Under The Old Dome

[RoyC]

Here are a couple photos of a dissected, early style AR dome tweeter.

With the exception of the very fragile aluminum lead wires, they were very well built. (Tom Tyson has posted very interesting information about this in the past.)

The fiberglass under the dome was quite compressed, and literally "poofed out" upon lifting the dome (NO deteriorating foam as in the later 3/4" black dome tweeter). The suspension, which is similar to the later tweeter, appears to have been very carefully manufactured, and was still compliant.

A strong case can be made that the early alnico magnet drivers, with no foam used in tweeter or woofer construction, were AR's most consistent and durable products. Given that they produced the sound upon which AR's reputation was founded, and were mounted in vault-like cabinets finished in beautiful thick veneer, the pre-1970 era could very well represent AR's "finest" hour.

Roy

http://www.classicspeakerpages.net/dc/user_files/1398.jpg

http://www.classicspeakerpages.net/dc/user_files/1399.jpg

[dynaco_dan]

Hi Roy;

As always, an excellent commentary and great photos.

Was this a natural or man made eruption?

Was it a DOA?

Can it be put back together again and function correctly?

Thank you again, Roy.

[tysontom]

>Here are a couple photos of a dissected, early style AR dome

>tweeter.

>

>With the exception of the very fragile aluminum lead wires,

>they were very well built. (Tom Tyson has posted very

>interesting information about this in the past.)

>

>The fiberglass under the dome was quite compressed, and

>literally "poofed out" upon lifting the dome (NO

>deteriorating foam as in the later 3/4" black dome

>tweeter). The suspension, which is similar to the later

>tweeter, appears to have been very carefully manufactured, and

>was still compliant.

>

>A strong case can be made that the early alnico magnet

>drivers, with no foam used in tweeter or woofer construction,

>were AR's most consistent and durable products. Given that

>they produced the sound upon which AR's reputation was

>founded, and were mounted in vault-like cabinets finished in

>beautiful thick veneer, the pre-1970 era could very well

>represent AR's "finest" hour.

>

>Roy

This is a nice report by Roy showing the great images of the dissected AR-3 1-3/8-inch “super tweeter.” That tweeter was one of AR's greatest accomplishments, and it represented a quantum-leap forward in tweeter design when it was introduced in October of 1958. The voice coil was aluminum and there is *no* voice-coil former or bobbin. The light-weight, free-floating coil was bonded directly onto the treated-phenolic-plastic dome itself, with fiberglass compressed underneath as a cavity-damping element. As Roy noted, the fiberglass is under considerable compression, and it actually caused many of the domes to “pop” out of their suspension, causing failure of that tweeter. This was a bigger problem with the earliest 2-inch domes, and the fiberglass in front of the dome (under the screen) was put there primarily to counteract the fiberglass under the dome. Another form of failure with the dome was the tensile breakage of the lead-in wire itself, actually part of the voice-coil winding. The brittle aluminum wire, if not looped exactly right, could break at the point where it entered the dome itself. Thermal “burn out” was probably a less-common form of failure, but all forms of failure in the AR-3 were actually less than the failure-rate of the shipping cartons themselves.

The suspension system of this dome tweeter was the novel urethane-foam that was poured into slots on the top plate. This foam was then coated, as well as the gap around the top of the voice coil, with butyl rubber to form an air-tight seal around the dome edge. With the big (3 lb.) magnet assembly and the lightweight dome, this carefully designed and tested tweeter was capable of electrostatic-like transient response and extremely flat on- and off-axis performance. It was about as near “perfect” as the state-of-the-art permitted, and to this day it is still considered an outstanding tweeter.

--Tom Tyson

[dynaco_dan]

Hi Tom;

The icing on the cake, so to speak.

You just seem to always come in with some very vital information to add to the historical information which is always so interesting to read.

Thank you very much to both, Tom and Roy for this very interesting topic which is a part of AR history.

Again, without both of you, this would be un-documented.

Thank you.

[RoyC]

>The brittle aluminum wire, if not looped exactly

>right, could break at the point where it entered the dome

>itself.

That was the cause of death in this case...note only one aluminum wire in the photo. The other is on my work bench. I believe I am the guilty party.

Vern, it would have taken a neuro-surgeon to repair it, but thanks for the kind words.

>The suspension system of this dome tweeter was the novel

>urethane-foam that was poured into slots on the top plate.

So much for my "no foam in the construction" comment:-). At least the urethane foam held up better than the foam later used under the dome of the 3/4" tweeter, and that used for woofer surrounds. I guess the butyl rubber coating ultimately succeeded in preserving the integrity of the suspension.

Great info, Tom...

Roy