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Art-13 questions in a 8xx modulated by 811's




 
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Author Topic: Art-13 questions in a 8xx modulated by 811's  (Read 2603 times)
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KA3EKH
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« Reply #50 on: April 03, 2018, 08:51:36 AM »

Shipping may be a issue but this is a good source of parts.

https://www.ebay.com/itm/E-F-Johnson-Viking-II-transmitter-chasis-PARTS-ONLY/232716440668?hash=item362efb145c:g:M88AAOSwWyRaovoO

Not a big Viking 2 guy so donít know if those are the plate transformers or modulation transformers.
Agree with you about amplifiers, in order to get the linearity you need from an amplifier it has to run in class A or something similar and the efficiency sucks. No substitute for good old plat modulated AM, just donít over think it. Math is nice but you would be surprised how far you can get with just bodging things together.


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KA2DZT
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« Reply #51 on: April 03, 2018, 09:05:55 AM »

Any idea what the current and voltage is??  Maybe I'll check eham try to see what it is.

Fred
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w9jsw
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« Reply #52 on: April 03, 2018, 09:10:00 AM »

he just said this in a follow up note...

901414-501....grv 80310.  I remember looking it up but I canít find it either...used the supply to run a pair of 813ís to 400 watts....somewhere around 300 mils...it didnít even sweat

said it weights around 20 lbs.

This is looking promising. Maybe I should go directly to a pair of 813's? will the modulator have enough kick to keep up?

update - said it would cost around 30-40 bucks to ship. it is 3200V CT and he us only using 1/2 of it at 1600V.

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KA2DZT
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« Reply #53 on: April 03, 2018, 09:48:48 AM »

John

I looked on eham but couldn't find the xfmr.  I saw from the pic that he was using only half of the secondary.  A 20lb xfmr is not a big plate xfmr.

Better to use the whole secondary with a FW rectifier.  With a choke input filter the voltage will come down.  You can use a variac to bring the voltage down more or just run it at 1400-1500 DC volt output.

It may be a good one to get,  depends on what he wants for it.  It won't run a pair of 813s modded by a pair.  Probably will be ok for single 813 by 811as

Depends on what he wants for it.

Fred
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KD6VXI
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« Reply #54 on: April 03, 2018, 09:58:10 AM »

Get ahold of Gary w1zm at TRS.

I've bought a couple things from him over the years.  All exceeded expectations.

He was selling off a huge pile of old stock transformers a while back.  Might have JUST what you need.

--Shane
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w9jsw
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« Reply #55 on: April 03, 2018, 10:41:08 AM »

Fred,

$100 bucks for all that is pictured, plus 30-40 for shipping. I think I will hold off for now. Got 2 hamfests in the next 2 weeks and who knows, I may get lucky.

John
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KA2DZT
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« Reply #56 on: April 03, 2018, 11:17:20 AM »

Fred,

$100 bucks for all that is pictured, plus 30-40 for shipping. I think I will hold off for now. Got 2 hamfests in the next 2 weeks and who knows, I may get lucky.

John

Good idea,  I would wait for the hamfests.  At a $100 it's not a bargain.  Should be able to do a lot better for much less at a hamfest.
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KC2ZFA
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« Reply #57 on: April 03, 2018, 11:55:38 AM »

one of these will provide 1600VAC up to 0.5A. Use two, one for the modulator and one for a 400W output PA.

http://www.antekinc.com/as-8t800-800va-transformer/
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w9jsw
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« Reply #58 on: April 03, 2018, 04:21:56 PM »

Very interesting!
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w9jsw
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« Reply #59 on: April 04, 2018, 10:20:21 AM »

While I await a few hamfests and the hopeful discovery of the perfect transformer on this Easter egg hunt...Can someone teach me about balance between the modulator and the RF tube?

As I read the datasheets, the power of the RF tube is variable based on what I determine the HV to be. So, let's say that I use that spiffy 1600V transformer and get a solid 1450V of power for the RF deck. The data sheet for the 813 lists data for 1250v and 1600v for CCS of a Plate modulated RF amp. That is great. 1450 splits it down the middle. That leads me to believe that the Rf deck will produce 140W of carrier at this voltage. A pair would go 280W. (BTW, a pair is looking more interesting)

Now, looking at the 811, remembering that I have a marginal mod transformer, at 1450V we are in ICAS territory. And the 2 tube modulator will be producing over 330W of audio. I think I see smoke coming from the modulator! I think the 811's should be designed for CCS operation, right?

I read somewhere that I should design for the modulator to give me 50% of the RF wattage. No more than 100W, and 75W may be better. Is that correct? I am confused... This goes against my understanding when I tune my SDR that can deliver 50W that for AM mode I should set the carrier at 12.5W. That means 75% of the power goes to the sidebands and 25% goes to the carrier. Would that mean that I need to be delivering 400 watts of audio? Huh?

I have this other transformer that can give me 750 volts of power at 200ma. I could use it as a separate supply for the mod deck and get 175 watts out of the modulator. That still seems like too much for the modulation transformer, but that is another discussion.

So, how much audio for carriers of 140W and 280W? using this 1450V example.

Second - we had discussed using a single power supply of around 1100-1200V. Reading the data sheets that shows 120W of carrier and 300W of audio. Is that a good balance?

Help!
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KA2DZT
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« Reply #60 on: April 04, 2018, 10:48:06 AM »

John,

You're over thinking everything.  At 1450VDC you'll run the RF and modulators off the same supply.  The 811s will need about (-)4 volts of bias (a few diodes in the 811 filament xfmr CT to ground).  811s might produce more audio than you need.  So, you turn down the audio control to produce no more than 100% modulation.  You always need more audio power than you think.  There are losses that you need to overcome.

Your ART-13 mod transformer was designed for a single 813 and 811s.  While the ART-13 ran on about 1150 volts from the two dyna-motors it used the mod xfmr can handle much higher plate voltages.

Fred
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w9jsw
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« Reply #61 on: April 04, 2018, 11:39:50 AM »

Fred,

What transformer specs should I be looking for? Say 1300V ct to 1600v ct with .5A? Can I do with a bit less current, say .4A or is .5A a solid minimum current spec?

Also, I am going to drive the AF with a audio chain from the small transmitter I am building. After I get it running I will split it and use the RF side for exciting the big transmitter and the audio chain for driving the modulator. Right now the final stage is a single 6V6 running at 300V. It can put out 5 watts. I think I need to redo this stage and make it push-pull. That will give me 14 watts. I will then use a matching transformer to match it to the modulator.

Will 14 watts be enough drive if we are running the mod at 1450V?

John
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KC2ZFA
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« Reply #62 on: April 04, 2018, 11:49:51 AM »

What transformer specs should I be looking for? Say 1300V ct to 1600v ct with .5A? Can I do with a bit less current, say .4A or is .5A a solid minimum current spec?

you need a 2600 vct to 3200 vct (or a 1300 v to 1600 v no ct) transformer to do what you want.
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KA2DZT
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« Reply #63 on: April 04, 2018, 01:55:43 PM »

John

Current should be at least 300ma,  400-500ma is better.  I'm driving my 811a modulators with a single 6L6 through a 25watt mod xfmr.  Class B modulators (811a) will draw grid current so you need a step-down driver xfmr.  Hardest part to find is the right driver xfmr.

Plate xfmr should be just what KC2ZFA said.  A xfmr that is only 1300vct to 1600vct will only give you half the voltage you need.  A xfmr like that can be run with a FW bridge rectifier but the current capacity will be half of whatever it's rated for.  In addition not all xfmrs can be run with a FW bridge rectifier.  Some have to have the CT grounded.  Depends on the xfmr.

Another thing is you need to know how to understand the ratings and wording you may see on a xfmr or choke.  Things like RMS TEST voltage, means the  high voltage used to test the unit.  Say you see 5KV RMS TEST voltage.  This dosen't mean the unit is good for 5KV. General rule is you subtract 1KV and then divide by two.  So a choke with a 5KV RMS TEST voltage is good for 2KV.  You may see a voltage rating that says "working".  Say you see a rating that says 2500v working.  That means you can put up to 2500 volts on that unit.  Other times you may see an insulation rating.  No idea what exactly is meant by that wording, but generally you stay way below that insulation rating.

You may see these type ratings on chokes and on xfmr windings.  Most all the standard catalog xfmrs and chokes from the major manufacturers have this info in their catalogs.  Military and special xfmrs and chokes may have more info printed on the unit.  A lot of units you might see may be specials even though you see a familiar manufacturer's name on it.  Usually xfmrs and chokes with 5 digit part numbers are specials and you're not going to find any info about it in any catalog.

Fred
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w9jsw
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« Reply #64 on: April 04, 2018, 02:34:29 PM »

Fred,

What windings are you using on the driver trans. I take the plate voltage divided by the plate current for a 6L6 to get the plate impedance of 3.5K. Is that correct? How do I calculate the input impedance of the mod pair. I don't see input data for the 2 tube circuit. The 813 circuit that K1JJ published takes a 2K winding, as I read in a note.

Thanks for the trans info. That helps.

John
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KA2DZT
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« Reply #65 on: April 04, 2018, 04:48:30 PM »

I had a 25watt Merit mod xfmr that has a single PP primary and a secondary that has taps.  Using different taps reflected different impedances back to the PP modulators.  That is how the xfmr is suppose to be used.

I used it in reverse, the pp side I used to drive the 811 grids and used the other side as the plate load for the 6L6.  You need a step-down going to the grids.  Impedances for class B driver xfmrs are calculated from the primary to 1/2 the secondary.  I used all of the tap side for the plate load except for the last end.  So the plate was connected to the tap next to the end of the winding.  I used the end for feed back to the 12AU7 driving the 6L6.  The 6L6 runs with 300 volts on the plate and the screen is tied to the plate through a 100ohm resistor.

I tested this circuit by loading down the grid lines with 130ohm resistors, one on each grid line to ground.  Don't know what the grid impedance is, so I just guesstimated 130ohms. 

I used a audio signal generator and a scope to see how much audio voltage i could get across the 130ohm loads before it started to distort.  The undistorted audio voltage was way above the audio signal needed to drive the 811s to full output.  That grid audio voltage is shown in the tube specs.

When you build from scratch some of the details have to worked out as you go along.

It's nice to do a lot of calculating and figuring but, non of that is going to do you any good if you don't have the right parts to build what you calculated.

You have to build and make work a xmtr with the parts you have or are able to find.

Fred.
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KA2DZT
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« Reply #66 on: April 06, 2018, 10:26:26 PM »

John

I checked my notes on the driver xfmr I used in my HB xmtr.  I used a Merit (A-3109) 25watt mod xfmr.  I have 5000ohms on the plate of the single 6L6 driver and 3000ohms CT on the grids of the 811a's.  I used the 10k end of the primary winding for feedback to the cathode of the 12AU7 that drives the 6L6.

It's the turns ratio of the driver xfmr that is important.  The actual impedances don't mean anything as the load that the 811 grids reflects back to the primary is not known and is not a constant load.  The actual load impedance varies with the amount of grid voltage.

To figure the impedance ratio, 5000ohms/1500ohms = 3.34,  the turns ratio is the sqrt of the impedance ratio.  Square root of 3.34 = 1.83

So the turns ratio is 1.83,  turns ratio of Class B driver xfmrs is the primary to 1/2 the secondary.

2:1 turns ratio is about right for 811a's

This will give you some idea what's needed for a driver xfmr.

Fred
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w9jsw
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« Reply #67 on: April 07, 2018, 08:11:52 AM »

That's what I had calculated ... hihi ... just kidding  Roll Eyes  Actually I got 3500 on the 6L6 and 2000 on the grids, which after doing the math give a turns ratio of 1.87.

Got a schematic for that circuit? I wrote out a design for my 6L6 audio amp and would like to compare them. I don't have a feedback line. I got my 6L6 circuit from an audio amp and joined it up to my mike preamp/amp. The article described a grid tap of 43% on the audio transformer for ultra-linear mode. I don't know what that means.

Off to the ham fest this morning. Taking my grandson with me. He is a bit young (9mo old) but we will have some fun anyway. At this age he is a bit of a YXL magnet ... hihi

John
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WU2D
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CW is just a narrower version of AM


« Reply #68 on: April 07, 2018, 11:47:40 AM »

An ART-13 can be made to sound as good as a stock commercial ham transmitter (Apache, Ranger etc...)at the 120 Watt out level, with no modifications other than in the speech module, and by adding some feedback. The output transformer is not the limiting part in terms of fidelity, it is more related to the input transformer and the anemic and restricted driver circuit. So get a spare speech module and start playing. There is a dandy article in 73 that talks about running 500 Watts input power with an ART-13 in CW mode. The limiting factor to AM output power are the 811's and the mod transformer, not the 813.


* ART13Audio_New_WU2D.jpg (390.5 KB, 2274x1706 - viewed 37 times.)
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w9jsw
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« Reply #69 on: April 07, 2018, 06:57:13 PM »

Went off the reservation. No transformers other than a Dahl for $350. Ouch. Did buy a Drake L4B wo PS for $200. I still have a PS to build...
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KA2DZT
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« Reply #70 on: April 07, 2018, 07:59:52 PM »

Maybe the next fest you'll find a good xfmr.  When you're at the next fest tell the sellers you're looking for xfmr.  Lot of times the guys don't want to drag out heavy xfmrs but may have something they willing to sell.
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KD6VXI
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« Reply #71 on: April 07, 2018, 09:02:30 PM »

There is a Denton clipperton xformer in qrz for 75 bucks right now I believe.

I offered 50, he balked.

--Shane
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KB2WIG
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« Reply #72 on: April 07, 2018, 09:51:53 PM »


Look under the tables and under the cardboard boxes. You never know what lurks there.
 

klc
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w9jsw
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« Reply #73 on: April 08, 2018, 05:51:03 AM »

There is a Denton clipperton xformer in qrz for 75 bucks right now I believe.

I offered 50, he balked.

--Shane
KD6VXI

I saw that Shane. You think that would make a dandy replacement/substitute? I think it has the right voltages for the hi/low voltage doubler circuit but could not determine if it has the same current ratings. I was going to add up the current demands of the 4 572B tubes and see how they compare to the pair of 3-500's in the Drake. I would use a heathkitshop L4 board also.

Interested in your thoughts...

I am also investigating doing the grounded grid and zero bias mod. The bias mod would eliminate the requirement for the 120V line.

John
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w9jsw
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« Reply #74 on: April 08, 2018, 06:01:18 AM »

Maybe the next fest you'll find a good xfmr.  When you're at the next fest tell the sellers you're looking for xfmr.  Lot of times the guys don't want to drag out heavy xfmrs but may have something they willing to sell.

This was a real small fest. Probably a dozen tables. I asked around and only one guy brought any transformers. That was the Dahl guy. If anyone wants that level of iron, he had the goods. He had that monster 3500v transformer, a matching choke and a 6000v oil filled cap. Makings of a maul for sure. He wanted good money, though. $350 for the transformer, $150 for the cap and I did not ask about the choke. I can pass on his name if anyone is interested. Was way out of my league in cost and scope.

I did dig into every cardboard box I could find. And struck up quite a few conversations with the old iron guys, but no firm leads. Will go to another fest this next weekend. Half of the fun is the hunt.

John
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