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811 bias for Mod




 
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KB2WIG
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« Reply #25 on: December 29, 2018, 12:46:35 PM »



J,

Throw a few high Wattage lamps in // and test away.

Wall Whirl has bulbs (don't task for lamps as no one knows what one really means).......

GE LIGHTING Incandescent Light Bulb,A21,200W 200A/CL-1     $2.97.

 Get 'em while they're hot.


klc

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« Reply #26 on: December 29, 2018, 12:54:16 PM »

Phil,

On the grid meter. A8 is a voltmeter. My bad. I fixed it to properly be identified as such.

On the knee limiter, what is the target knee voltage to shoot for at 1700v?

Tom,

I added a ball gap on the mod trans.

I also added PTT of the filaments. I guess this also gives me a soft start on the filaments as well?

KB2WIG - I have a few 300W Farm bulbs that will work nice for a dummy...I live on a farm...

New schematic...

John

* 8000 Transmitter.pdf (950.7 KB - downloaded 18 times.)
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« Reply #27 on: December 29, 2018, 01:05:26 PM »

Phil,

On the grid meter. A8 is a voltmeter. My bad. I fixed it to properly be identified as such.

On the knee limiter, what is the target knee voltage to shoot for at 1700v?


New schematic...

John

The rule of thumb is 1.2XHV B+ so in the new schematic I posted the limiting starts to occur at ~ 2000V.


Phil
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« Reply #28 on: December 29, 2018, 01:41:57 PM »


Tom,

I added a ball gap on the mod trans.

I also added PTT of the filaments. I guess this also gives me a soft start on the filaments as well?

John


Good on the ball gaps. They gets adjusted by trial and error. You want no arcing during normal speech but yell and depending on your low level limiting, the gaps may arc. Or they may just arc occasionally during PTT unkeying. All adjustable to suit your fears.  :-)



Filaments keying, soft start???

You meant you are keying the center tap of the fil transformer which is cathode current, right?  The filament voltage  from the transformer goes directly to the tube fil pins and stays on, no keying. The fils do not get keyed, just the fil xfmr CT.  Maybe that's what you meant. Look at my 813 schematic for an example if not clear.    

https://www.amwindow.org/tech/htm/813/813.htm


UPDATE: I just looked at your schematic and you have it correct. It was just your description that was unclear to me.

T
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« Reply #29 on: December 29, 2018, 06:31:54 PM »

John,

Some more suggestions...  I took a look at your power supply schematic....

You COULD put the HV filter choke in the plate transformer's  center tap to ground. It will do the same job but the high voltage to ground/frame will not be there.  It will still experience current induced voltage spikes as usual across the windings.  If you keep it in the HV lead, then let it sit on some Plexiglass or a block of wood to isolate it better from ground. You never know how well old choke internal isolation to frame is holding up.

Another point - about your ART-13 mod transformer:   It has a tiny core and is  an amazing transformer for the job it does. The WWII guys really knew their stuff. It's an anomaly.  I once had a pair of 813s modulated by a pair of 813s using a 50 pound broadcash mod transformer with a Heising choke. It ran 3KV and sounded great.  One day I decided to clip lead in an ART-13 mod xfmr for a test. It did well and sounded fine except the lows were greatly lacking for reasons I will explain. It did not blow up...

Anyway, you are driving the 811As with a backwards hi-fi 8 ohm audio transformer and system so it will have great audio except for the ART 13 bottleneck. If you cannot find a better mod xfmr, then it's highly recommended you add a Heising choke to the circuit. Say 20H or higher - with a 1-2 uF 4KV coupling cap will keep the DC off the ART-13 xfmr and improve its saturation figures greatly. Better low end and safety.  If interested, look at my 813 schematic for the hook up details.  

In the meantime, saturate the media with some "wanted" ads on the various ham websites for a broadcast mod transformer.  BC 250w or 500w xfmrs with matching Heising reactors are available. There are lots of older hams who have them in the cellar and will never get around to using them. They'd love to have your strong back take one away. :-)   You will never regret taking the time and effort to locate a quality xfmr. It will last a lifetime, let you run hi-fi audio with a big low and high end - and do well with NFB.  There is probably someone on this BB willing to help you out.

T
  
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« Reply #30 on: December 29, 2018, 11:09:05 PM »

Tom's suggestion of a Modulation Reactor will insure the Modulation Transformer doesn't saturate easily.  Cool

Annnnndd, if just by chance you decide you need one I happen to have one from a
BC-1T 1kW transmitter.  Grin


Phil


* Modulation Reactor Pic.jpg (4179.77 KB, 3872x2592 - viewed 27 times.)
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« Reply #31 on: December 29, 2018, 11:25:50 PM »

Wow, that looks like a beauty, Phil!   Made for 833As X 833As from 1957 era.

I use a similar looking black "décor" mod xfmr/reactor set from a 1KW BC transmitter for the 4X1 rig. They are built like tanks.

Well, there ya go, John. That's the big league  - 1/2 way there!   Don't settle for anything but a matching 1 KW mod xfmr now.    

T
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« Reply #32 on: December 30, 2018, 01:13:15 AM »

FWIW, I’m running my 811A modulators at 1400v and -2.4V on the grids via battery bias. I’m using two 1.2 V rechargeable size AA batteries. I have about 40 mA of idling current. Very simple system and seems to work FB. The mod transformer is a UTC S-22 with a Dahl mod reactor (reactor highly recommended). The transmitter is currently running a single 810 in the final position.
This transmitter is my “learning curve” transmitter, and overall has worked much better than I could have hoped for. I am now in the process of planning my next transmitter which will be PP 254W’s (I think) modulated by 838’s. I’m drawing the schematic now, will post it when ready. Good luck John. Yes, triodes are nice.😉
Ron W8ACR
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« Reply #33 on: December 30, 2018, 06:30:15 AM »

I really appreciate all of the help I am getting here!

Ron, your thread history is the genesis of my circuit design, along with the circuits posted on Jack's K9ACT web site. And with all of the help I am now getting, the circuit is getting better and better. This is a great group of hams!

Phil, want to make a deal? You go to any hamfests in the area? We live 275 miles away from each other.

If not, I am finding a few chokes at 12H and ~300ma. None in the 20H range that are not horribly expensive. If I don't go to a BC class choke, can I pair 2 relatively inexpensive filter chokes together to get around 20H? I don't want to spend a couple hundred bucks on an interim solution.

John
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« Reply #34 on: December 30, 2018, 10:45:29 AM »

John,

I have a spare mod reactor that is in tough physical shape, but i believe it is in working condition. I believe it is either 40 or 50 HY. I can measure it if you like. I have been keeping it as an emergency spare, but I’ll probably never need it. I also have a single spare 8000 tube that I know has a good filament for sure, and I’m also pretty sure it is a good tube. Again, I was keeping it as a spare, but I now doubt that I’ll ever use it. You can have these if you want them. I’m in Montana now at my cabin, but when I get home on Tuesday, I can dig them out and send them your way. They will be very affordable.😉 just let me know. Do you need any other parts? Do you have parts or tubes for trade?
73, Ron W8ACR
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« Reply #35 on: December 30, 2018, 11:40:53 AM »

I have some stuff to trade. Send me an email to Ke5ssh@gmail.com. My old call sign.
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« Reply #36 on: December 30, 2018, 11:47:54 AM »

Hi John,

I have the matching mod transformer as well. Sent you a PM.


Phil


* Modulation Transformer.jpg (3599.22 KB, 3872x2592 - viewed 18 times.)
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« Reply #37 on: December 30, 2018, 12:04:27 PM »

I really appreciate all of the help I am getting here!

If not, I am finding a few chokes at 12H and ~300ma. None in the 20H range that are not horribly expensive. If I don't go to a BC class choke, can I pair 2 relatively inexpensive filter chokes together to get around 20H? I don't want to spend a couple hundred bucks on an interim solution.

John


Hi John,

Yes, chokes in series will work to increase inductance as you would expect. However, as I understand it, the chokes designed and wound specifically for audio work, like Phil's BC beauty, will do a better job with audio (30 Hz-12 KHz) vs: common power supply chokes designed for 60 Hz.  At least this is what I have been told over the years. Is it winding placement or gapping, I dunno.   So go with Phil's BC iron and you will be covered.

Don't worry about impedance ratios on these big BC mod transformers as long as they are reasonable like 1.1:1,  1.5:1, 2:1, etc.   The optimum match for a single tube  final modulated by a pair of the same type  (one 813 X 813s) is a 1:1 ratio.       For a pair X a pair (813s X 813s) the ratio becomes 2:1.    The lower ratios usually mean bigger audio peaks, though sometimes less efficient from my own experience.

Anything between 1.1:1  >  2:1  step down ratio is common and will work FB in the many different rigs you may build over the years.  IE, don't get hung up on perfect matching of ratios. It will affect the peak audio capability, but with the right tubes, you will have plenty of clean audio available.


Probably you will get a 1KW set of iron, use it for this rig and later build a bigger 1KW rig.  Then use this iron for the bigger rig - and later find a smaller set, like 250W/ 500W, etc, for this present rig..


Update:
 I see Phil has offered you the 1 KW matching mod iron!  You don't know how lucky you are, OM... :-)   What is the impedance ratio on the mod xfmr, Phil?  The BC-1T web notes show 1.12:1 and 1.33:1   (I'm assuming they mean impedance ratio, not turns ratio, but not sure)


Here's a 2012 AMFone thread that has an interesting mod xfmr and reactor discussion:

http://amfone.net/Amforum/index.php?action=printpage;topic=31556.0


BTW, Your idea about using software (in another thread) for most low-level audio processing is a great idea. This is the present trend with SDRs and will eventually be the only way to go in the future - so knock yourself out... :-)


T
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Favorite Song - Trololo thru the years:  
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« Reply #38 on: December 30, 2018, 01:50:02 PM »


...What is the impedance ratio on the mod xfmr?  The BC-1T web notes show 1.12:1 and 1.33:1..


That sounds about right. Here is the simplified schematic which also shows the tertiary secondary winding for the driver stage:


Phil

* Gates Modulator with 833s.pdf (99.94 KB - downloaded 18 times.)
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« Reply #39 on: December 31, 2018, 12:12:05 PM »

OK, Rick. I have it correct now.

On the grounded variable cap. I have a 10-100 breadslicer cap in the junk box. Will that work as a direct replacement in a single ended LC or will I have to recalculate the LC and also center tap the inductor? I have this 21uF Air-dux that just works out single ended. I did not want to go with a center tapped inductor due to ease of build for 4 bands. I will if it makes better sense. The input LC calculations were pretty easy using a LC calculator and 15k for RL. Not sure how to model the other circuit.

Pls review my rewiring of the bias. I forgot to bring the +V off of the supply. I think I have it correct now.

John

The grid meter circuit looks ok now.  The only confusing thing is the "A" on the meter.  Makes it appear at first glance to be an ammeter, where a V would indicate a voltmeter.  It is common to have a grid ammeter, but rare to also have a voltmeter. 

As for the cap, if you use a split stator, that is actually two capacitors in series.  To obtain the same capacitance in the parallel resonant circuit, each half of the split stator should be double the pF value of a single capacitor.  Remember how to calculate capacitors in series......

Look at the diode string in the filament circuit of the modulator.  If these are standard diodes, then you have the symbol reversed.  If they are zener diodes, then the symbol is in the correct direction.   For standard diodes, they should have the cathode end grounded.
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« Reply #40 on: December 31, 2018, 12:51:24 PM »

Look at the diode string in the filament circuit of the modulator.  If these are standard diodes, then you have the symbol reversed.  If they are zener diodes, then the symbol is in the correct direction.   For standard diodes, they should have the cathode end grounded.

Rick,

Glad you caught that. That was my fault. John followed my description from a post I made.  Been corrected.



John,

If you later decide to get rid of that driver transformer, (phase distortion) consider this WA1GFZ-designed MOSFET audio driver that will drive your 811As or even 833As directly - later on. It will handle NFB right at its input. Cleanest driver system I ever used on my 4-1000A plate modulated rig.  You then have just one transformer, the mod xfmr, to worry about.

http://amfone.net/Amforum/index.php?topic=23632.0

T


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« Reply #41 on: January 01, 2019, 06:40:02 AM »

Made all of the changes noted in the last few notes and removed the voltmeter. Did not have one anyway.

My latest quandary is metal layout. I am not sure how the modulator will lay out if I add a large reactor. I may just have to lay them both out on separate 17x12 chassis and forgo the cabinet that I have that I thought was large enough. Finding rack style cabinets in southern Illinois is almost as hard as finding broadcast iron!

If I add another 8000, how much more power can I expect? I have the iron for a separate 1050V power supply at 300ma. I could run the mod on that. I have a Thorardson 1180-0-1180 transformer and a 12H 400ma Thorardson choke. Would just have to find some caps. If I use cap input before the choke I could get 1400v with less regulation.

What else in the overall design would have to scale up?

Scope creep is occurring!

John

Update - if I don't find a rack, I have a backup plan. 2 37in high storage rack end frames from Menards ($17 each), then mount 20U rack rails ($19 pair) to them. Put the power supplies on a wood shelf on the bottom then 2 7U high spaces for the mod and rf deck. Going to keep the exciter and AF amp outboard of the rack, probably next to the computer that will run my audio processing chain and the SDR RX. 

* 8000 Transmitter.pdf (938.56 KB - downloaded 10 times.)
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« Reply #42 on: January 01, 2019, 07:44:54 AM »

Mention of the ART-13 transformer being subbed in made me think  - humorously, that if transmitters looked like this, one of those big blue levers would hot-switch from the BC transformer for fidelity to the ART-13 transformer for punching through the QRM.
(The red levers?  NEVER PULL THE BIG RED LEVERS!)


Dennis W5FRS uses one of those ART-13 units to mod a 250TH running just about 2KV and it sounds great. Think of it as high-level analog signal processing. Another transmitter uses a BC-610 transformer. All that stuff works great.

There was a transformer article by Menno Van Der Veen a while back that stated generally that if the audio transformer's power rating is doubled, the low frequency limit is divided by SQRT(2). Another reason to use a large size unit.


* 20071111-1171-e1465872445709.jpg (136.32 KB, 1000x769 - viewed 21 times.)
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« Reply #43 on: January 01, 2019, 11:53:00 AM »

John -

You were looking for an actual negative feedback circuit for the modulators. I didn't realize it, but this MOSFET audio driver includes the working NFB circuit I currently use in my 4X1 rig. It will give you a good idea what is needed for your rig.  
http://amfone.net/Amforum/index.php?topic=23632.0



BTW, the way you're scaling that rig up - and you're just in the schematic design phase, why not bite the bullet and  build a pair of 813s X 813s for a 500 watt carrier and 140% modulation?  (3KW+ pep)  Phil has the perfect mod iron for you.  A single bigger plate transformer would be easy to find. (2500-3KV DC is robust for 813s) You know how hobbies go -   a year from now you'll be thinking bigger and wished you had... :-)

Can you hear it whispering in your ear?
https://www.amwindow.org/tech/htm/813/813.htm


The JJ promise and money back guarantee:  Wink

Build this rig using 813s X 813s, 2800VDC, with BC mod iron, the GFZ MOSFET audio board with NFB, low level audio processing/ limiting, proper voice EQing -   and you will be in the top 1% of the cleanest AM plate modulated rigs on the air.   I did just that and have the actual IMD spectrum and triangle audio wave tests to prove it...  This combination works like magic when set up right.  The 813 modulators in triode config with the GFZ audio driver board  and NFB are a "perfection anomaly."

Clean test audio 20 Hz > 12 KHz =  transparent audio =  Great sounding audio on the air

T
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« Reply #44 on: January 01, 2019, 12:26:20 PM »

Then I will have to replace the S-49 HV trans, build a bigger bias supply, buy different tubes, etc.

Where does it end???

John
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« Reply #45 on: January 01, 2019, 12:28:41 PM »

Commit to build it as described above and I will send you four NOS 813s and four NEW white ceramic sockets for FREE.  (free shipping)

T
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« Reply #46 on: January 01, 2019, 12:33:27 PM »



J,



Its a Don Corleone offer.....  how can you refuse?

KLC
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« Reply #47 on: January 01, 2019, 02:58:30 PM »

Is there a board or gerbers avail for the mosfet driver?
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« Reply #48 on: January 01, 2019, 03:38:18 PM »

Hi John,

The MOSFET driver board thread had talk about making circuit boards but it never happened as far as I know.  I did the only prototype as pictured below.
I think Frank/GFZ has built one for his Valiant too.  There is a PCB layout by Rich/W7SOE shown below. It probably works. Needs to be checked and tested. He mentioned traces need more spacing.

But point-to-point wiring like I did is an easy build and is well worth the effort. Mine worked right away. Been in the rig for 8 years, no problems. All parts from DigiKey, etc. Cheap $3 MOSFETS.  This driver will drive most ANY modulator, big or small effortlessly, so is good for a lifetime.  1 volt in, mucho watts out, no transformers, clean as a whistle.

Now ask around for a HV plate transformer good for 500 mA and capable of generating 2800 VDC and you're dancing.


***  I'll even sweeten the pie and throw in FOUR brand new white ceramic 813 plate caps with silver plated inside cups for FREE.


If you're game, I'll give you up to 60 days (or sooner, your choice) to show some solid progress on the project as described -  and then all parts are yours, OM. (4) 813s, (4) sockets, (4) plate caps

T




* GFZ MOSFET AUDIO BOARD.jpg (311.5 KB, 1280x960 - viewed 25 times.)

* GFZ audio_driver PCB.gif (34.57 KB, 485x822 - viewed 14 times.)
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« Reply #49 on: January 01, 2019, 07:52:42 PM »

Under serious consideration
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