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Author Topic: exciting transmitter project  (Read 14444 times)
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NR5P
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« on: October 29, 2014, 11:57:37 AM »

I've been wanting a cool home brew transmitter for some time. I found a free craigslist find to start with, much better than what I had. I already checked the transformer and took the power supply apart to clean it up. Of course making schematic as I go. There's a few changes and additions I will make as I go to make it better. It has a 6kv center tapped transformer unmarked that is 53lbs. Choke input so I guess about 2500vdc. Rf deck had a 4-250 family tube. Already setup with forced air cooling pressurized.  No modulator yet and I need to find a mod transformer. With a transformer that size I'm thinking a200 watt carrier and enough left to run the modulator as well? I do have a couple chokes that would be great for modified heising to keep DC off the transformer. What wattage rating could I get away with on the modulation iron? That's the hard thing to find so I'll be building based on what I can get I guess. I'll post a few more pics.


* IMG_20141027_194507274.jpg (2164.66 KB, 2432x4320 - viewed 550 times.)
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NR5P
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« Reply #1 on: October 29, 2014, 12:01:49 PM »

A couple more pictures of HV section. I'll have to take some of the rf section


* IMG_20141027_204242847.jpg (2734.59 KB, 2432x4320 - viewed 518 times.)

* IMG_20141027_204242847.jpg (2734.59 KB, 2432x4320 - viewed 497 times.)
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NR5P
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« Reply #2 on: October 29, 2014, 12:20:12 PM »

A size comparison of transformer


* IMG_20141027_194620730.jpg (2208.47 KB, 2432x4320 - viewed 533 times.)
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N2DTS
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« Reply #3 on: October 29, 2014, 01:45:05 PM »

Looks like a nice start.

A separate power supply for the modulator works much better then using the same power supply for both.
But, you have to work with what you have sometimes.
2500 volts is a hard voltage to work with for modulators, push pull or push pull parallel 100TH's could work at 2500 volts, or 4-125/250/400's would if run as tetrodes, but you have to regulate the screens.
811a's want 1200 to 1500 volts, 4x150/4cx250b's will do 2000 volts, and I am not sure what other cheap easy to get modulator tubes are around these days.
813's can be run triode connected or as tetrodes at 2000 volts.

I would try and tap that psu down and fit an 813 or a pair, and run them at 2000 volts at 300 watts carrier for one, 600 watts for 2.
Ditch the noisy blowers and go silent convection cooling.
813 modulated by a pair gets my vote.

Good mod iron is hard to find.
I did not like the cvm-5, but the old Thordarson mod iron seemed to be very good, and the old RCA iron is very good.

I like AB1 modulators with a LOT of headroom, but low voltage stuff is easy to do, 1200 to 1500 volts works with all kinds of common tubes in various combo's, 811a. 572b, 4x150/4cx250b, 4D32, 812a's, 813, etc.
 

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NR5P
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« Reply #4 on: October 29, 2014, 02:00:20 PM »

Yeah. If I can't find a decent transformer I'll just build a linear out of it. And just build one of those low power cathode modulated transmitters. Then I can use it did sideband as well.
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N2DTS
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« Reply #5 on: October 29, 2014, 03:10:32 PM »

There are some modern screen modulator circuits around that work REAL well.
No mod iron and some very good fidelity can be had.

I have worked a number of guys running some sort of screen mod that sounded fantastic.
There is a solid state circuit that is around here someplace that works with 4-400's and is very easy to adjust and to get clean audio out of.

I should order the parts and try it myself.

A single 4-250 is not much of an amp.
A pair of 4-400's is not much of an amp for AM...
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NR5P
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« Reply #6 on: October 29, 2014, 05:16:41 PM »

N2DTS let me know if you have a link to the screen modulation circuit.  I'm open to different options.  I finally have a little shop to work in and I'm not really looking for huge power.  I just want a homebrew that I can say I built myself with decent power.  It's nice to finally have a good chassis to use and some of the work started.  I have started a few other projects but never could get really excited about them, so I've got some other parts around, such as other HV transformers(just not as big).  I'm not even stuck on a certain tube. 
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N2DTS
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« Reply #7 on: October 29, 2014, 07:01:26 PM »

Info here:
http://amfone.net/Amforum/index.php?topic=35375.25
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« Reply #8 on: October 29, 2014, 09:04:24 PM »

That transformer looks a good 2.5KVA.

A pair of 3-500's makes a nice modulator on 2500VDC. I use a pair of those Chinese ones and run a 300-40W carrier full modulation as easily on 2500V as 3000V. Just saying don't overlook some of the Triodes. caveat - best to drive 3-500's with a cheap cathode follower like a 6080 tube, or with transistors - most driver iron won't match those low-Z grids too well -they seem to be in the 500-2000 Ohm range.

I guess that with the RF tetrodes in class C running 2500V, you ought have a good AM signal if a plate modulator is used. Space issue/ trade up to a 6 FT rack. Nice thing about 4-250's.. 4-400's fit the same socket and filament power.. so you can use either.
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NR5P
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« Reply #9 on: October 29, 2014, 10:44:06 PM »

Got the power supply completely taken down and started cleaning up the transformer and choke.  This thing only had 3 uf of capacitance after the choke.  I've got 2 large 6uf oil filled caps that maybe will squeeze in.  One of them has a leak on the top seal but I think it's still full as it puddles a little at the top even when standing straight up.  I ran the fan and it's really not that loud at all with the muffling of being pressured in the cabinet.  I just took off the cover to make a schematic of the rf section.  Not sure what I'm going to be doing with it yet but we shall see.  If I get a mod transformer I was just going to build the modulator in a different chassis, right next to the other one.  I'd still like to use the same Plate supply for both to make it simple, also to have that current draw from the rf amp for voltage regulation. 
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N2DTS
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« Reply #10 on: October 30, 2014, 10:26:32 AM »

Hope the leaking cap has no PCB's in it!

Look at the plate tuning cap and see if the voltage rating is good for AM.
Needs to be at least 6000 volts I think, at 2500 plate volts.
Also, the grid bias, how is it done?
It had one or two 4-250's in it?
How is the screen voltage got?

Screen modulation allows lower voltage components, you get less power out, no mod iron and can use small/marginal plate caps...plus no iron in the audio path is possible.


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fg5fc
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« Reply #11 on: October 30, 2014, 11:05:13 AM »

Hello a good design from KB3AHE

* SCREEN MODULATED.pdf (37.78 KB - downloaded 255 times.)
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W3GMS
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« Reply #12 on: October 30, 2014, 11:46:51 AM »

Hello a good design from KB3AHE

Frank used a good scheme with modulating the screen grid and the control grid thru the tertiary winding of the ART-13 xmfr.  It definitely stood tall among the best of the rigs. 

ART-13 xmfr's can be found pretty easily. 

Joe, W3GMS
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WD5JKO
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« Reply #13 on: October 30, 2014, 12:02:40 PM »


  That design of Frank's sure looks interesting. The modulation transformer listed in the attachment is an ARC-5 mod transformer. Do these have a similar tertiary winding like the ART-13 mod transformer did?

Jim
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w8khk
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« Reply #14 on: October 30, 2014, 12:54:21 PM »


 The modulation transformer listed in the attachment is an ARC-5 mod transformer. Do these have a similar tertiary winding like the ART-13 mod transformer did?

Jim
Wd5JKO

According to this document:  http://tonnesoftware.com/Manuals/ARC5manual.pdf
on page 27 the schematic for the ARC-5 modulator shows a center-tapped primary, two secondaries to modulate the plates and screens, and a third secondary for sidetone monitoring in "tone" MCW operation.

This winding could be used for feedback in an amateur modulator.
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Rick / W8KHK  ex WB2HKX, WB4GNR
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My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
NR5P
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« Reply #15 on: October 30, 2014, 09:02:25 PM »

N2DTS.  They do have pcbs I'm sure.  I have always heard as long as I don't eat it or touch it it's ok.  I always wear throw away latex gloves when handling them....

Plate cap is fine.  The spacing is .25"

grid voltage is oa2 voltage regulator

screen voltage is fixed....I want to change this to variable.

It had a 4-250 family tube.  It has no tube right now.  It used to have 2.  One of the holes has a plate bolted over it but I can tell it has the holes for another socket. 

Grid input tuning is a parallel circuit and has 5 position switch with coil selection for 5 bands.  I've got a very very terribly drawn schematic so I may do some searching to see if maybe this transmitter is a copy from a book.

I wish I knew more about the builder who had it at one time.  Unfortunately it is one of those things where a guy bought it off a friend who's dad built it and this guy had it for 30 years he said in his garage.  He wasn't ever very active and from talking to him I don't think even has a license anymore.  Of course he had no idea who the ham was who had it and wasn't in contact with the friend anymore. 

I'm going to post some more pictures of the inside of it.  I'm pretty sure I'm going to try to work with what I have.  Add adjustable screen voltage and grid and modify it so I can adjust for linear use grounded cathode for qrp input.  Then some day when I find a mod transformer it won't be a big thing to use for both.   I do realize it wouldn't be a very high power AM linear.  I'm ok with that though. 

If anyone can give me an idea of when this might have been built by looking at the components let me know.  pictures to follow in a few minutes
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NR5P
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« Reply #16 on: October 30, 2014, 09:36:53 PM »

Well I'll have to upload tomorrow. tomorro my satellite internet interne is over on bandwidth again and it's taking forever to upload
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NR5P
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« Reply #17 on: October 30, 2014, 09:40:11 PM »

I'm thinking of something like this. Another tube and adjustable grid and screenscreen voltage.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http%3A%2F%2Fham.homelinux.org%2Fschemi-pdf%2F59hb210.pdf&ei=MOlSVPTaNcyxogT6joLoCw&usg=AFQjCNGJMs3UsP1CbVDJT_6OYYRMBTZWgA&sig2=UXmo0EgsI5AEqhvUdBolmQ
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NR5P
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« Reply #18 on: October 31, 2014, 07:04:12 AM »

Inside rf deck


* IMG_20141030_200244309.jpg (3516.37 KB, 2432x4320 - viewed 570 times.)
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NR5P
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« Reply #19 on: October 31, 2014, 07:05:22 AM »

.


* IMG_20141030_200310664.jpg (2770.04 KB, 2432x4320 - viewed 562 times.)
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NR5P
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« Reply #20 on: October 31, 2014, 07:16:37 AM »

What would be a conservative estimate for the type of filament transformer I have there by weight?
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N2DTS
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« Reply #21 on: October 31, 2014, 12:29:10 PM »

Looks to have a vfo in it!
That was unusual back in the day.

Can not tell what transformer is what, the loading cap looks small, and that roller inductor may not be good for two 4-250/4-400 AM type power.

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NR5P
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« Reply #22 on: October 31, 2014, 12:40:23 PM »

It's just a parallel tuned input. All those coils are link coupled to the input.
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N2DTS
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« Reply #23 on: October 31, 2014, 10:33:19 PM »

Oh, it looks like a reduction drive going to a little cap and coil like a vfo would have.
Seems like a lot of stuff for just a grid drive setup...
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WBear2GCR
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« Reply #24 on: November 02, 2014, 08:13:28 AM »

A few thoughts.

A lot depends on how deep your "war chest" is for this project.
Also, how patient you are willing to be.

If ur on a "shoestring" then patience is going to be needed in order to get the "right stuff" to come your way.

----

...your question on the rating of the filament xfmr?
No need to guess.
A resistive load placed on the secondary is what you need to make this determination. When you get to 10% of unloaded voltage dropped by some resistor value, that is your rating (or when it gets to the target secondary voltage with the primary voltage that it is spec'd for - usually not the >120vac we have today). Do ohms law and that's the current rating.

Same thing with the HV iron.

So, just go over to that box of 100w wire wound resistors, or that giant tapped one, and have at it? So, if you don't have a box of big wire wound resistors, then you've got a mission for the next hamfest.

I think that homebrew building to a given specific target set of parts or BOM is very difficult unless ur going to spend $$ (like online) to get exactly what you want when you want it.

---------------

Assuming there is enough space 4-400s fit where 4-250s went (different chimney, hole in chassis - if there is a hole). Fairly similar results given that you'd expect a big differernce between them


---------------

It might be possible to modulate the thing with a reverse transformer and a big arse solid state power amp - it's been done. It's fast and pretty simple. One way to go while you find parts and build a modulator.


-----------

I'd scout around old handbooks and articles to find the exemplar for this build. Probably it was in an old magazine. That would help if it was. You'd know all sorts of things about the design and the specs.


------

You probably are now interested in a somewhat taller rack, to fit the modulator and its power supply. Racks are pretty easy to find, and get thrown out quite a lot.

--------


No matter what, if ur going to work in HV, you'll need assorted HV gear and parts. Starting with a HV probe for some meter... Shorting stick. Other stuff.

HIGH VOLTAGE IS DEADLY

Knowing this and knowing how to work around it are two different things.
In case you or anyone reading needs this reminder?

-------

Personally, I'd not change out anything at this point.
Not the filter caps.
Nothing.
Fwiw, the PS could be "tuned" in which case bigger caps will possibly give worse performance.
First steps for me would be to try to find the article that this was based on; failing that trace the schematic, clean the unit and the chassis. Then post the schematic, and determine what to keep, what to change, what the options are, and what is needed.

Possibly test everything and fire up the stock unit and see if it works and what it does.
Certainly test the power supply and spec it.


                    _-_-


PS. just got the image of the top deck to open - guessing it looks like a 6m single band amplifier, guessing by the coil and the size of the caps. So best revise ur plans if this is correct, and I am correct n my guess. In which case you've got a PS and some parts for an RF deck that may or may not be really useful. Btw, that does not look like an "air system" socket.
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