AR 4x Pair: 1 with Fiberglass, 1 with Rockwool

[Guest Islands Innovation]

AR Fans,

I have some information to add to Carl's recent stuffing study:

http://www.classicspeakerpages.net/dc/dcbo...ing_type=search

I just finished rehab'ing a pair of AR 4x's that I rescued from the dump. They are not a matched pair. One has a larger cover on the woofer magnet. Also I noticed that the inductor coils were different colors, like perhaps they had a different flavor insulation on them. The serial numbers are: 119043 and 0121?? (paper torn).

What's most interesting is that one had yellow fiberglass, and one had brown rockwool.

I weighed the 2 for comparison to try to add some data to Carl's study. I used a food scale, so take the readings with a grain of salt, I'd guess +/-20g: Fiberglass = 200g, Rockwool = 500g

So this roughly verifies Carl's estimate that the performance of rockwool is matched with about 1/2 it's weight in fiberglass (actually 0.4 in my case), at least in how AR figured it with my pair.

Looking at the interior cabinet volume (ignoring the driver's volume) of about 0.6 cu ft, it works out that AR used about 0.7lb/cu ft of fiberglass stuffing in my AR 4x. I think Carl's data pointed to about 0.9lb/cu ft...so mine was a little lower but in the same ballpark. Figuring in the driver volume would raise my figure slightly. Perhaps AR used different stuffing densities in different model speakers?

I put them back together with 200g of fresh pink OC fibergalss. That filled up the cabinet pretty well without compressing it too much. I left some room around the woofer also. Does anyone have any thoughts on the paper used to separate the stuffing from the woofer? Mine only had it in the rockwool cabinet. I used a piece of burlap as a barrier.

Another observation was that the pots were in about the same (bad) shape. I noticed a very thorough study on pot corrosion that indicated that the sulfer in rockwool might be a factor in rapid corrosion of the pot centers. I did not see a big difference to my untrained eye. However, I did note that the rockwool pot seemed to have deeper gouging in the face under the wiper. I wonder of abrasive properties might be a factor? Or that could be a sign that in fact the corrosion was worse in terms of depth, while looking similar on the surface.

Another pot difference I noticed was that one of them (don't recall if newer or older one) had a little "post" or "stopper" (wish I took a picture) that protected the wiper from being bent if the pot shaft was pushed in. Seems like a failure mode on those pots could be that if they are pushed in too much, the wiper could easily get bent back and loose contact with either the center or coil, or both. I wonder if that "shaft push stopper" was a later design to address this failure mode?

Thanks, Jeff

[Carlspeak]

"....I put them back together with 200g of fresh pink OC fibergalss. That filled up the cabinet pretty well without compressing it too much. I left some room around the woofer also. Does anyone have any thoughts on the paper used to separate the stuffing from the woofer? Mine only had it in the rockwool cabinet. I used a piece of burlap as a barrier....."

Thanks Jeff for taking the initiative and the time to add to the database.

With regard to the paper put behind the woofer, I wonder about it's effectiveness. Perhaps it was very useful at one time when vulnerable, Alnico magnet woofers were used.

Maybe one of the AR experts/historians could help here.

Along those same lines I also wonder about the screening glued to the back of some vintage woofer frames. Was it intended to accomplish the same effect as the brown tissue paper? Don't know. My guess is there was some concern about fine bits of fiberglass getting into the VC air gap somehow. Then again, the high permeability of the brown paper and screen wouldn't provide effective protection - at least in my mind. The only other thought I had was perhaps there was concern about clumps of fiberglass getting wedged between the woofer frame and cone thus interfering with the cone's motion?

It's all about the music

Carl

Carl's Custom Loudspeakers

[Carlspeak]

Yesterday, I finished assembling an AR3a speaker from stock parts. I stuffed it with about 1 lb of pink OC fiberglass. The box Fs measured at about 40 hz - pretty darn bang-on spec. The refoamed woofer had an Fs of 17 hz. The woofer coil was a standard #9.

It's all about the music

Carl

Carl's Custom Loudspeakers

[tysontom]

>Does anyone have any thoughts on the paper used to

>separate the stuffing from the woofer? Mine only had it in the

>rockwool cabinet. I used a piece of burlap as a

>barrier....."

>

>Along those same lines I also wonder about the screening glued

>to the back of some vintage woofer frames. Was it intended to

>accomplish the same effect as the brown tissue paper? Don't

>know. My guess is there was some concern about fine bits of

>fiberglass getting into the VC air gap somehow. Then again,

>the high permeability of the brown paper and screen wouldn't

>provide effective protection - at least in my mind. The only

>other thought I had was perhaps there was concern about clumps

>of fiberglass getting wedged between the woofer frame and cone

>thus interfering with the cone's motion?

>

Carl,

I think your last intuition is basically correct: the screens on the woofer frames, or the KemPac used beneath the woofer frame to cover the exposed fiberglass, was there primarily to keep pieces of fiberglass from wedging between the woofer cone and the frame, thus affecting performance. When the woofer cone is oscillating at low frequencies (see attachment), the fiberglass is literally bouncing back and forth slightly, and could easily lodge under the woofer frame.

http://www.classicspeakerpages.net/dc/user_files/1803.jpg

AR-2 woofer excursion at 20 Hz

--Tom Tyson

[JKent]

>>Does anyone have any thoughts on the paper used to

>>separate the stuffing from the woofer? Mine only had it in

>the

>>rockwool cabinet. I used a piece of burlap as a

>>barrier....."

>>

I don't think that paper is available anymore. John O'Hanlon identified it as a special paper made by Kimberly-Clark. OTOH, I noticed that my Allison: Fours used what appears to be common fiberglass window screen to keep the stuffing out of the woofer--not glued to the back of the woofer as mentioned, but just placed behind it, the way the paper was in the ARs. I bought some screening at my local hardware store to use on my next AR 4 project if I can't salvage the paper. The burlap sounds like a good choice, too, although the purists would say to only use the original paper ;-)

[Guest matty g]

Jeff

Did you notice a date stamp on the magnets or inside on the back of the cabinet? I noticed that feature on a set of 3A's from July 22 1972.

Also noticed my 4X's from '67 didn't have that feature. Yet another set of 4X's I have kicking around here (don't know the date, but serial #FX229535) have red stems on the pots - kind of strange. As I recall, the '67 models had fiberglass in them.

I've got three sets of AR4's here and I've had a couple of others - from various eras - and to me they all sound about the same...GREAT!

BTW - someone on Ebay has a VERY old set of 4X's - with a cross hatch grill design. I can't recall ever seeing that before!

Take Care

Matt

[dynaco_dan]

>>>Does anyone have any thoughts on the paper used to

>>>separate the stuffing from the woofer? Mine only had

>it in

>>the

>>>rockwool cabinet. I used a piece of burlap as a

>>>barrier....."

>>>

> I don't think that paper is available anymore. John O'Hanlon

>identified it as a special paper made by Kimberly-Clark. OTOH,

>I noticed that my Allison: Fours used what appears to be

>common fiberglass window screen to keep the stuffing out of

>the woofer--not glued to the back of the woofer as mentioned,

>but just placed behind it, the way the paper was in the ARs. I

>bought some screening at my local hardware store to use on my

>next AR 4 project if I can't salvage the paper. The burlap

>sounds like a good choice, too, although the purists would say

>to only use the original paper ;-)

Hi Kent;

I wrote previously about the use of gray plastic fly screen pieces glued on some AR woofer housing openings.

There were glued to the frame openings, possibly with rubber cement or 2 part epoxy.

It appears now that this was a cost cutting measure used to prevent the possible rubbing of insulation against the woofers cone movement and also the wires from rubbing and causing a buzz.

AR may have had a number of complaints from the Kim Towel shifting and perhaps wires rubbing and causing warantee repairs.

I do not know which sizes or models but I have seen some on 10" and 12" woofers.

If the brown Kim Towels are not available, I would take cutting up of the plastic fly screen and glue them to the frame openings.

My next choice would be be the burlap sacking laid in behind, the same as the original Kim Towels.

Loose burlap sacking may not cause a buzz but it still may shift in the enclosure and wad into woofer frame openings.

I would not suggest using a sheet of the fly screen in place of the Kim Towel because if it shifts, it may buzz being plastic.

I do remember buying a box of Kim Towels from a local plumbing wholesaler decades ago in lieu of wiping rags.

I have a few remaining pieces but it was not a great wiping rag for my particular work at that time.

It is fine to be a purist, Kent, but, availablity is maybe out of reach.

We must use the next best all round material for that job.

The work requires removing the woofers, all the rest is gravy.

Cutting and laying or glueing, with insulation and wire isolation as the required endall.

Either method is not expensive, nor too time consuming, but worth every penny of your effort.

Good luck and please let me know of your thoughts.

[Guest Islands Innovation]

> Did you notice a date stamp on the magnets or inside on the

>back of the cabinet? I noticed that feature on a set of 3A's

>from July 22 1972.

>

> Also noticed my 4X's from '67 didn't have that feature. Yet

>another set of 4X's I have kicking around here (don't know the

>date, but serial #FX229535) have red stems on the pots - kind

>of strange. As I recall, the '67 models had fiberglass in

>them.

Matt,

I looked for date stamps on the woofers and cabinets, but no luck. It was interesting that both tweeters were stamped mid-1969, indicating that at least one must have been replaced because there appears to be a few years between the speakers.

I think it was the newer pot that had the little stopper. Both were black stems, and looked otherwise identical. Seems like that stopper might help prevent an open pot resulting from that push-turn-grind that it takes to make them function again after sitting.

Thanks, Jeff

[dynaco_dan]

Hi there;

After giving it some more thought, I now feel that the glued on screens were prior to the Kimpack sheets.

My reasoning now is that, it was more labour intensive to apply four plastic glued strips than punch a finger hole for 2 wires.

[Carlspeak]

I agree Vern based on the numerous AR woofers I've worked on.

It's all about the music

Carl

Carl's Custom Loudspeakers

[JeffS]

>>>Does anyone have any thoughts on the paper used to

>>>separate the stuffing from the woofer? Mine only had

>it in

>>the

>>>rockwool cabinet. I used a piece of burlap as a

>>>barrier....."

>>>

> I don't think that paper is available anymore. John O'Hanlon

>identified it as a special paper made by Kimberly-Clark. OTOH,

>I noticed that my Allison: Fours used what appears to be

>common fiberglass window screen to keep the stuffing out of

>the woofer--not glued to the back of the woofer as mentioned,

>but just placed behind it, the way the paper was in the ARs. I

>bought some screening at my local hardware store to use on my

>next AR 4 project if I can't salvage the paper. The burlap

>sounds like a good choice, too, although the purists would say

>to only use the original paper ;-)

When I worked in a plastics factory a few years ago, we used Kimpak to soak up hydraulic fluid. The protective sheet inside the old AR cabinets appears to be the same stuff. A quick search shows that it is still available if someone is willing to purchase a roll of it.

http://www.brasspack.com/tl/Kimpak.html

I refinished a pair of Allison:Six speakers last year. They had burlap behind the woofer for protection.

Jeff S.

[dynaco_dan]

>Hi there;

>

>After giving it some more thought, I now feel that the glued

>on screens were prior to the Kimpack sheets.

>

>My reasoning now is that, it was more labour intensive to

>apply four plastic glued strips than punch a finger hole for 2

>wires.

I wonder if they had any speaker returns with wires touching the plastic shielding and buzzing, perhaps?

Hi there;

I was just at our local lumber yard yesterday and saw something very familiar.

In the cleaning supplies section was 3 cartons, 100/box, and $10.00 CDN per box.

Kimberley-Clark Professional 47026 Wypall L20 wipers.

100 per box, Tan colour, bar code 036000470260.

9.1 x 16.8 inches 23.1 x 42.6 cm

2 layers thick with a similar look and feel of Kimpack hand wipes I bought many years ago.

If you have a pair of AR-4X's, you can use 2 and have 98 for another project. LOL

[johnieo]

Jeff:

I missed your original post. Do you recall the part number on the two magnets? Either a #4 or #5 in black felt pen. My guess is that both are #4. At some point around serial number 200,xxx (fourth quarter, 1968) AR switched to the #5 coil. Your tweeter dates of mid 1969 are not in sequence with the serial numbers, which would be in the 25x,000 range by that date. Perhaps both tweeters were blown and replaced?

At another time, AR reduced the stuffing weight (FG or Rockwool) from 18 oz (510 g.) to 12 oz (340 gr), so if you found only 7 oz in one of your cabinets, I'll bet it has been opened and not reassembled properly--or your scale is way off! The stufing weight was reduced to increase the system Q factor. Best to describe stuffing in units of weight per AR-[xx] model. The stuffed density (wt/cuft) does not scale between speaker models; it is only one of the factors that determines overall Q. Best to use the same weight (18 or 12 oz) in both cabinets so the system Q is the same.

Cheers,

[ironlake]

Speaking of buzzing wires, maybe that explains my very 1st generation large advents having pink fiberglass glued to the speaker wires all the way to the voice coil solder joints. My advents are late 69 early 70;s without the tweeter protection grill.