Vented dust caps

[Guest dogmeninreno]

Was there ever a verdict as to AR using "vented" dust caps or were they all sealed? I will also direct this ? to the refoaming guys OK? Dale

[Carlspeak]

>Was there ever a verdict as to AR using "vented" dust caps or

>were they all sealed? I will also direct this ? to the

>refoaming guys OK? Dale

Dale:

Dust cap materials varied with different models. Most of the woofers I've refoamed which don't have vented pole piesces have a porous dust cap so the air can circulate thru the cap back into the voice coil area to cool it. Many with vented polepieces have nonporous dust caps. It's been my experience the vintage AR woofers did not have vented pole pieces.

Carl

[rrcrain]

Oddly, the woofers in my AR90s have a double dust cover. The outer cover is vented, the inner is some form of plastic.

[Rlowe]

I have also noticed that some AR 10" speakers have 2 dust caps. The outer cap was pourous , the inner one, under it was like compressed pizza cardboard.

The attached renovation picture also shows the remnants of 2 dust cap circles. The inner circle (off centred) used that shiney black glue leading to the speaker wires.

What would be the purpose of providing a non-porous dust cap along with an outer cap?

[Guest Brian_D]

When I replaced the surrounds on the AR17's I got, they had the normal visible dustcap, and then a flat, very thick and stiff cap directly over the coil. It's just like you guys are describing.

The second pair of 17's was the same, but the odd thing was that the 18's I did for a friend (with the same woofer part number) did not have this cap.

I did not replace the cap on either pair of 17's, I just made sure the visible cap was good and sealed.

I wish I would have measured the response before I did the replacement, but I could swear that after re-foaming, the 17's had more bass extension. I always attributed it to the deteriorating surrounds, but perhaps this changes some charicteristic of the motor structure? (Mass, I'm sure to a tiny degree, but maybe even resonant frequency as a result of the re-sized air space behind the pole?)

-Brian

[ninohernes]

My AR-4x woofers have vented dustcaps. However, my AR-2's have a flat thick cardboard dustcap. My AR-3's have a cloth dustcap which appears to be sealed with the same type of coating used on the cloth surrounds.

[tysontom]

>Was there ever a verdict as to AR using "vented" dust caps or

>were they all sealed? I will also direct this ? to the

>refoaming guys OK? Dale

Dale,

Nearly all of AR's woofer voice coils have holes in the top of the voice-coil former, or "bobbin," that would allow air to escape from under the the dust cap and above the pole piece, and on through the spider into the enclosure. A small amount of air can pass through some of the dust caps, but many were coated like surrounds (and others were solid cardboard); therefore, I believe that most of the air movement above the pole piece was contained inside the speaker cabinet and not out into the air. The dust cap on the AR-4 and AR-4x was porus, but I believe those speakers did not have the holes in the bobbin if I remember correctly.

--Tom Tyson

[Rlowe]

Tom,

Does the attached picture show the holes you are refering to in your comment?

Therefore, if the holes/vents are in the cylinder, I should use a solid dust cap?

[johnieo]

Might this explain the use of one of two inductors in the AR-4x? My son purchased an AR-4x a few years ago that used a #4 coil. Its dust cap was solid, if I recall. I remember that the open ends of the magnet were covered with tape. Recently, he acquired a mate to this. It has the vented dust cap described here and also has mesh covering the magnet sides. This speaker uses a #5 inductor. (Both use a 20 uF cap).

I assume that the two AR-4x speakers are identical. Do I also assume that the #5/vent and #4/no-vent combinations are intentional?

[tysontom]

>Might this explain the use of one of two inductors in the

>AR-4x? My son purchased an AR-4x a few years ago that used a

>#4 coil. Its dust cap was solid, if I recall. I remember that

>the open ends of the magnet were covered with tape. Recently,

>he acquired a mate to this. It has the vented dust cap

>described here and also has mesh covering the magnet sides.

>This speaker uses a #5 inductor. (Both use a 20 uF cap).

>

>I assume that the two AR-4x speakers are identical. Do I also

>assume that the #5/vent and #4/no-vent combinations are

>intentional?

>

Nearly all AR-4xs, to my knowledge, had the 8-inch woofer with treated-cloth surround and the "pole-piece" magnet assembly. These woofers varied somewhat: some woofer cones had ribs; some had hand-applied reinforcing glue lines on the cone; some cones were smooth; some cones were black; some were grayish-black; some woofers came with a masking-tape cover over the magnet assembly; others had a molded screen wire over the opening on the magnet assembly. There were two different pole-piece magnet styles, one larger than the other, but with the same approximate flux density/strength. Perhaps different vendors for the magnet/frames. Nearly all of them, to my knowledge, had a porous-type dust cap. Some of the early woofers were similar to the AR-4 versions, which were also treated-cloth surrounds, only yellow-orange in color, whereas the AR-4x had a lamp-black, treated-cloth surround. The AR-4 with the 3-1/2-inch tweeter and 2000 Hz crossover was introduced in 1964. A year later AR introduced the 4x with the improved 2-1/2-inch tweeter with a 1200 Hz crossover. The 2-1/2-inch AR-built tweeter was superior to the 3-1/2-inch version, and could go lower in response, thus the lower crossover. Considering that there were many tens -- perhaps hundreds -- of thousands of AR-4xs built (second only to the Advent I believe in total production), I would be surprised if there weren't some slight variations in the crossover itself, but I am not sure.

The AR-4xa came along towards the end of the classic-AR period, and it had the AR-6 tweeter and the newer-style ferrite woofer (it ultimately had the woofer with a solid dust cap and foam surround, I believe). The service parts for the AR-4 and AR-4x from the early seventies, onward, called for the ferrite woofer with foam surround, as the earlier cloth-surround woofer was discontinued. Many 4xs were retrofitted with this replacement woofer. Check to see if your son's AR-4x versions had serial numbers far apart. It could be that the coil was changed to accomodate a changed and improved version of that woofer.

Sorry I don't have a more definitive answer to your specific question.

--Tom Tyson

[tysontom]

>Tom,

>

>Does the attached picture show the holes you are refering to

>in your comment?

>

>Therefore, if the holes/vents are in the cylinder, I should

>use a solid dust cap?

>

Yes, those shown in the image are the vent holes in the former. You could use a solid dust cover, but try to replace it with the original-style if possible. If there is shown a second, larger dust cap, someone most likely retrofitted it to the driver. Most of the original AR drivers had only one dust cap.

--Tom Tyson

[tysontom]

>Most of the original AR drivers had only one dust

>cap.

Let me correct this to some extent: in the late 70s and early 80s, AR began to use some larger plastic or formed-paper dust caps on many of the speaker drivers. On the 8-inch and 10-inch woofers, such as used on some of these newer speakers, cosmetic changes were made to the drivers, and I think different dust caps were applied over the existing flat-paper/cardboard dust cap originally seen on the AR-2ax in 1970.

--Tom Tyson

[johnieo]

> Check to see if

>your son's AR-4x versions had serial numbers far apart. It

>could be that the coil was changed to accomodate a changed and

>improved version of that woofer.

Thank you Tom.

One is in hand -- the other enroute. Both units have 2-1/2" tweeters (4x tweeters). I will look more carefully at the arriving unit and see if it could possibly be a 4 and not a 4x. Perhaps the 4 had the smaller coil?

[r_laski]

>>Let me correct this to some extent: in the late 70s and early 80s, AR began to use some larger plastic or formed-paper dust caps on many of the speaker drivers. On the 8-inch and 10-inch woofers, such as used on some of these newer speakers, cosmetic changes were made to the drivers, and I think different dust caps were applied over the existing flat-paper/cardboard dust cap originally seen on the AR-2ax in 1970.<<

Tom,

I think your explanation is right on. I’ve re-foamed several AR9 and AR90 woofers and lower midrange drivers. When re-foaming, I remove / cut open the dust caps to shim the voice coil. To add to your description:

The original AR 8” lower midrange (200027) has two (2) “dust caps”. An outer, porous (flimsy) cloth dust cap and an inner solid dust cap. The outer dust cap looks like it is there for aesthetic (cosmetic) reasons. The material of the inner dust cap appears to be a round piece of woofer paper cone material that is secured to the cone by the same adhesive used to secure the voice coil former (bobbin) and the lead wires to the cone. This inner dust cap lays on the top edge of the voice coil bobbin. To me, because it contacts the voice coil bobbin, it acts as both a sealed dust cap and an anti-resonant plug to cut down on resonances (ringing) from the pole piece and the aluminum voice coil bobbin.

BTW, the voice coil bobbin is also vented on these drivers.

Rich

[Rlowe]

Thanks for the reply Tom.

Was the solid dust cap ever used on the 12" woofer?

Richard

[r_laski]

>>Was the solid dust cap ever used on the 12" woofer?<<

I've never seen one. My experience with AR 12" woofers is from AR11s and AR9s, and a pair from AR LSTs that were poorly re-foamed.

The dust cap on the 12" woofer is really too small to have another one underneath it. If the person/machine applying adhesive around the voice coil / cone or to the dust cap was a little sloppy, parts of the dust cap will actually be glued to the voice coil

Also, although they are cloth, the dust caps are sealed. Maybe Tom or someone knows what AR used to seal the dust caps - lacquer, polyurethane?? Whatever it is, the dust cap is non porous.

Rich

[johnieo]

>> Check to see if

>>your son's AR-4x versions had serial numbers far apart. It

>>could be that the coil was changed to accomodate a changed

>and

>>improved version of that woofer.

Tom: The two AR-4x in question have quite different dates of manufacture. The one I have had for a few weeks is s.n. 346,xxx, and was manufactured first week of 1971. The unit which my son sent was manufactured mid August, 1967, s.n. 118,xxx.

The drivers are the identical in the two units; however, there are other differences. Most notable, the early 4x has a #4 (0.88 mHy) crossover coil and contains 18 oz of Fiberglass, whereas the later unit has a #5 (1.2 mHy) coil and is filled with 12 oz of Fiberglass.

The cabinets underwent slight material changes from chipboard and plywood to MDF and particle board. The interior volumes are identical.