Polk 7B's - need to know what Mid Woofer Model?

Gardenstater · November 2019

Hello,

New to the forum although been reading for years. I'm the original owner of a pair of 7B's that I bought in 1979 for my college dorm room. Love 'em! I have a bit of an occasional *chuffing* sound in the right speaker's Mid/Woof. I discovered several months ago that the spider had a crack in it and I repaired it (successfully I think) with Super Glue (Gel), and recently did a 2nd more thorough treatment evenly all around where the spider meets the cone when I thought I heard some chuffing return. Anyway, heard it again last night and thoroughly examined the repair and couldn't for the life of me see where I could justify throwing more super glue at the situation since it appeared all good! However, I could actually hear a bit of a chuff when I manually exercised the cone/voice coil and I noticed that when I pushed on one quadrant, less noise or very little noise occurred. Well, these speakers were positioned vertically for part of their lives and horizontally tilted downward mounted from the ceiling for part of their lives, so various positions of the voice coils! I decided that maybe I'd reposition the mid/woof so that the quadrant that produced the least noise was facing up and I have to say that it has been about 95% effective at eliminating the noise. I'm sure it will come back because that's the way of things !!

I need to get a replacement so that it is available to me for a swap out, should the need arise (which it will). I don't see any model # designation on the driver whatsoever. I've heard that they are the MW6502 but someone here on the forum said that the very early ones were MW6500 so I'm not sure what to do. I've posted a picture below but I'm not sure it will help.

Can anybody advise me? I see used ones for sale on eBay but I don't really want one that could very well have the same problem and would prefer a new one. When I spoke to someone in the Polk Audio room at the New York Audio Show the other day, he said that I would have to go through an authorized repair person to get a new one

Is that so, even for a new Club Polk member

AND an original owner? Thanks.

dkfreebird · November 2019

That is a 6500. Says so on the top right hand.

pkquat · November 2019

Welcome to the forum. The driver is a MW6500. In your picture right under the polk audio logo, is "Monitor Series Driver MW6500" The bad news is this driver has not been made for a while and there are no direct "new" replacements. Your best bet is ebay.

These are early "silver basket" MW6500s. In the early 80's polk made "black basket" MW6500s. In theory there is no difference other than appearance. Their specs should be the same, however some people feel there is a difference and don't like to mix the two.

I have a feeling the chuffing is the voice coil rubbing on the magnet assembly. This is more likely caused by the repairs than the driver being in a certain position, although drivers being affected by gravity has been known to happen.

The MW6503 is considered the closest replacement for a more recent model, and one that is being reproduced. These should be replaced in pairs. Polk is no longer making these. New reproductions can be purchased from Midwest Speaker. The quality is assumed to be as good or possibly better than the OEM replacements from Polk. Supposedly they are being made at the same factory.

I should note, that a number of forum members prefer to stick to the originals even for the 6503 which can be found on ebay, vs the OEM replacements.

The polk OEM tweeters that replace the SL-2000, 2500, 3000 are considered better than the originals. You have the peerless tweeter which is many people feel is the best or most pleasing of all the tweeters polk used.

pitdogg2 · November 2019

I would absolutely be all over thay one on ebay if i was you. I'd also buy used on eBay before anything from Midwest speaker. Not that there is anything wrong with Midwest but I'd buy original until there is no more

pitdogg2 · November 2019

Gardenstater wrote: »

@ pitdogg2 - Thanks, That's the kind of input I'm looking for Are you referring to the used silver basket pair or the old stock open box black basket OEM Polk replacement? I see that some of what I thought was rust may be a brown shadow.

Old stock open box
Then used silver basket.
These are hit or miss. Many months can go by with absolutely none on ebay.
Ask questions of the sellers as how they plan to ship etc. When you get them test them then get some power grab all purpose adhesive and glue them up.

boston1450 · November 2019

Did some of the gasket come up thru ?

F1nut · November 2019

That's the gasket.

Gardenstater · November 2019

Thank you! I guess my picture was so out of focus my eyes just glazed over that (what a dope)! So, I'm wondering if I absolutely have to have a MW6500 or, to put it another way, what the difference is between the 6500 and the 6502. I only seem to see the 6502's being offered on eBay.

Gardenstater · November 2019

Well, I take that back. I see a bunch of MW6500's on eBay. Just need to confirm that I can't get a new one from Polk before I take a chance.

Gardenstater · November 2019

What would people here think of this one? : https://www.ebay.com/itm/New-Open-Box-Genuine-Polk-Replacement-BD0600-W-MW6500-Driver-6-5-NOS/223742838696?hash=item34181cc7a8:g:4BAAAOSwYUpdx0vu

Gardenstater · November 2019

Thanks for putting so much thought into that response pkquat, and I'm grateful to have joined a forum with so many knowledgeable members! So, as usual, I guess I have a bit of a dilemma because it seems everything about my particular vintage Polks was made better than later models! Only see one pair of silver basket ones on eBay that unfortunately looks rusted which to me is a no-go: https://www.ebay.com/itm/Polk-MW6500-monitor-series-6-5-woofers-pair/264475870600?hash=item3d93fd8588:g:AwQAAOSwhGBdiXdr

Gardenstater · November 2019

This must be what you're referring to, from Midwest Speakers. They say closest match to MW6500's. I wish I knew what the specs are for the MW6500's. I just see that the resonant frequency was tested and found to be 22Hz. I suppose I can verify the impedance with a meter. Or can I, since impedance would be different than DC resistance? https://www.midwestspeakerrepair.com/shop/home-audio/woofers/polk-audio-1/mw-audio-mw-6503-8-6-5-inch-polk-audio-copy-woofer/

Gardenstater · November 2019

@ pkquat I have no doubt that my repairs with the gel superglue weren't perfectly even but the original condition that prompted me to investigate and discover the cracked spider was that I heard the same *chuffing* sound, only it was much more severe pre-repair. It went several months without any issues, except maybe at fairly high volumes with demanding bass material.......the kind with sustained low base notes. I later heard it more often and figured that it might need to be revisited and I was concerned about unevenness throwing off the voice coil movement and then I tried to do the entire circumference of the spider/cone area, figuring that there might be other cracks or areas about to crack.

Gardenstater · November 2019

@ pitdogg2 - Thanks, That's the kind of input I'm looking for

Are you referring to the used silver basket pair or the old stock open box black basket OEM Polk replacement? I see that some of what I thought was rust may be a brown shadow.

Gardenstater · November 2019

OK, thanks! I need to do the adhesive treatment on the magnets on my existing ones also. I have Dynamat and have done the troublesome mid/woof thinking that it might help, but still have to do the other mid/woof and the passive radiators.

Gardenstater · November 2019

Picture of my setup. The amp is also 1979, Crown D-150 that a buddy sold me for $50 on my last Birthday. I never had enough damping factor to make the Polk 7B's shine before with just a Technics SA-500 reciever, damping factor 24 vs 200+ lol. The homemade ribbon cables are a gamechanger. I'm making some larger gauge shorter ones. The sub is a Mordaunt-Short that a buddy lent/gave me. He'll have to pry it from my dead fingers until I can get a Carver Super Junior on the cheap.

pitdogg2 · November 2019

well if you didn't buy the NOS in the box it is now gone. you get on those quick or they are GONE.

Gardenstater · November 2019

Ha ha. No worries it was me! I quickly asked him if he would use plenty of cushioning and he said absolutely. So I snatched it.

F1nut · November 2019

It's not the damping factor because the difference between a factor of 10 and 1000 is so small it's laughable.

Gardenstater · November 2019

I guess it depends upon who you listen to. This article says he noticed a difference at 25 and my old Technnics SA-500 was only rated 24. When I first hooked up the Crown D-150, even with only 16 gauge cables, the bass was far punchier than I ever remembered. When I made some Litz speaker cables from 3 Cat 5e cables that were only 6 ft long and were 13.5 gauge, the bass really got tight and punchy. I think the acoustic isolation system as part of and underneath the stands I created made a huge difference as well. At first I said....whoa.....where'd my bass go, but further listening revealed how much more articulated and tight it was. Speaker cable gauge and damping factor do make a difference in my opinion. http://www.bcae1.com/dampfact.htm

boston1450 · November 2019

There ya go folks. Cables do matter

Great thread btw

F1nut · November 2019

It's current, not the damping factor.

Gardenstater · November 2019

Perhaps current and damping factor are related? The article I posted has a calculator at the bottom that shows the effect that the speaker wire (length and gauge) has on the effective damping factor.

I will say that the other day, I polished the 3 plugs in the chain to my amplifiers: subwoofer plug, power strip plug, and main amplifier plug, with well worn 1500 grit sandpaper followed by some Mapleshade silver platelet grease and damned if my first reaction wasn't that my bass had gotten a little bit better.

pkquat · November 2019

Gardenstater wrote: »

What would people here think of this one? : https://www.ebay.com/itm/New-Open-Box-Genuine-Polk-Replacement-BD0600-W-MW6500-Driver-6-5-NOS/223742838696?hash=item34181cc7a8:g:4BAAAOSwYUpdx0vu

You will want to check the package over thoroughly when you receive it, or possibly contact the seller immediately before it ships and ask for picture of exactly what he will be sending, and some type of history with paperwork if possible. I saw a couple of issues

.

#1 The first picture with the speaker on the box. The box does say MW6500, which is a plus, however the speaker shown has a black basket and a paper ring on the front. To the best of my knowledge polk never made this combination. Only silver basket speakers had the paper ring. (It is possible polk had some transitional black basket models with a paper ring. I never saw one in production, so maybe they got relegated to replacements).

#2 The following pictures do not match the first one. There is no paper ring anymore.

#3 The 2nd picture shows the last digit of MW6500 covered with whiteout and a 0 written over it. The MFR date is 1999, and it is made in Mexico. I have a feeling this is really a OEM MW6503. I am not sure if these were made in Mexico at this time or if it is an OEM speaker or OEM replacement. @F1nut may know better. I also don't know when they stopped making MW6500's. I could be possible it is a MW6500 and the whiteout was done by polk because they did not have any MW6500 labels. That said it could even be a MW6503 that polk re-labeled as a MW6500 since it was considered the replacement to the MW6500, but I doubt this since polk has been upfront that they are different speakers and for the last 10 years or so has stated the differences. I don't know what things were like 20 years ago.

Silver basket speakers
I may have a pair. I was looking black basket and got sent silver basket ones. I recall the drivers move, but I did not hear them play. I got them in good faith on a transaction that went sideways. I'll check if I still have them. I don't remember if I traded them.

The rust on the one is not great, however the other one does not look too bad. They most likely were in a wet or southern environment. The seller is in FL. Considering the rust, the price is not that great, but eventually they will sell.

Gardenstater · November 2019

Yeah there's always something huh

? I took screenshots of every picture and if something is different than shown I will have a serious cause for complaint with the seller, or eBay if it comes to that! I did notice that the paper gasket was removed in a couple pics however I was put at ease by the fact that it is visible on the table in the lower right corner, so I just assumed that the seller removed it to show that the surround was in good condition, which it certainly should be for NOS, open box or not. Yeah I was a bit concerned about the handwritten "0" however I decided to take a chance and I probably should have asked the seller about it. I figured that the box stating "MW6500" was sufficient protection for me.

I wish I had a list of specs for the various drivers; MW6500, 6502, 6503. Does anyone have this or is it posted somewhere on this forum? I heard Polk customer service is pretty unresponsive possibly due to the release of their new flagship speaker, which I saw at the New York Audio Show. At first I was unimpressed with the boominess of it from outside the room in the hall but nobody uses acoustic isolation which still amazes me! I must say that when they played some pipe organ music on it, it was quite an experience......but I digress....

I would be interested in those silver basket 6500's provided they move freely without any chuffing or the like. I want to have backups so that I can play tough music with high sound levels without always being on pins and needles. These have held up remarkably for 40 year old speakers so far but how much more can I expect?

Thank you.

Gardenstater · November 2019

I think I found a specs list here in the forum. Hopefully it's accurate. MW6503 is pretty close to MW6500:

https://forum.polkaudio.com/discussion/133992/thiele-small-parameters-for-mw6500-drivers

pitdogg2 · November 2019

The paper on it looked to me to be a MW6501 as my MW6501's looked excatly like that and were also made in the 90's I had bought more than one from polk where they whited out the last part of the number and hand wrote in the digits. Some were shipped with the fiber gasket to protect the face of the woofer as I had some shipped from Polk that way. Polk then I believe redesigned the box and they were no longer necessary. Back about 2015 Polk was still supporting legacy drivers although the MW6500 was not one of them at that time. I'd speculate that they were still in the 90's with a few others. Here was the current divers in 2016 Polk was still having made at that time frame.

1 Driver Type Q BL Compliance DC Resistance Fs

3 MW 6501 1.47 5.96 N 3.25 E - 3N/M 7.75 Ohms 31 Hertz
4 MW 6502 1.97 5.16 N 1.47 E - 3N/M 3.52 Ohms 44 Hertz
5 MW 6503 1.82 5.24 N 3.55 E - 3N/M 6.54 Ohms 29 Hertz
6 MW 6509 2.1 4.57 N 3.54 E - 3N/M 8.97 Ohms 30 Hertz
7 MW 6510 1.39 6.32 N 3.41 E - 3N/M 6.57 Ohms 31 Hertz
8 MW 6511 1.405 4.34 N 3.496 E - 3N/M 3.13 Ohms 29 Hertz
9 MW 6512 1.905 4.99 N 1.588 E - 3N/M 3.466 Ohms 40 Hertz

pkquat · November 2019

TY @pitdogg2 for the added info. It looks like there is less to be concerned about.

Gardenstater · November 2019

Looks good! This puppy is definitely brand new. Of course I've been playing "Welcome to the Machine" at high volume and the repaired and re-oriented old driver in the right speaker has had not a single glitch. Probably sensed something and decided to be on it's best behavior

pitdogg2 · November 2019

Only thing different from original is the leeds on the cone are under the dustcap. Just like my 1992'ish 6501

Gardenstater · November 2019

Yeah I noticed that. The whiteout / relabeling looks very old. You can tell the ink is very old. The DCR needs to be measured with a higher resolution instrument than either of my DVOM's are capable of. As near as I can tell it's around 6.5 Ohms. I need to dig out my old Radioshack non-digital model and check it with that, to try to distinguish between a MW6500 and a re-labeled MW6503.

Gardenstater · November 2019

Well my old Micronta VOM is reading 7.0 Ohms and my DVOM is reading 6.5, 6.6, 6.7 (fluctuates) so probably 6.6 Ohms. Probably need a miliohm meter to get a more accurate reading. Pretty hard to distinguish between a MW6500 (6.50 Ohms) and a MW6503 (6.54 Ohms) with these meters.....errr uhh make that impossible.

Gardenstater · November 2019

I wonder why this used black basket MW6500 has silver screw holes? It's almost as if this was painted black with the screws installed. Mfg date 1981, which is just 2 years after my silver basket ones that came with my 7B's.

Polk 7B's - need to know what Mid Woofer Model?

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