ra.ra
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Posts posted by ra.ra
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Rather long-winded here, but....
My two take-away thoughts from this experience are: 1) you just never know what might turn up in the most unexpected place, and; 2) sometimes there is comedy and tragedy within the same tale.
Yesterday I was exploring a South Boston warehouse for modern furniture, and very soon thereafter stumbled upon a totally different industrial building with a hand-painted "Saturday flea market" sign leaning against the fence. Unlike so many places within urban Boston, parking was easy and available, so what the heck, let's take a look-see inside.The place was cavernous and gloomy with no natural light, and the only other things that held any attraction for me were a full set of 50's metal kitchen cabinets and a pre-owned golf club, both of which I passed on. But what really caught my attention was a lonely orphan AR-MST speaker, sitting rather uncomfortably amongst some hideous Rococo dining room hutches and buffets.Upon closer inspection, this really isn't much of a speaker to speak of.....so to speak. Curiously, this "find" seems to be an original four-tweeter MST cabinet which I am sure never left the factory as a finished product. The 3-sided hinged linen grille is present (condition: decent), as are twenty T-nuts, and the cabinet has a layer of thin fiberglass stapled to most facets of the interior. Otherwise, no drivers, no crossover components or rear panel, no wiring, no switch, no terminals or front badge. Speckled paint on backside, most likely intended for Euro market.Oops, I stand corrected by my own pics.... what I meant to say was "no original drivers". This specimen, which I did purchase (no sympathy required - - - it was almost free), had a single 8" co-axial driver which had zero screws holding it in place, flopping around like a fish in a boat, but at least it did have a long tail of attached brown lamp cord (huh?). The tweeter openings were all covered with what looked like war-zone bandages, swatches of fiberglass insulation taped over the cut-outs. Yikes!!! ...... although it's been well-documented that classic AR speakers are not particularly attractive with their clothes off, this one just might raise the bar for ugly.Appalled and aghast yet?.... but truth be told, that was actually the comedy portion. The tragedy now appears in Act II. Gleaned from rapport with the merchant, I soon learned that this semi-skeleton cabinet was a loner on the sales floor solely because it was so very fortunate to include this single "speaker". Mouth agape, jaw dropped, eyeballs rolling backward, I was informed that the other three similar (and entirely NOS, still in plastic wrap) cabinets were recently relegated to the dumpster (long gone, yep, I was willing to dive) because each of them did not possess said desirable speaker driver, and therefore, were deemed worthless. Sigh...What's past is history, so let's look to the future. The central purpose of my post here is that I am just fishing for thoughts about how I might best use this unique cabinet. I am reasonably familiar (but no expert) with the 3-tweet and 4-tweet versions of the MST, but clearly, this speaker model had its (limited) success and failure in two-channel stereo mode. With only one cabinet, there simply ain't gonna be no typical stereo with this project.My thoughts, thusfar. With a four-tweet cab, I do have more options than a three-tweet cab might offer, at least regarding tweeter placements. My limited inventory does include several versions of 8" AR woofers (cloth and foam, alnico and ceramic) as well as several 1.25" tweeters (ferro-fluid and not), so I do have some in-family drivers that will fit the baffle cut-outs and just might "play well with others". With only one cabinet, one consideration is to construct a symmetrical "center" channel which could then fit to my 60's HH Scott tube amp which does have taps for this feature. I really just don't quite know what to do with this, but I'd like to make it operable within the general spirit of the original MST products. If this center channel idea might just be a feasible notion, how might I try to fabricate this completed speaker - - - more like the 3-tweet version (two x-o frequencies) or more like the 4-tweet version (one x-o frequency)? .... or something else unique in between?Thoughts and ideas welcomed and appreciated.
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I was totally unaware of JKent's post-edit in post #17 until just now, but Bingo!!!! .......... I do think he has solved this puzzle. These colorful caps are indeed small things of beauty, but really, who wants to have to decipher this code just to ascertain the microfarad value of these teeny components? I was flummoxed by the seemingly apparent lack of five stripes, but indeed they are there - - - there just happen to be two adjacent red stripes that appear to be one rather wide one.
JKent's math calcs have totally convinced me - - - these Italian flag caps do indeed have a 2.2 uF value. You almost have to work from the bottom up to decipher this color code: determine the voltage and tolerance first, then figure out the capacitance with the remaining stripes. Anything that represents basil, tomato and mozzarella is always OK with me.
Fortunately, I've not yet ordered a quantity of 2uF caps for this restoration, but will instead focus on purchasing 2.2uF caps if the Mullards do not measure up to snuff.
Below is a bit of colorful intrigue. Pics shown, described as follows:
1. (3) "tropical fish" caps - five bands distinct
2. (3) "tropical fish" caps - five bands distinct
3. mullard color code map
4. .0022uF cap (note here that there is a triple band of red at top, then white, then red)
5. .022uF cap v.1
6. .022uF cap v.2
7. 2.2uF cap (subject of this thread, and the largest uF value in this series)
At least for this pair of Mini-III's, I am now convinced that the original cap values were 2.2uF and 90uF.
For more info, go here:
file:///Volumes/GWA%20OFFICE/mullard%20caps/Mullard%20Magic.htm
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to alkermes: I first learned of these striped Mullard caps reading a thread on AK about highboys, and my pics below show two of the three grille cloth fabrics (JKent's original off-white being the third choice) offered on Mini III's.
to fisher400: your comments in post 14 make sense to me now - - this pair of speakers has the mids adhered to baffles with a stiff yellow glue (that looks similar to Titebond) instead of the flexible clear silicone that was used in my first pair.
to JKent: thanks, yes, I will conduct a more serious woofer-push test when I give these a more thorough evaluation. After pulling the woofers to peek inside, I found that the original rope caulk is so crunchy (unlike similarly aged AR's I have dis-assembled) that it can no longer provide even a temporary air-tight seal, so I will have to provide a good temporary seal at the woof opening to conduct this test.
Images below show two types of grille cloth, both speckled - - what I call mid-brown and dark-brown. Backside of grilles shows that mid-brown was single layer cloth, but dark-brown also had the second layer of black scrim (note here that position of tweeter has changed). Backside of cabinets depicts two minor, but obvious, differences - - in these pics, more recent speaker is on left; earlier speaker (with missing control knobs) is on right. The first notable difference is the change of speaker wire terminals. The other difference is the cabinet perimeter assembly detail. On the earlier (right) speaker, the back panel is flush with the four sides - - probably a simple rabbeted joint. On the later (left) speaker, the back panel is recessed from the four sides by about 1/4", and while this does provide a nice little finger grip for lifting the cabinet box, I think its primary purpose is to keep the rather large tone control knobs from protruding beyond any part of the wood cabinet box, which does happen in the earlier version.
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This post about caps only. More thoughts later.
While they will never surpass AR's as my preferred speaker manufacturer, Rectilinears seem to be the Rodney Dangerfield of 60's-70's east coast speakers - - - "they don't get no (much) respect!" But at least there are three or four of us here on CSP who seem to have an appreciation.....
No question, I will be replacing the 90uF caps - - - can't seem to locate any single 90 uF NPE cap, which is fine, so am thinking about using Erse 40uF + 50uF - - - both their 6% and 10% tolerance caps seem to be superb price values (pennies!!), and are all almost the exact same dimensions. Easy-peezy parallel install. Would not know what to think of mixing NPE with the surplus polypropylenes - - - am curious, what's to gain with this method? As I've noted in other posts, I do like these 10 uF surplus caps a lot and am grateful for having had them brought to our collective attention here on CSP - - great, great value .... but really, does anyone truly know how long these will last as compared to NPE's? They do measure very accurately when new, but then, what should we expect from this little known Taiwanese manufacturer regarding longevity?
I do not mean to knock these caps at all, but these thoughts are from my cynical devil's advocate voice. My experience with these caps to date has been positive, and I expect to purchase more in the future at such a bargain basement price. Despite the attractiveness of their ultra low price, my own replacement solutions are largely driven by a preference for simplicity plus a realistic understanding of my own soldering skill level. Issues of capacitor longevity aside, I remain unconvinced I'd be able to detect audible differences between film and NPE caps, even tho' I do understand the huge importance of the midrange driver in the Mini III and want to restore its function properly. Final point about the multi-cap solution: at what point does a large wad of small cap values begin to compromise the necessary internal volume of a speaker cabinet, particularly with small cabinets?
About the 2 uF cap. I had originally planned to simply replace them with a good (Dayton? Erse?) film cap, but with JKent's comment about Mullards, instead I will remove and measure these before deciding on replacement. And the color coding? It does sound fairly straightforward, but there is no way I was able to come up with a value of 2uF for these caps trying to interpret the tropical fish chart.
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Yet another corollary to the post started by JKent which was then resurrected years later. This model of speaker (and others) from Rectilinear Research Corporation do perhaps seem to fly under the radar a bit, and I continue to be very impressed with their overall quality - - robust construction, very good drivers, straightforward crossovers, elegant appearance, and oh yeah ........ excellent sound reproduction. (Not to mention.... very often available for extremely reasonable cost value.)
Recently, I've come into possession of another pair of Mini-III's.....very similar, but slightly different from other pairs noted here. Walnut cabinets are in very good shape, and the grille fabric is the speckled mid-brown flavor. My other pair (circa 1968) has cotton batting for stuffing; this pair has fiberglass (circa 1974). Previous pair had mids attached with silicone adhesive; this pair has mids that seem to be secured with a more rigid, yellowish glue. Not apparent to me upon first inspection, but it now seems that the internal speaker wiring is an unusually heavy gauge, altho' I have not done any physical measurements or side-by-side comparisons to confirm this.
Crossover. It has been pretty well documented that the capacitors for these speakers are 90 mf and 2 mf, but this is the first time I've actually seen these striped capacitors with the secret color coding. I don't know anything about this Mullard-Tropical-Fish code, but still I find it rather interesting. Where did these components come from, and why were they coded this way instead of having alpha-numeric labels? Also with these crossovers, are there any advisable maintenance procedures for these 32 ohm pots other than a spritz or two of De-Oxit?
The mid-range drivers are nominal 5" Phillips with whizzer cone. Unlike JKent's mids that had (paper?) pleated surrounds, this pair of mids, while ever slightly different from each other, both have rolled fabric surrounds. No ports or intentional air leaks on these speakers - - full acoustic suspension - - so my question is: would it be beneficial for this restoration to include a re-doping of not only the (inverted) cloth woofer surrounds, but also a similar treatment to the mid-rage drivers?
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Huh?
Are you saying that you obtained two of these rare AR-11 "visible" demo speakers and then replaced the purposely dissected woofs with working 3a woofers, and also patched in the transparent side panels in order to create another pair of usable hybrid speakers? OK ..... I guess. I'll admit that I've also been known to try to give old speakers a new life with refreshed parts.
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Thank you, Tom, for yet another enlightening peek into the mystery of the history. Sounds like by the time AR had developed manufacturing facilities in England and Holland (when did that occur?), there was a need for additional support (Delrama) in the customer service and parts distribution areas that AR had already well established stateside.
Those cut-a-way speakers are so cool, as well as extremely educational, for anyone with an interest to understand all of the components and some of the phenomena that constituted the design, construction and performance of these speakers. I'd never seen that AR-11 with plexiglas before.
Just so we do not confuse any followers here, in paragraph 2 of your response, I believe you meant to say "amplifier" instead of "turntable". About the phone jack, I agree it must have been a later production modification, but I can't be sure it was only intended for foreign markets. I tend to think that since the phone jack was most probably well-received on the AR Receiver, perhaps it was just another case of "parts bin engineering", where someone said, "Hey, we got these 1/4" jacks here, people like 'em, let's add one to the amplifier." And if the jack-amps were only intended for international markets, wouldn't all of them be of the 220V variety?
Your explanation of the inclusion/exclusion of the "INC" on the AR amp front panel does seem to make good sense following a corporate timeline (Cambridge to Norwood), but would it then follow that this explanation is also the rationale why the AR Receiver and AR Tuner also had no "INC" on their front panels?
Your comments about the heat sink enclosures make perfect sense, but if a little perforated metal was creating such a problem, why didn't AR engineers just increase the metal mass in the heat sink fins?
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Thank you, Robert_S, for the response. I had also noticed the two small distinctions you mention - - - the "INC" on the front panel, and more than one type of enclosures at the rear heat sink. To get really picky, I also noticed differences in the way the color coding is communicated at the RCA jacks.
Your confirmation about the existence of the headphone jack model affirms that my pics were, in fact, not some elusive unicorns; and your serial number authentication suggests that indeed this feature was most probably a late model design modification.
Also, your comments about the Delrama copy, although not fully authoritative, at least tend to suggest that this company really had no substantive business venture with AR, which is pretty much what I tend to believe until another voice states otherwise. This information, however, was found on the copy in the forum library, which is why it gave me pause to question any possible association between the two companies.
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Interesting that the Delrama name was even recognized. I've glanced at this piece of literature several times before and never noticed that name, and now that I have, I'm sure I've never heard of it before.
I'm perplexed about the amp, too. Another pic attached : different amp with headphone jack and matching tuner.
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Speaker forum, yes, but we occasionally see posts regarding associated electronics, and I've just got a couple questions regarding the original AR amplifier. I've dug around a bit thru forum archives (perhaps not deep enough to answer my own question?) and I found the response below to another member's question almost ten years ago. Tom clearly dates the introduction of amplifier-receiver-tuner, but I'm curious about any original electronics products released after the tuner (1970) and before 1973, as noted in the reply.
1. Regarding the headphone issue, did AR produce a later release of the amplifier with the headphone jack and switch, as suggested by the attached pic? It sure looks factory original to me.
2. On page 2 of the original tech sheets for the AR amp, at the bottom there is mention of a NYC company called Delrama International. Who are they, and how were they involved with Acoustic Research?
Posted 01 June 2004 - 12:51 PM
>Did the AR receiver pre-date the AR amplifier, or were the
>tuner and amplifier released after the receiver? Why did AR
>include a headphone jack on the receiver, but not on the
>amp...I had to use one of those goofy Koss headphone boxes
>with my old AR amplifier!
The AR Amplifier was the first, and it was introduced in 1967, the same year the AR-3a made its debut. The AR Receiver was introduced in 1969, and the AR Tuner followed in 1970. AR originally felt that a phone jack for headphones was not necessary with the Amplifier, but the demand for it was answered in the AR Receiver. The last of the original AR electronics came in 1973.
--Tom Tyson
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I will admit that I am trying to buy into this minimal diffraction theory, but when I see:
* the deep lip on the AR-3a improved speaker, or
* the sequential progression of the AR-18 (tweet originally offset, later centered) series speakers,
I begin to think that the theory and the practice of speaker design had some degree of disconnect.
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Thanks, Carl, for the interesting weblink. Even though some of the information might be a bit over my head (when I started reading about 2pi and 4pi space, my first thought was "uh-oh"), after reading it a couple of times, looking at the graphs and the baffle layout diagrams, I think I get the basic concept.
Curiously though, when I looked at the writer's own built speaker projects, most (but not all) of his designs more closely followed his own second option - Tweeter shifted up (or in some cases, down) with the tweeter centered equidistant from the long edges, rather than his third option - Best case scenario - Golden Ratio, where the tweeter is offset from the centerline of the baffle.
This guy makes some beautiful speakers and I have no doubt they sound as good as they look - - the build quality appears robust, the craftsmanship is high quality and creative, and the engineering seems well considered. Not unlike your original question about the EPI-100, one of this designer's primary objectives appears to be minimizing these unwanted diffraction effects, which is further evidenced by his (typical) lack of speaker grilles and frames, the flush mounting of driver faces, and frequent radiusing of baffle perimeters.
Looking at this issue with regard to an AR-3a, I'll have to assume that the tweeter placement, which is almost crawling out of the cabinet way up in the corner of the baffle board, has been thoroughly engineered with regards to these diffraction issues. Still though, when trying to understand high frequency dispersion from this driver, I remain perplexed by the severe frame edge (note deep shadow) immediately adjacent to the tweeter.
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Carl raises a very good point and there appears to be no obvious answer to this positioning of the woofer with regards to the depth of the baffle board. Not only is the woofer recessed, but the tweeter is actually proud (in front) of the baffle board with its 1/4" masonite. I have included a pic that I believe represents the situation Carl has described. A similar head-scratching could be attributed to the AR-2ax's shown here, even tho' the tweeter cutout has been beveled.
[proud [praʊd] adj (of a surface, edge, etc.) projecting or protruding from the surrounding area]
Aside from the issue of driver placement in the depth dimension, and using this simple EPI speaker as an example, I've never really understood what benefit is gained by placing the tweeter cutout off the vertical centerline of the speaker cabinet. I've raised this question before in the AR forum with respect to the AR-6 woofer's placement, and there has never been a satisfactory rationale offered.
BTW, I really like the early walnut EPI 100's - - - great tweeter, great woofer, simple x-o, robust construction.
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genek and Anthro's posts frame a very interesting discussion, particularly about the SO who acknowledges and appreciates the history and aging (provenance) of older objects. My own (sometimes overly eager) inclination when I obtain an "aged" collectible of any sort, is normally to grab the chemicals and abrasives to begin a "restoration" or "rejuvenation", but I have learned that it is often better to live with the piece for awhile and consider other more delicate or gentler approaches toward giving the item a new and prolonged life.
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Hi Charles, and welcome to CSP.
This thread has helped me understand this Mini III speaker much better, and I am about to soon replace the capacitors in mine. Although I have never seen it identified or confirmed as such, it appears to me that the mid and tweet drivers are secured to the baffleboard only with what appears to be clear silicone caulk - - - a chemical bond rather than a mechanical one (screw, bolt, etc.). I don't know exactly when clear silicone became readily available in the consumer market, but these speakers probably date from the very early 70's and I know that silicone caulk was frequently in use in the construction industry by the mid 70's. It is fairly easy to use from a tube (with a typical caulking gun), has some odor which off-gases with the curing process, and is an extremely strong adhesive.
As for the cabinets, I have done many amateur wood refinishing projects but never knew about the Howard's products until I began reading these forums. Now that I've tried the Restor-a-finish, I will always consider this product and wouldn't hesitate to recommend it for your walnut veneer - - - it is what many readers here seem to prefer. It comes in many flavors - - - you may wish to try natural, walnut or maybe even mahogany if you like the enhanced red qualities in some walnut veneer.
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Kent:
Great advice, thanks so much. I really don't know much at all about the wide, wide world of capacitors, and it all baffles me a bit frankly. Still, I agree with you that if you have the patient opened up on the operating table, why not check the liver while you're transplanting the kidney? NPE's are so inexpensive, it makes sense to replace both. The thought that Rectilinear may have used a standard 2.0 uF cap in several models sounds logical, so that's what I'll do for the small cap.
The note about the KLH speakers is interesting - - - so paralleling this discussion to AR, why is this not a similar case with the impedance of the tweeter in the AR-3 being the same driver as the tweet in the early AR-2ax? Am I correct that these two speakers had different drivers (although they look the same) based on the 3 being 4 ohm and the 2ax being 8 ohm?
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Hmmmm ...... four years lag time - - - any chance of reviving this thread?
First, some initial observations. I am very impressed with the build quality of this vintage of Rectilinear speakers - these little guys are heavy, dense, and compact. But actually, despite my calling them 'little' and Rectilinear titling them as "Mini", these speakers are not really so tiny at all. While JKent first likened them to AR-7's and then updated his comparison to AR-4x, these speakers are actually the nearly identical dimensions (albeit 2" deeper) as the AR-6. Even so, while a 4x weighs in at 18.5 lbs. and an AR-6 at 20 lbs., these brutes tip the scale at 25 lbs. apiece.
Next, a couple of questions. When all but one of the other Rectilinear speakers from 1971 have 8 ohm impedance, why was this speaker product designed at 4 ohms? Following this, why does the tweeter cone have 8 ohms stamped on it? And the tweeter cap - - - thanks to JKent and RoyC, I see his measured at 1.8 uf and was replaced with 1.9 uf (where does one find this cap - which brand makes 1.9?), but would it make more sense to pursue a replacement cap of 2.0 uf per the comment that it was a typical value in other Rect. products?
While making comparisons to AR products, it appears to me that this woofer is nearly identical to that in the AR-4x. The basket, cone and magnet appear exactly the same, but this woof has an opaque felt dust cap and an inverted roll profile on the fabric surround. The spider has the same profile, but the material has a different texture and color.
A couple other peculiarities. My version has the dark brown Rectilinear grille cloth over the thin black scrim (2 layers), but strangely, the grille frame is made from two layers of 1/8" masonite. And just like an orphan Rectilinear XI that I have, the woofer is secured to the baffleboard with a rather odd fastener (see pics 1 and 2). No T-nuts and machine screws here - - - this device is basically a headed and threaded nail, punched thru from behind, with threads which accept a standard hex nut. This spike has a dangerous and sinister point which is no fun to work around.
AR-MST, ideas?
in Acoustic Research
Posted
A few other thoughts and pics....
Cleaned up the cabinet just a bit, and then loose-dropped in a few drivers just to generate ideas.
It occurred to me that I've never seen any company literature or schematic for the 3-tweet MST (MST-1), but I am now attempting to put together a sketch for comments, based on pics I've seen of the crossover components. But still, there seems to be no information on the amount of cabinet stuffing or even the cap values for the tweeters.
Not particularly germane to my particular objectives but equally curious to me, AR literature suggests that the cabinet volume of the MST is equivalent to the cab volume of the AR-7. At first I thought this couldn't be true; then I placed an AR-7 atop the MST and saw that their cabinets depths are identical and their horizontal cross sections appear to be nearly the same. But then I reconsidered again - - when the third dimension (height) is accounted for, the MST cabinet is more than 4 inches taller than the AR-7. Are these published cabinet volumes fact or fiction?