The AM Forum

Ive been talking to a couple of companies that have the ability and experience. I was thinking 250, 500, and 1000W of audio capability. What other requirements such as insulation voltage (trying to eliminate a mod reactor) impedance taps, etc?

This would be part of the ongoing buildup of the old RadioKit.

Carl

Phase shift over the frequency range is a big issue for setting up feedback.

What product did that?  Any links?

I don't know of any but if I was setting up a spec that is a consideration that effects the amount of feedback you can apply around a modulator. The driver I did for Tom has around 20 degrees phase shift through the frequency range but Tom had to roll off the feedback around 7 KHz if I remember. We suspected that was due to the phase shift through the iron since the feedback source was the secondary.
I'm not sure but leakage inductance could also be a consideration for high frequency response.

To design a good mod iron it more or less has to be approached like a hi-fi transformer.

That means interleaves and attention to capacitance and leakage inductance... it gets harder and harder as the power goes up. There is/was an article about (iirc) and RCA high power modulator iron's design around... I may have taken a copy of it. They showed what they had to do with the winding layout to make it fly...

                    _-_-bear

Id appreciate a copy if you can find it.

From the slow response on here I guess AMers dont want to spend real money to be heard ;D

are you surprised?

Loy Barton, who really started the change to class B modulators in the 1930s, wrote some good stuff on this. His first publication is in his Univ of Arkansas thesis where he built one of the first modulation transformers for high level AM. I posted this on the AM Window link, under the technical section. It gives a lot of interesting details into what to look for, from a rather primitive starting point.  Loy was hired by RCA and helped move them over from grid and Heising modulation to P-P class B modulators in their rigs. He also wrote a very good paper in QST about high level class B modulation at the time.

The requirements are similar to audio amplifier transformers with the additional need for DC bias on the secondary (cancels flux on the primary being center tapped). Or none, when used with audio reactor and coupling cap. There have been some good papers and references on the web on audio transformers, google "audio transformer design".

Some universal mod xfmrs have 6 terminals on each end -  giving a wide range of ratios depending on which sections are used.

I got a 600W Stancor A-3899 rated 2000-20,000 Ohms pri. or sec. and rated 500mA DC on the primary and secondary and rated for 3KVDC supplies. I've used it several times in experiments and the multi-match taps are great. It also two windings on the primary and two on the secondary.

Those are my only suggestions. I know better than to comment on transformer design except to say the more versatile it is the better it will sell.

Good info John, I'll look at that and Ive read the QST article several times, I have 1926 to now complete.

So lots of impedance options of course how high should I go?

For say 3KV on both windings what level of insulation?  Might as well push those 304TL's, 250T, 4-400, etc to the limit. Might get real expensive at the 4-1000 level. Id like to eliminate the need for a reactor. I guess Don has some experience with shorting transformers ;)

My experience has been about 5 years with simple plate and filament transformers up to 4CX-5000 size as well as 3 phase for the shoot out crowd. Unfortunately the company owner dropped dead at the bar after work :'( and the company dissolved, I couldnt come up with the money to purchase plus hire a manager and engineer.

Tertiary winding for a NFB tap

There's a company in NJ that makes toroid power and audio output xformers; they're quite inexpensive for what they are and I believe they'll do custom work as well.  No reason they couldn't do mod xformers as well.

If not you might just want to have the vendor copy 1 each of the popular mod xfomers at a particular power level - ie, Ranger, Apache, Valiant, Glob Queen 400, KW-1/Desk KW (also good for the GPT-750).  That way you get business for replacements as well as new construction.  Guys doing new projects have a blank canvas, they can design around the mod tranny availability.

I would like to hear from Gary Brown and Tom Hand about this ....

The problem about replacements is they are mostly custom with special brackets, end bells, cans, wattage, etc that really jumps the prices. Let Gary rewind those. I'll stick to the builder end.

Ive emailed that NJ outfit twice about a year ago and heard nothing. I suspect they dont want to play with low volume and HV insulation.

Low volume transformers cost a lot. We used to wind 6 at a time back in the PS days.

I have been unable to locate the article I recall, probably maybe could have been Loy Barton... dunno, can try to look on an older machine/drive but I have no way to get the article off that machine at present... but I could cite it if I can find it.

IF you know the winding pattern you want, there are winders who will do the job... some will put it on the core, otherwise you might be able to do that. PM or email me if you wish to discuss offline?

                             _-_-bear