450 lb. field coil

[Jan Daugaard]

I have just read this article about Rudy Bozak:
http://en.wikipedia.org/wiki/Rudy_Bozak#Loudspeakers

At the 1939 New York World's Fair, a tower topped with a cluster of eight 27" Cinaudagraph loudspeakers in 30" frames with huge 450 lb. field coil magnets covered low frequency duties for a 2-way PA system at Flushing Meadows. The loudspeakers were mounted into horns with 14' wide mouths and were each driven by a 500 watt amplifier derived from a high-power radio broadcast tube. In June, 1940, Electronics magazine published an article that Bozak had written about the design of the 27" loudspeaker.

Does a member of this forum have the June 1940 issue of Electronics? It would be intriguing to read a detailed description of these woofers.

[Ducatista47]

The loudspeakers were mounted into horns with 14' wide mouths and were each driven by a 500 watt amplifier derived from a high-power radio broadcast tube.

The amplifier grabbed my interest, too. Steve Schell says those who have gone that way still prefer large directly heated transmitting triodes over receiving tubes. I would like to see something about the amps, but it sounds like the article is only about the speakers.

For those to whom this is all Greek, all the audio tubes we usually encounter are receiving tubes - for receiving a signal, as in a radio, or for amplifying that signal for a speaker. Look up the old term detector. (Remember the RCA Receiving Tube Manual?) Transmitting tubes were found in radio stations, not in radios and amplifiers. The T-1610 manufactured by KR is either a new model transmitting tube or something in between.

Clark

[Steve Schell]

Rudy Bozak is one of the largely unsung heroes of high fidelity sound, every bit as deserving of study and praise IMO as Jim Lansing. In my informal study of his work some surprising facts have come to light. He was perhaps the first to incorporate edge wound voice coils in production cone loudspeakers, many of these being edge wound aluminum... in the 1930s! I have a pair of Cinaudagraph 18" drivers, designed by Rudy, that have 3.5" edge wound aluminum voice coils and a claimed response to 10kHz. The cones are in tatters but the highs from the voice coils are indeed very good. He also pioneered the tapering of cones in thickness and density to optimize smooth wide band performance. His Cinaudagraph and later Bozak cones were thin and hard near the apex, graduating to thick and soft at the perimeter. This encouraged good high frequency performance from the cone center and discouraged reflections from the outer rim back to the voice coil.

That 27" wide range cone driver with the 450lb. field coil magnet was the result of Rudy's uncompromising pursuits. It reminds me of some of the mind roasting stuff produced by Bell Labs during the same period. I would recommend reading the Wikipedia entry referenced by Jan as well as the article by Peter Ledermann linked to in the references section. Peter indicates that the 27" driver at Bozak was destroyed, though I understand an example of this driver is on display in the Klipsch museum in Hope, Arkansas. Here are scans of the article written by Mr. Bozak for Electronics magazine:

[Titanium Dome]

Look at the size of that MF. (monster fixture, of course ) Holy crap!

[Ducatista47]

I am enjoying the nomenclature that was not chosen by history. "Centering net" for spider, much more descriptive, and "rim felts" for waffle surround or the more generic compliance.

I also notice that 12 inch is not large after all for a full range cone.

Clark

[louped garouv]

too cool!

[Doc Mark]

Hi, Steve,

I remember that some of my older "how to build speaker enclosure" books offer quite a few Bozak designs, and that they were considered very, very good, back then. I seem to remember that some of those cabinets were quite large, and had multiple woofers. As I was a "JBL man", even back then, I very probably didn't give Mr. Bozak's designs the proper respect that they deserve. But, I thank you, very much, for shining the light on the man and his wonderful accomplishments! He was, truly, a pioneer in his field, just as was James B. Lansing. Thanks, again, and God Bless!

Every Good Wish,
Doc

[iain42]

I thought that sounded familiar.

They have one of these at the Klipsch factory museum in Hope, AR. It is a monster. I tried to gently lift the back of it LOL....... It was slightly damaged in shipping.

[Jan Daugaard]

Steve: Thank you very much for the reproduction of the article in the June 1940 issue of Electronics.

Iain42: Thanks for the picture. It's sad to see that the (only?) extant woofer is in such poor shape.

[iain42]

Steve: Thank you very much for the reproduction of the article in the June 1940 issue of Electronics.

Iain42: Thanks for the picture. It's sad to see that the (only?) extant woofer is in such poor shape.

Yeah that is how it was delivered unfortunately. It caused quite a stir in the lab when it was delivered or so I heard.

[jcrobso]

The amplifier grabbed my interest, too. Steve Schell says those who have gone that way still prefer large directly heated transmitting triodes over receiving tubes. I would like to see something about the amps, but it sounds like the article is only about the speakers.

For those to whom this is all Greek, all the audio tubes we usually encounter are receiving tubes - for receiving a signal, as in a radio, or for amplifying that signal for a speaker. Look up the old term detector. (Remember the RCA Receiving Tube Manual?) Transmitting tubes were found in radio stations, not in radios and amplifiers. The T-1610 manufactured by KR is either a new model transmitting tube or something in between.

Clark

The tubes in my transmitters are directly heated pentodes and are rated at 30kw, the plate voltage is rated at 10KV. I wonder what kind of audio amp I could make with them? The down side is that they cost $6000 each. John

[Ducatista47]

That 10kv plate rating is typical for the type. In home audio use, they would be loafing along and easy to keep in the linear portion of the curve.

I know these amps only by reputation, but Steve is, I believe, aware of some of the current users (no pun intended). He posted these thoughtful replies to one of my ignorant inquires.

Clark, those large transmitting tubes with their white hot filaments do look grossly inappropriate for the job at hand, but I have heard some of the most sweet, detailed, tactile, believable sound from them. I'm no theorist, but I have read many times that the simple triode is the most intrinsically linear amplifying device known to man. So far I have not seen any transistor guys rushing in to argue the point. Apparently, for whatever reason, the directly heated tubes have the lowest measured distortion of the breed, and this extends further to the very high current filament jobs.
I have found with receiving tubes that the high current, lower mu DHTs tend to sound more clear and lively. The 26 was the first voltage amplifier designed to run either AC or DC on the filament, and with its high current filament it does sound better to me than either the earlier starved filament battery set voltage ampflifiers or the later indirectly heated cathode tubes. Why exactly? I dunno. There does seem to be a correlation between high filament current and dynamic sound, but I'm not the one to explain why.

Clark, maybe part of the reason those big transmitting tubes sound good is that they are being run at a tiny fraction of their capabilities for hi fi, like the high efficiency speakers most of us like. They have such long, relatively straight plate curves that they can be set up with low distortion loadlines at many different voltages. I heard two different pairs of single ended 750TL amps at RMAF last weekend, and they both sounded wonderful. Though the 750TL is rated at 10,000 maximum plate volts, these amps had only about 550 and 1,200 volts on the plate.

Found here: http://audioheritage.org/vbulletin/s...ld+coil&page=3

I can personally vouch for the benefits of increased heater current. I have a little EL84 amp that sounded phenomenal. When I rolled a Soviet era tube in that draws much more filament current, the sound quality and power advanced to the nirvana class. Unfortunately, most amps do not have the iron to handle higher currents than what the standard tubes are designed for. In this case, the designer did and recommended the tube to me. Go Alan Kimmel!

Clark