.
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ka1tdq
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« Reply #12 on: October 17, 2020, 10:04:20 AM »

All very true.

However, it could be a moot point. Nobody knows what Heising is anymore, let alone something as basic as a vacuum tube. Unless one of you guys get a wild hair across your a**, this is probably going to be the last 4-1000 rig built by a ham in America.

(Driven by a class E rig to close the books on American ham home brewing).

Sorry guys, I am Revelations.  Grin

Jon
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« Reply #13 on: October 17, 2020, 12:38:25 PM »

It *is* a bit persnickety, but to not mention it would be like letting someone walk around all day with his fly open...
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« Reply #14 on: October 17, 2020, 12:52:05 PM »

I'm hoping someday we can end the use of "Heising" where it neither belongs nor makes sense. Unfortunately, the incorrect use is widespread so it's worth addressing.

Heising is a form of constant current modulation where single RF final and single AF modulator tubes alternately draw current from a power supply at the rate of the modulating waveform. https://www.w8ji.com/Heising%20modulation.htm

From the 1976 ARRL handbook:




It feels to me that the more common usage in everyday ham circles came with Bob Heil's QST article for the pine board transmitter, which should have been modeled after the handbook circuit you posted.  He left out the C1/R1 element, which made for a transmitter that couldn't modulate well. But I am with you -- using the wrong terms makes for confusion, especially as one searches for ideas/circuits/examples. If I google a concept like Heising, I get ten circuits, half of which aren't Heising. As a late-comer to tube AM, learning is everything....but bad source material slows me down or leads to making mistakes.

Ed
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« Reply #15 on: October 17, 2020, 12:55:59 PM »

Well stated, Clark.  


Now a comment on a subject from previous posts:

There are probably 5-6 classes of AM rigs...  and they all have degrees of difficulty to get sounding good. Some will never sound great while others always sound great.

The simplest is an SDR driver like a Flex into a linear. No effort. They always sound good.  Then there's the class E rig. The most popular is the QIX "kit" rig which is already engineered and sounds fantastic. Not a lot of effort AFTER it's built to get it running perfectly.

Next we have the many, many ham commercial rigs like the DX-100, Apache, B&W 5100, Valiant, Ranger, etc.. etc.  They always sound restricted until some work is done. This is possibly the most difficult project compared to the ones already mentioned.

Then we have broadcash transmitters that are already engineered and for the most part, sound great. Minimal effort except for the hauling into the basement...  Wink

The most difficult rig to get running correctly and sounding great is a big homebrew big iron plate modulated transmitter. To get it sounding like a class E rig or a PDM tube rig, and it can, requires mucho work. Lots of testing, optimization and using the correct parts and techniques. Skip some important refinements and it will never approach the sound of a clean, transparent class E rig.  And yes, I can hear the difference when conditions are good.

I can definitely hear the difference on the air of a homebrew plate modulated rig where the owner has built it but then stopped optimizing and refining it. The class E rigs show a transparency that is unmistakable in comparison.  Then there's the guys who have spent several years in continual big rig refinement and sound better than most, approaching and even equaling a class E or SDR rig... doing it with plate modulation and mod iron.  But it takes an understanding of the fundamentals and persistent testing and improvements.  Building a big plate modulated rig that sounds great and passes a triangle wave at 50 Hz is a difficult venture, but the refinements  can be heard!

The difference is like a professional body man looking at a car fender that has just been fixed and filled with Bondo. He can see all the waves and ripples, but to me, it looks FB. I just don't have the eye for bodywork perfection.   There is always a lot going on in the finer details.

We all have different standards and audio goals. Some are happy with stock ham transmitter audio and some need to push on to near perfection. Some cannot sleep knowing that there is something else that can be added or improved on.  On a quiet day with S9+50 carriers I can certainly hear the difference between one of the exceptional plate modulated rigs and one that is just "good enough."

So I say do what suits you and makes you happy.   There's no free lunches.  If we wanted it to be easy, we'd all be communicating on Skype and throw out the ham gear.

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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« Reply #16 on: October 17, 2020, 03:10:27 PM »

Schau zum Fenster !

http://www.amwindow.org/tech/htm/shuntfedmod.htm

http://www.amwindow.org/tech/htm/heisingorwhat.htm
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« Reply #17 on: October 17, 2020, 03:19:01 PM »


Tom,

All great points about tweaking a given design till it is fully debugged, and working as desired. Only a few of us are up to that task, and many of those refuse to go down that path. All too often, me too, one stops mid way down that path. The rig then is used as is, or becomes a shelf queen.

As to a single ended class A modulator, I attach a link to the old CQ article about a sliding bias 304TL modulator used to modulate a 1KW DC input RF final. It is actually pretty ingenious!

http://www.813am.qsl.br/artigos/moduladores/hileman/the_bias_shift_modulator_cq_apr_54.pdf

Jim
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« Reply #18 on: October 17, 2020, 04:57:55 PM »


Tom,

As to a single ended class A modulator, I attach a link to the old CQ article about a sliding bias 304TL modulator used to modulate a 1KW DC input RF final. It is actually pretty ingenious!

http://www.813am.qsl.br/artigos/moduladores/hileman/the_bias_shift_modulator_cq_apr_54.pdf

Jim
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Hi Jim,

You're always coming up with novel stuff...  Grin

At least it's not a controlled carrier scheme. What do you think is the downside?  

I'm curious cuz right now I'm about to wire up the new series modulated rig and the modulator is running in pure class A.

I once ran my big linear using a sliding idling bias between words. It went from near cutoff to normal class AB1.  Worked FB with much less heat overall.   If this works well, I wonder why it is not more popular?

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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WD5JKO


« Reply #19 on: October 17, 2020, 09:09:47 PM »


At least it's not a controlled carrier scheme. What do you think is the downside?  

I'm curious cuz right now I'm about to wire up the new series modulated rig and the modulator is running in pure class A.


Tom,

   Looking at the schematic, and the text by Bill Orr, the audio grid drive to the 304tl could use some more beef to it such that audio peaks could provide some grid current without distortion. Said another way, allow the 304TL to operate class A2 once the bias is shifted over on an audio peak. The 6J5 driver could be a triode connected 6V6, or some such thing, Parafeed, etc.

Jim
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