UTC Mod Transformer Connections

[KD1SH]

  Much of this seems pretty much self-explanatory. In the "Primary" column, for example, P-B-B-P would imply that the modulator plates connect to the "P" terminals, while the modulator B+ goes to the combined "B" terminals. Likewise for the "Secondary" column: you'd join the terminals indicated, and connect the B+ supply to one of the remaining two terminals and the PA tube's plate choke to the other.
   But what does the "line to RF" section mean? I'm probably overlooking something embarrassingly simple, but it's escaping me. It refers to connecting 500 ohms, apparently, between two terminals, or is it to those two terminals tied together? It doesn't mention modulator plate to plate load impedance at all, and there's nothing about modulator plates. What am I missing here?

[K8DI]

Back in days gone by, audio systems often delivered signal on a 500 ohm balanced line.  This row is matching a 500 ohm impedance to varying rf section (finals) loads. Audio amps could be bought /built/equipped with output transformers with low (4-8-16 ohm) and 500 ohm taps. So if you already had a PA system amplifier with a 500 ohm line output, you could run that into the mod transformer without modifying the amplifier, which would keep it suitable for feeding speakers when you weren?t on the air?

Ed

[KD1SH]

  Excellent! Thanks, Ed; that makes more sense to me now. After reading your explanation, I found a brief description of the 500 ohm balanced line approach in some old Thordarson documentation. I'd never heard of that before.
  I've found that a stumbling-block to visualizing these "multi-match" or "vari-match" or "universal" mod transformers is not only the lack of representative diagrams and schematics from the manufacturers, but a lack of application examples as well. I sometimes come across old modulator plans using them, but they most often don't show any terminal numbers. I have found a few of the old Thordarson Transmitter Guide books that do a nice job, though. The UTC documentation I've found (like that clipped piece I attached) gives connection tables but no diagrams.
  Thanks again. Sometimes I figure I can get rid of all the hammers in my shop, and just bang nails in with my thick head.

Back in days gone by, audio systems often delivered signal on a 500 ohm balanced line.  This row is matching a 500 ohm impedance to varying rf section (finals) loads. Audio amps could be bought /built/equipped with output transformers with low (4-8-16 ohm) and 500 ohm taps. So if you already had a PA system amplifier with a 500 ohm line output, you could run that into the mod transformer without modifying the amplifier, which would keep it suitable for feeding speakers when you weren?t on the air?

Ed

[KL7OF]

Tim, Wa1 henry yellar  told me once how to hook up a "multimatch" Mod xfmr to produce super modulation.. I don't know the specifics any more , but it had to do with using multiple sections of the primary side in series (or something like that) to get maximun positive peaks..  Ask Tim for specifics..  OR...TIM, if you read this maybe you could re-enlighten us....I still have a CVM-5 on the shelf for a project coming up....

[KD1SH]

I have a CVM-5, too, and I'd love to read about some 'Tron multi-match magic!

Tim, Wa1 henry yellar  told me once how to hook up a "multimatch" Mod xfmr to produce super modulation.. I don't know the specifics any more , but it had to do with using multiple sections of the primary side in series (or something like that) to get maximun positive peaks..  Ask Tim for specifics..  OR...TIM, if you read this maybe you could re-enlighten us....I still have a CVM-5 on the shelf for a project coming up....

[n5ama]

While we are on the subject of mod transformers, I remember having a pair of Thordarson transformers. One is the T11M78  C.H.T series rated 300-500w and the other is the matching driver T-15D76. Both are NOS with the driver transformer in its original box.

They were purchased for a never to be built modulator and purchased with a pair of Aerovox 88mfd/1200vdc, a pair of Aerovox 2400mfd/450vdc and a single G.E. 16mfd/4500vdc. They are all non-PCB oil electrolytics.

If there is any interest in any of this stuff, I have pics that I?ll be happy to email. I?m in the Houston Texas area.
Thanks,
Tom N5AMA

[w8khk]

Tom, that Thordarson iron is some good stuff!  I am running the exact same driver and modulation transformer in my legal-limit rig, and it has been working well for a long, long time!

My grandfather, W8YNG, bought a home-crafted transmitter in a 6 foot relay rack cabinet in the late 30s, I think around 1937.  It ran legal limit, 1000 watts DC input, to a push-pull pair of Eimac 250-TH triodes, modulated by a pair of RCA 810 triodes, using the iron you have listed above.  He operated it until 1958, when he became a silent key.  My dad, W2DU, moved it to NJ, and operated it until around 1970, when he disabled the modulator, and used the 250-TH deck as a linear amplifier for single sideband.

I restored the transmitter to its original configuration sometime around 2010, adding fuses, circuit breakers, overload relays, and such, just to protect the original iron.  It is now my main transmitter, running in its original configuration, with the final amplifier and modulator configured exactly as they were in 1937.....  The restoration is documented here:
http://amfone.net/Amforum/index.php?topic=38055.0

That iron just keeps plugging away, reliable, and sounding great.  Quite an amazing feat to be used by three generations of hams, over an 88 year period!  Don't try that with a plastic radio!

You are probably aware that the "CHT series" is their high-end iron that sports Chester H. Thordarson's "autograph", a superlative endorsement indeed.  The iron is "potted", so it should not be subject to moisture issues during years of storage.  Interconnection data to configure taps for the desired modulation transformer impedance ratio is readily available.

 I hope you are successful in finding a good home for that iron, where it may grace the HF airwaves with Angel Music.

[n5ama]

Wow Rick, that?s one great story and only wish we still had the skill and knowledge that your ham family had in the past.

I?ve got mostly hollow state gear such as the Hallicrafters HT-9, the BC-610E and more recent T-368 but also have a homebrew transmitter that came out in a series of 60s vintage QST articles for a multi band 500w transmitter modulated by a pair of 811s. The builder constructed that transmitter to the exact article description to the last detail. A true work of art that motivated me to drive from Houston to Michigan to bring it home. I?m still in awe of the design and his work.

The Thordarson iron has always impressed me and I?ve bought and used it in a bunch of projects. When I purchased this set many years ago, I thought myself fortunate to run across it at a fest and planned to use it in a future project that never materialized.

I hope to run across someone that recognizes the quality of this iron and buys it to build a great modulator. If I can?t find a buyer, I?ll be forced to build one just because it needs to be done.LOL  I do have the the 810 tubes and sockets for a modulator but don?t have the rf deck needing that much horsepower. I?d probably go with some 828s or 100-THs.

Thanks for your reply!!
Tom N5AMA

[w8khk]

Tom,

I would personally encourage you to hang on to that iron, and the capacitors.  Consider an RF deck with a pair of 813s, plentiful and inexpensive, easy to implement a pair in either push-pull or parallel configuration, plenty of headroom.

No need to use all the power available from a pair of 810s in the modulator - - run it conservatively, with lots of headroom, and it will last forever.  1500 to 1800 volts on the plates will give you all the modulation you desire, safe and sane.

Rather than settle for the artwork someone else built, there is nothing greater than the satisfaction of running a rig that you designed and built on your own, bar none!  No need for another power supply, leverage one in an existing rig, just by turning off the filaments in the idle transmitter.   

The modulator could not be simpler, and you have all the key components to make it happen.  If you have the time and inclination, put that dream rig together.   

Down the road, marketing a working rig instead of some precious iron will be much more practical, and then a younger amateur will have the opportunity to enjoy the pleasant sound of natural AM audio.  Please give it some serious thought before you divest those treasures!

Best regards,
Rick

[n5ama]

Good morning Rick,
Sounds like great advise. I?ve got plenty of 813s and all the necessary hardware to assemble a 813 or just about any other rf deck using 40s vintage vacuum tubes, just missing the ?time? component needed to do the project.

Actually, I have already built two rf decks, one using horizontal mounted 813s on a 6? relay panel. The only downside is side there is the tubes aren?t visible through a window (which isn?t a major problem with carbon plated tubes if I elected to use 813s.

I have another rf deck I built using a single 833. It is nice with vacuum variables and a large inductor. I modified a toroid transformer for the filament. The only issue there is the 833 not capable of doing much above 80 meters. That is my preferred band but it would be nice have it work up to 20m. I?ve got the space on the chassis to replace the 833 socket with another socket and redo the filament transformer for the new final. It?s just that the 833 looks so cool all lit up.LOL

Food for thought. Thanks
Tom N5AMA

[n5ama]

Thought I'd add a few pics of my 833 rig if I can figure out how to attach them.
Tom N5AMA

It seems I can upload only one pic at a time.

[Pete, WA2CWA]

It seems I can upload only one pic at a time.

Not true.
You can add three attachments.
First picture below is your screen just before you clicked the SAVE field.
On the left side, see ATTACHED which indicates one file is attached.
Look below that and see ATTACH and to the right of that, the link to click "more attachments"

NOTE: The third picture shows what three attachments look like before you click the save button. The third picture I reference is labeled on the screen shot att_bigbutton_phone because that was the name of the picture before I substituted the one you see.

[n5ama]

Thanks Pete. I?ll give it another try from my desktop. When I tried to add an attachment this morning, it would replace the first one I selected with the second. I was obviously doing something wrong.
Tom

[w8khk]

Tom, that is some neat fabrication work on the 833A deck.  Nice touch with the trim around the viewing window. 

With all the tubes, sockets, and other parts you have, it would be a shame to let that mod iron sit idle or depart company.  A pair of 813s, or anything else unique, would be a fine addition to your AM station.  When you build, post the progress and final result for all of us to see.

Best regards,
Rick   W8KHK

[n5ama]

Another attempt at pics OF MY 833 AMP..

Pete, It didn't work this time either. My reply has a "choose file" box which takes me to the files. I click on the file I want to download and it selects it and places it to the right of the Choose file box. When I go to (more attachments) and click on it, it takes me back to my files. I select the next image to download and it replaces the first downloaded file. It doesn't add the next file to what has already been selected. It replaces it.

I'm fairly familiar with the process of adding images to email. I expected that selecting (more attachments) would add to what I have already uploaded. It just blinks red and replaces the existing file.

What am I doing wrong? 

Tom N5AMA

[Pete, WA2CWA]

Are you using a smartphone or a typical computer?
What is your browser?
This web site, I believe, has not been tailored to be 100% compatible with all the various smartphones that are in the world. Fonts can be screwed up and I guess it's possible that some Fields and Actions may not be recognizable by actions issued by a smartphone.

I saved your pictures to my desktop and then attached them to this post.
The third picture was at least 2 Meg in size.

See my next post for additional info.

[Pete, WA2CWA]

(See the attached below) This is what the bottom part of your post should look like when you attach three pictures. 3 is the max per post.

After you post the first picture the Browse field button will redraw itself to the next line with the "(more attachment)" to the right. You click on this Browse field to attach your second picture. After you do that, the Browse field will redraw itself to the next (and final line) with the "(more attachment)" to the right. You click on this Browse field to attach your third picture.

NOTE: If you click on the first or initial Browse Field again after you pull a picture into the post and want to post a second picture, it will overwrite the first picture in the post.

[n5ama]

Another try.

Pete, that is exactly what I have been doing all along. After attaching the first file, I click on "more attachments" and it doesn't do anything. I have tried both left and right mouse buttons. I'm on a desktop Windows 11 home computer, not a cell phone or Ipad.
Tom

[n5ama]

I guess I could do this one at a time since I only have two more images I want to send.

[n5ama]

One more.

[KD1SH]

  That is really odd; I'm using Windows 11 myself, with Firefox, and I've never had that problem.
  Otherwise, that's some impressive workmanship with the 833 rig. I've got an 813 rig, modulated by 810's, in the works myself, and I hope it turns out as nice as yours. It's a slow process since I've got multiple projects going on at the same time, and I'm afflicted with chronic project-hopping disorder.
   Another thought regarding modulating your 833 rig: check out the March and April 2009 issues of Electric Radio (the April issue corrected some errors in the March issue). There's an article in those issues by Dan Krupp, W8NWF, where Mr. Krupp describes a method of series-cathode modulating an 833 RF deck. It's essentially the same approach as described in the older "West Coast" handbooks (page 297 in the 15th edition, for one). The efficiency will be a bit lower than with plate-mod, but Krupp says he gets 300 watts output with his, and he claims the audio is great.
   It's just a thought, another possible route you might be interested in, but not in any way intended to supplant or distract you from Rick's suggestion; he's absolutely right, you've got some awesome mod iron there, and it would be a shame to waste it. The biggest problem with plate-mod is finding the iron, but you've already got that, so you're halfway there!

Another try.

Pete, that is exactly what I have been doing all along. After attaching the first file, I click on "more attachments" and it doesn't do anything. I have tried both left and right mouse buttons. I'm on a desktop Windows 11 home computer, not a cell phone or Ipad.
Tom

[n5ama]

Thanks Pete,
I don?t recall the ER article but since I?ve got a full set of them, you can bet I?ll be looking up Dan?s article.

I struggled with using the 833 or something like the 304TL but even though I had sockets for both, the 833 socket is a bit harder to come by so I used the 833. I know that?s a strange reason to use a tube but even thought the 304TL has a little frequency advantage, I don?t operate above 40m on AM.

The reason I used the 833 is about as strange as the story about how a found the socket. Believe it or not, a guy assembled a single triode receiver using the 833 and I don?t think it turned out very well. He brought it to a fest and I saw it under a table (yeah, I?m a fest bottom feeder) and asked him what the hell it was. He replied and I bought it on the spot. It was mounted on a 8?x8? chassis and had some other usable parts on it but I was really interested in the socket. There should be a million of them out there from the broadcast world, but I had never seen one for sale.

Anyway, I?ll probably wind up building a modulator but I?m 78 and it won?t be long before I?m unable to lift and move this old iron. I?d better get with the program soon.LOL

Tom N5AMA

P.S. never had problems adding pic files to emails, QRZ ads, forum's etc. I have no idea what I?m doing wrong.

[Pete, WA2CWA]

Thanks Pete,
I don?t recall the ER article but since I?ve got a full set of them, you can bet I?ll be looking up Dan?s article.

Anyway, I?ll probably wind up building a modulator but I?m 78 and it won?t be long before I?m unable to lift and move this old iron. I?d better get with the program soon.LOL

Tom N5AMA
P.S. never had problems adding pic files to emails, QRZ ads, forum's etc. I have no idea what I?m doing wrong.

As you can see from your own post, when you type an apostrophe that you enter, it appears in your post here as a question mark.

I have win11 installed on a new desktop that I bought in December 2024, but it's still sitting in the box. Haven't had time to play with it.