Heil AMT Prototypes? Anyone ever seen anything like these units? - diyAudio
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Old 18th March 2008, 12:37 AM   #1
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Default Heil AMT Prototypes? Anyone ever seen anything like these units?

I recently picked these up from a local craigslist ad. Paid more than I should have for them, considering the diaphragms are pretty much destroyed. But I couldn't pass them by because I've never seen an AMT that looks anything like these, (and I've seen many AMTs, considering they've been my favorite toy for three decades!)

The man I purchased these from had little information. He'd got them in a bulk purchase of speaker parts. He previously had never seen an AMT, and was suprised to find out they were so unusual. The man he purchased them from told him they were early prototypes built by Dr. Oskar Heil. Also, they had served for years as part of the sound system that toured with the Greatful Dead. (Who knows if there is any truth in this, but it does make for a nice story.)

Each unit is aprox. 4"Hx 8"Wx4"D, and weighs aprox. 20-25lbs. The massive "U" magnets appear to be Alnico. The U mags attach to very deep V-shaped focus plates that form the gap. The Focus plates also serve to hold the diaphragm in place, with a little help from the good ol' blue ESS putty-goop (Though in these, someone had used gobs of some horrid black gunk that had long since turned hard as a rock....Maybe ESS didn't have the blu goop yet).

The diaphragms that were in these, the poor things, we're almost certainly early ESS....They were/are totally undamped (no little fish net glued on), have the blue and purple lead wires and the original ESS production circuit pattern. However there are subtle differences that I have never seen before. Still, I'd bet that they are one of the early AMT 1 diaphragms, which changed design several times, mid-run, as the good people at ESS worked out the bugs. They also are obviously not the diaphragms originally intended for these units, as they are 3/4" taller than the magnetic gap.This unfortunate detail has led to their destruction... Seems poor handling devistated the exposed areas, even ripping off one of the lead wires.

Oddly enough, one of the diaphragms still worked, at least at the time when the man selling them to me insisted on showing that one did indeed work. (He hooked it straight to his amp, no crossover) It sounded quite good, even in it's tragic state....As a matter of fact, it sounded amazing.

When I got the diaphragms out, I was suprised to see that there was none of the "twisty-wear" you'd expect to see on a well used (or even moderately used) undamped diaphragm. This is odd. Especially if you consider they were suspposedly used in pro-sound reinforcement for many years! I suspect the gap flux density may be unusually high ( given that Alnico, oz. per oz, is around 3x stronger than ceramic magnets), but I have no device to measure the magnetic flux at the moment.. If the gap strength is greater than other AMTs, this extra Bh may have something to do with the lack of wear on the diaphragms. Perhaps it even explains the almost hypnotic sound I briefly experienced during the sellers percarious demo. That, or it could just be the golden sonic signature of Alnico V. (Could also have been no-crossover thing).

For now, they sit silent, as I wait for the universe to lead me to diaphragms at a reasonable price. I could canibalize a couple of my other Heils, but I don't want to risk it as diaphragms are so very hard to come by these days, at any price. (Thanks a lot Audio Int'l! To think I was originally excited to hear you'd aquired ESS.). I just might be able to repair the diaphragms that came with them....I've managed to bring back ones that were nearly as badly damaged. (Hmmm...How sadly desperate that sounds...Oh well, I was lucky enough to grow up down the street from ESS, so they've been a wonderful part of my life almost as far back as I can remember. Now some German kid will get to have the same great experience I did...Provided he's filthy ritch!)

Meantime I'm wondering just what I have here...An early AMT made by or for another brand? A custom order for a rather ecentric rock band? A pro-sound product? A DIY Project? Or a real honest-to-goodness Heil AMT prototype, actually touched by Dr. Heil himself! (Oohh...the goose bumps!). Never mind if Jerry Garcia had his mits on them.

So...If anyone has ever seen these, knows anything about them, or can just venture a good guess as to their origins, I'd love to hear from you.
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Old 18th March 2008, 04:00 AM   #2
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Hi Matt28sr - you might try these guys for replacement ESS AMT diaphragms: http://www.simplyspeakers.com/12diaphragms.htm (Look about 2/3 down the page.) They are listing two types and one is a bit shorter than the other. You could email Nelson Pass at Pass Labs as he worked with the good Doctor H while at ESS. You can also email him her at diyAudio as he often visits and offers kind words of wisdom and advice. (Look him up on the Pass Labs Forum) If those are early prototypes of an Heil AMT I would think that he might be interested just for S&G's

As to this device being used by the "Dead" - John Curl - who also wonders into these forums - might know. I went to quite a few Dead concerts and my buddy and I were both very much into the tech that went into the "Wall of Sound" (we worked club PA's) and I don't recall seeing anything like this device sitting around. Take a look at http://dozin.com/wallofsound/ for a diagram of the stage and PA layout.
To me they look like someone's attempt to DIY a set of AMT's - but heck - you never know.
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Old 18th March 2008, 04:16 PM   #3
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Are they AMTs or just Ribbons?

Big **** horseshoe magnets there!

How do they open up??
Is that WELDING I see on them??

Clearly homebrew... imho.

http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
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Old 19th March 2008, 08:50 AM   #4
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Hey c2cthomas and bear..... Thanks for the input!....The contact information is especially great....I'll follow those leads and report back what, if anything, comes of it.

To answer bears questions: They are pleated diaphragm AMTs, not flat ribbons. They don't open up (I suspose you could pull the magnets from the focus plates, but it'd be a hell-of-an ordeal to do so). The diaphragms simply slide into place like all ESS Heils. (One of the magnets was appearanltly removed, or worse, knocked out of place at some time, and it is out of exact alignment with the focus plate. You can see this in the close up photo. I'm researching on how best to correct this.)

The stuff you see that looks like welding is the "horrid black gunk" that I refered to before. Actually, once I got all that off, the fit and finish is a little better than early production ESS AMT's, which had chipped magnet edges, so-so focus plate machining quality and all kinds of DIY looking variations of ways they tried to hold the diaphragm firmly in place....(As time went on they did get a better fit and finish, but, once you get them apart, the F&F was never all that percision. I once read, that because of the way they worked, super high manufacturing tolerances were unnecessary as the electrical signal across the opposing pleats of the diaphragm forced it into perfect alignment.)

Anyway I'm inclined to agree with your DIY conclusion. But there are a couple of clues that still make me wonder:

It's a little different than any DIY design I've ever seen. Specifically, the "slide in diaphragm space", and the way it holds the diaphragm into place, or actually dosen't hold the diaphragm into place. You'd think that anyone technically sophisticated enough to design and make something like this would go to the trouble of including a system to hold the diaphragm. The only way this exclusion makes much sense is if you were considering the ease and economics of mass production, or that you weren't quite sure what the diaphragm was ultimatly going to look like.

Another oddity is the focus plates themselves. At their narrowest point, they are more than 1" deep, not at all an ideal (or even reasonable) way to let the sound radiate from the diaphragm. From looking at the AMT 1, it's obvious that Dr. Heil went to extrordinary trouble to open up the radiating space for the diaphragm. If a DIYer was using an AMT 1 as a design example, this is quite a deviation from it. Again, the only way this design makes much sense is if the designer was just trying to get the damn thing to work right in the first place.

Finally consider the use of Alnico, by the mid/late 60's it was fast becoming economically impractical for use in mass production loudspeakers. But back up just a bit on the timeline and it was the magnetic material of choice for audio. That might just place it around the time Dr. Heil was most likely first tinkering with the AMT idea.

I once saw a late prototype AMT built by Dr. Heil. Though it looked very much like a taller version of the first production ESS AMTs, it was very home-made looking. He used huge nuts and bolts of different lengths to hold it all together, and didn't bother to trim the excess that stuck out 2 or 3 inches above the unit. Compared to that, these AMT's look pretty sophisticated.

So we have a bit of a mystery here, and I do love a good mystery. Let's see if Mr. Pass can shine some light on the matter. Or anyone else. I plan to attempt to contact Mr. Pass in the next couple of days. Stay tuned!

Again, thanks so much for your input guys.

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Old 22nd March 2008, 09:28 PM   #5
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OK, Just saw this thread and did see identical magnet
assembly on ebay a few months ago for prototype Heils
that match your pics.
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Old 1st April 2008, 09:19 PM   #6
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Default Heil

Looks an awful lot like an old Litton magnatron.
The folks at Heil, would have likely know the folks at Litton's Grass Valley Group.
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Old 17th September 2009, 07:15 AM   #7
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Those came from the ESS Blue Ox. I also have a pair I took out from them. Here's what they look like.Click the image to open in full size.
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Old 17th September 2009, 11:34 AM   #8
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Originally Posted by alxpen View Post
Those came from the ESS Blue Ox. I also have a pair I took out from them. Here's what they look like.Click the image to open in full size.
A (long!) time ago Audio Amateur featured an article on how to build your own AMT's including a diy diaphagm. If you want to look at that I can look it up, I have it in my files (question only is where ).

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Old 17th September 2009, 05:47 PM   #9
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Hi janneman

I think we are a lot here that would love to read that article.
If it's that old I think you can just post it here. Otherwise a mail would be super
Looking farward to that.
Thanks in advance and best regards Baldin
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Old 18th September 2009, 02:57 PM   #10
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Originally Posted by Baldin View Post
Hi janneman

I think we are a lot here that would love to read that article.
If it's that old I think you can just post it here.
I just tried calling AudioXpress but no answer. I wrote that article many years, but I don't recall what sort of of agreement I signed, if any, to transfer the copyright. Actually, I don't recall signing any agreement at all, but that was about 33 years ago . I'll try calling again to see if I am allowed to distribute the article on my own. If someone already knows what the agreements were back then, please tell me.

I've got a pdf scan of the article, although it is a bit hard to read because of the amount of bleed-through from the opposing pages. The article appeared in the 2/77 issue. There was a follow-up letter in the 4/79 issue that described how to etch the diaphragm, and this method is MUCH easier, especially if you apply more modern etch resist methods.
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