The AM Forum

I've got a UTC VM-4 that, according to the UTC catalog, is "designed to pass the class C plate current", and no mention of a matching reactor is made. I'm assuming  that means a reactor isn't required.
I've also got a Thordarson T-21M65, and though the catalog doesn't specifically say that it's designed to be used without a reactor, it does specify plate current, and again, no reactor is mentioned.
I'm thinking that in both cases, operation without a reactor was the designer's intent. Still, I'd rather use one, for the safety and longevity of the old mod-iron. Next time I see a unicorn passing by, I'll ask him if he's seen any big mod-reactors around.
I've got those two transformers, a couple of brandy-new 4-250's, a B&W band-switching tank coil, some huge wide-spaced bread slicers, and a gonzo power supply, but no mod-reactor. All dressed with nowhere to go.

http://www.amwindow.org/tech/htm/modtran/modtran.htm (http://www.amwindow.org/tech/htm/modtran/modtran.htm)

Above is some data on some of the mod transformers that are in use and there are a few that I have used that are listed herein.

Just about all mod transformers that I have looked at in diagrams or in use in articles I have read that are in commercial and amateur designs that are rated for 500-600 watts of output audio do not require a reactor. The line where reactors generally come into use is in designs that are at 1000 RF watts output and beyond.

I seen one 5000 watts modulation transformer in person that didn't require a reactor and it was huge. It was about 4 1/2 feet long and about 24 inches square in the core. It probably weighed 2000 pounds. Can you imagine one built for a 50,000 watts rig or larger? I believe that going with heising was a way to improve efficiency and cut down on the size of the Magnitic's.

You can use common filter reactors in series to get you up to 20-30 Henries. You just need to put them up on some insulation materials to keep them above chassis ground, as most wont have 6-8 KV breakdown insulation in them like you may find in a reactor designed for the application.  There is some math out there for finding the perfect value reactor to match a given mod transformer so the magnitic's will be matched but for ham radio, just put together what you have and go for it!

Lastly, I have used transformers with and without a reactor and both configurations worked fine. There is one caveat for using a reactor where you can use double and maybe triple, the amount of audio you can run through the mod transformer as long as you don't exceed the current ratings of the xformer.  

After closing the rewind shop after 25 years. I did keep some rare stuff and one is a 100 henry reactor I did for a customer and was never paid. It is about 2 feet long and 14" square. Cast Iron endbells and was wound with 10kv insulation. I have no need for this and willing to let it go. Let the bidding wars begin.
Gary

Hammond 193Q: 10 Henry, 500 ma. Rated at 1000 V but, as you say, isolating them should deal with that. But at $125 a pop, stringing them together gets pricey.

Well who needs new stuff? Why not put something in the Wanted section of this site and see what you can find. There must be someone out there that will just give you what you need? Its all old stuff and there's lots of it out there and we can't take it to our graves. What are your requirements in MA. I know I have some reactors around or I may be able to find some for you? You would most likely need to pay shipping for them but otherwise should be cheap to free.


BTW, what is your name?  Me, I'm Terry

Hi, Terry,
  Name here is Bill. Yeah, by far most of my stuff is bought at hamfests, and I avoid new if I can. I suppose I'm just suffering - like most of us - from hamfest deprivation.  When things get back to normal I'm sure I'll find plenty of stuff at Nearfest or Boxboro or Sussex or whatever. Really, this isn't even a work-in-progress at the moment; I'll be starting the RF deck before the modulator, anyway, and I've got several other projects going. Both mod-transformers I picked up at Nearfest for well under $100, but never even thought of looking for a reactor until recently, after all the 'festers went down the drain.
 My plan, actually, is to make the transmitter "dual mode"; switchable between plate modulation and screen modulation, hence the 2X 4-250's, which would otherwise be overkill for plate modulation, and I don't ever expect to have a mod-transformer capable of modulating two maxed out 4-250's.

WOW, I would like to see a picture of that reactor. That 5 KW mod transformer that I was talking about is up at WBCQ in the BC transmitter room not to be confused with the SW stuff. Here is a pic of that transformer which is not hooked up and is burned out. The gray one behind it is now in service and is hooked to an old 5 KW RCA BC transmitter.

Sounds like a fun project Bill! I have some tubes that look exactly like a 4-250 but they are high mu triodes. I thought about putting them in my BC-610D in place of the 100TH's tubes but if I do that, it will have to wait till next spring as my ham shack is seasonal up in the Adirondacks. I have access to the place but I can't stand the cold and using the wood stove to stay warm. It keeps the place warm but it's my old age creeping up on me that is the problem.

Gary,

Slightly off topic - did the fellow who "acquired" your rewinding biz surface? Sink or swim?
I've not seen anything...

                       _-_-

Bear:
After around a week or two, after buying my business,  he had a major heart attack and I have never heard from him since.
If I was 15 years younger, I would start up the business again.
Regards,
Gary

Since it was brought up, I'll add a little info here and be done. I have no further info than this.
A fellow at work has everything from the other guy you mention. I have talked with him on several occasions and he assures that he'll be ready to start rewinding as soon as he gets a new tensioner setup which he says is quite costly.

Two pieces of hardwood, two long screws, two springs to fit over the screws, two washers and two wing nuts is what I used as a tensioner. That must come to a total of five bucks. Worked great for 25 plus years.
Regards,
Gary

Any news on this?


--Shane
WP2ASS / ex KD6VXI

Hi Shane,
                   Weird I had thought to myself that I needed to check in for something. I'm sorry to say that I think it's probably not going to happen. The fellow had a promotion at work to supervisor, so I'm pretty sure that boat has sailed. It's a shame that it all will just set wasting away. If I were younger, I might would give it a go.

73, Phil