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Old 27th May 2014, 05:18 PM   #1081
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Right, that's the same information I have.

Good point about the resistance, that is one thing I haven't done that I should have (do'h). I'll do that soon as I get home and also see what is stamped on the baskets and report back.
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Old 28th May 2014, 04:20 AM   #1082
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Quote:
Originally Posted by Qwin View Post
Most of the info I have come accross says the same bass driver was used in 25's 44's and 66's. The very late 66's changed to a larger dust cap but the magnet, coil assemblies were not changed and there were no x-over adjustments made for them. That's the way I think things went anyway.

Maybe the drivers with the large caps are are in some way defective like you suggested. Maybe 16 ohm? Guessing.

Part/Model numbers on the drivers? The large dust cap version should be part No T.2619 which was usually printed on one of the chassis legs.

Have you measured the resistance of the drivers and compared the two sets?

Inductor part numbers for TBC and PCC boards were the same so you would think identical.

You have probably looked at all this, it just has me stumped, same as yourself.
OK - one measured 4.4ohms the other 6.6ohms. Looks like this accounts for part of the problem. One of them was repaired, and that is the one measuring 6.6ohms.

Good thing I had the spare drivers lying around.

They are marked T.2619 on the baskets.
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Old 28th May 2014, 08:36 AM   #1083
Qwin is offline Qwin  United Kingdom
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The part numbers are correct for 66 very late production with large dust cap, which is what you thought you had.

So it looks like these drivers are the problem.

I would concentrate on the other pair of drivers and the x-over which you seem happy with.

I think its worth mentioning, that in my view Polyprop replacements for electrilytics never give quite the same results. Sure they are more transparent and linear in performance, but if the engineers spent months developing the circuit using the parts they did you can bet that it is right for the job. The use of ESR compensating resistors helps but to me the values being used are only effecting the frequency range around the x-over point. This is where any changes will be most noticeable and the minor differences in what folks are using is probably down to fine tunning in terms of integrating their individual drivers and of personel taste. The problem is that it is a fixed value resistor effecting just a small window. The Electrolytics are effected by ambient temperature, current and frequency to a fairly large degree. The amount of variation in ESR over the range they are handeling will be large. This is why a Polyprop will never work like the electrolytic it has replaced and not allways in a good way.

I read a couple of threads earlier in the week on this very same topic, actually many hundreds of threads but these two grabbed my attention as they were from KEF owners working on speakers from the same period. Lots of similarities, two and three way designs using an ABR and electrolytics in the x-over. One of these chaps had replaced his caps one at a time and noticed the more Polyprops he added the worse they sounded. He ended up removing them, he was reasonably knowledgeable and done quite a lot of DIY projects on his amp etc. He was expecting great things from the new better quality caps but he just didn't like the way they changed the character of his speakers. There was pleanty that they did well but he just couldn't live with all that they changed. You see this time after time on recap projects. Initially the owner is full of praise about the clarrity, transparency, less muddy bass, but how many times do you see them come back and get into endless tweaking of parts and values because after living with them for a while they are not happy with the overall balance of the sound they are producing. The circuit was designed to work with these inferior and very differently behaved parts. If you want to get the best from the polyprops you may need to change the circuit design and taylor what is happening in terms of frequency response and impedence matching, a much more involved process.

The results you get with Polyprop replacements will very much depend on the circuit design and some swap outs will work out better than others. But if you like the sound of your speakers and don't want to "upgrade" the sound and spend lots of cash on bean can size components, consider just renewing the electrolytics to protect the drivers and sit back and enjoy the sound you appreciated in the first place.

This is just my point of view and there are bound to be those that disagree very strongly, but they can not argue with the fact that Llytics and Polyprops are different animals and behave very differently in a circuit regardless of quality.

Rant over
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Old 28th May 2014, 09:48 AM   #1084
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Never ignore > 100 V polyester . Although not relevant to this conversion polyester sometimes out ranks polypropylene ( memory effect ) . My brother laughs at polypropylene . His company uses it a lot , it is used for extreme cheapness .

Where polyester might work better is if using a Quad 303 as I do . When using the capacitor both as crossover and DC blocking ( 33.5 V ) an opportunity for a no money upgrade exists . That is to bi or tri wire and remove a mild demerit of the Quad ( Sugden A48 , and many more , the 2000 uF output cap in series with everything ) . For what it is worth the Quad might be the finest amplifier ever made . Mostly as it's low distortion also has ideal maths for it's distortion harmonics ( a super tube amp with 40 dB better distortion ) . This simple upgrade might be decisive . Being class AB sometimes means it looses detail . Even that is so close to not being so ( Output triples and good bias behaviour ) . If a Quad sounds less than wonderful look elsewhere or repair it . The 405 was not as good although a nice amp . It phase inverts so always have black to speaker red . No one ever says that , why ?

I note that EPCOS 100 and 250 V polyesters are very cheap . I suspect it is the end of the line for them . The 400 V types also . Don't use 63 V types . The build requirements for space dominate the calculations when 63V . The high voltage types as a byproduct have a better tan theta . The 63 V types for want of a better word sound bloated . You can use mains suppression caps . These sound rather good as they follow the same logical . A suppression cap might be 630 VDC and 250VAC ( that is about testing rather than RMS , it implies 445 VAC which would be right , 250 VAC is the UL rating ) . 1uF costs about 50 pence and most likely will be polypropylene . The cheap ones sold by Rapid Electronics ( e.g 10-2508 ) being fine . Having taken so many of these apart I have great doubts about Audio Grade caps . They all look the same to me .

If an electrolytic is replaced be prepared for not liking the sound . Sometimes the designer did have good ears and balanced off the vices . The special electrolytic's used in speakers are exceptionally good at low voltages . In fact they give benchmark figures up to 0.4 V ( - 150 dB distortion ) . I use them a lot in low level circuits where they can considerably outperform all other types . They are dirt cheap which makes people think them no good . After 0.4 V they become ordinary, high voltage types seem the best as before . When a tweeter 0.4 V is about what it might be . Never polarize an electrolytic if you can help it ( respect it's preferred direction as there is always some bias in an active circuit ) . Above 0.4 volts they start to behave as rectifiers . It is surprising how often in low level circuits 0.4 V is fine . And to think , put them in any way you like . I usually find a circuit with 100 % DC feedback sounds best . I make things for people and find the caps replaced . Never mind .
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Old 28th May 2014, 11:19 AM   #1085
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"If an electrolytic is replaced be prepared for not liking the sound . Sometimes the designer did have good ears and balanced off the vices".

Exactly
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Old 31st May 2014, 05:19 AM   #1086
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Qwin,

I definitely agree with you about the differences in the capacitors and their impact and effect on the sound in designs where a specific part was used (such as 'lytics). In almost all of my speaker restorations I have used electrolytics, and in fact most of my speaker restorations have been British speakers that used Elcaps or Alcaps and I replace those with Alcaps to get the absolute closest thing to the original. Such as my IMF RSPM Mk IVs - I used Alcaps and the sound is wonderful.

Back to the polyprops. I was concerned about the sound quality of my Celestion 66s when I through in my crossovers that I put together using the inductors from old 66 boards and Solen polypropylene capacitors for the mid and treble circuit and Alcap for the bass section. I know that even ESR cannot fully compensate for the difference. However, as I sit here and type this I must say the 66s really do sound wonderful. I took your advice and dropped in another 6uF Solen to bring up the midrange filter to 30uF. After some swapping back and forth between the other one with 24uF I decided that it sounds more balanced to my ear.

Unfortunately through this testing I discovered one of the MF500s has developed some resonance issues at very specific frequencies. A shame, but overall it is working well. However it will need to be addressed at some point. I think I will rather find some spare MF500s and do experiments attempting to fix this one before I bother pulling this one apart, as I fear if I mess it up I will be stuck without any midrange at all in one speaker and that won't do.

Another thing I wanted to mention was charge coupling - ever heard of this technique? You apply a 9volt battery to the crossover, add in resistors and double the capacitance - it results in electrolytics performing much better. I have had one pair of speakers with this modification done and I compared them directly against the same model that had just recently been recapped (using Alcaps to replace Elcaps) and couldn't believe the difference. It's a cheap but very effective way to improve the speaker's performance and may alleviate any concern about swapping out different parts.
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Old 31st May 2014, 06:31 AM   #1087
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If I can find it I will try to show how badly an electrolytic cap performs once charged , I have some data somewhere . Naturally when in a PSU or as the output device there is no choice . Folk-Law has it that charging the caps to half stated voltage is ideal . Inside the cap is a lattice of resistance and inductance . As 0.4V is reached a rectifier effect kicks in .This makes the open sound of the sub 0.4V become dull and pinched ( sorry to ascribe it a sound , best to say something to give a hint ) .

I suspect what the person thinks is that two standard electrolytic's will be cheaper or easier to get . To polarize them might help them carry more current .

I use a large number of non polars in my work . None of that is loudspeakers . I use them because they are very good . A cheap Panasonic or Nikkia non polar kept below 0.4V will out perform an expensive Black Gate cap .

One trick I have seen is a non polar and poly cap in parallel . About 90 % non polar and 10 % film type .

One cap you could try although I have doubts would be Rifa Evox paper X2 grade suppression caps . I suspect they won't be far away from the old paper caps JBL used . If paper it almost becomes possible to make them . I dare say if bulk is not a problem kitchen foil would be OK . I would use cling film also to compare as dielectric ( cling film and toilet paper ? ) . Mostly we see 10 V rms so flash-over should not be a problem . A cheap capacitance meter and reference cap will be needed and clone caps you have ( open one to see how it is done ) . The " bad " thing about being bulky might sound better . That is sound exactly like the electrolytic , yet maintain that sound at higher volume ( > 0.4V ) . The bad thing being loss of > 20 kHz perhaps . Reality say more like 50 kHz .
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Old 31st May 2014, 07:23 AM   #1088
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Thought I should say a bit more . With a non polar cap what we are getting is a similar thing to class AB amplifiers . That is we maintain class A as long as we can . This technique has been discredited by the " measurements are better than ears " brigade . Another thread I am on one guy makes an old fashioned AB and has pushed it to the right level to work . Here we have the same . The more critical levels are served by the cap at it's best .

In my work I use tons of measurements . Mostly I ignore them beyond a certain point . After the listening phase I measure again . Usually the design even in measured terms has advanced . The measurements from then are to say the amp performs to book spec . Now to the crunch . Any deviation will not sound the same . Why it sounds good is seldom totally in the measurements except to say it is or it is not working to book .

If interested . The ideal production test would be a final null test against the prototype . That would be a musical signal or another fed in to both that devices that will produce 0V output if in some form of anti-phase . Tools designed for testing should never do the listening . To stop daft designs they are invaluable . Many things I design have their daft moments .

The polarized caps being better might be simply be that they are better devices . Paper caps should win all the arguments the more I think about it . The difference in construction between paper and non polar is zero . The difference is the electrolyte . This for want of a better way of saying it mimics a battery . In a non polar it has to polarize every time the voltage changes direction/polarity . Inside the cap both foils are shiny . Take apart a polar electrolytic to see that one side of the foils is mildly matt . That capacitor must not be reversed . Up to 0.4V you can reverse polar caps with reasonable success . For example in an amplifier input long tail pair it probably will be OK ( feedback cap ) . As soon as current is involved you can't . Thus a battery is required . In theory 9V is just about enough . Ideally it should be more . The 0.4V is the tipping point . After that it is more about what gives the better protection of the caps . I dare say 63 V caps with 30 V polarization would be best . 63 V electrolytic caps have far better tan theta ( 0.2 reduces to 0.12 perhaps ) . I dare say 200 V types set to 100 V would be ideal if taking that route . To keep a cap safe at high voltage also makes it sound better , it is a byproduct . All of these things cost very little to try so might pass the winter months very well . If you take the crossover outside of the speaker to test it might sound slightly different when returned to the box . Most likely inductors close to big magnets cause that .
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Old 31st May 2014, 08:30 AM   #1089
Qwin is offline Qwin  United Kingdom
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Jeffrey88 - Not come accross Charge Coupling before, interesting.
Sorry to here about your MF500, try and find a replacement and do like you described.

Nigel Pearson - Thanks for the input on cap construction/workings.
I will be keeping my mods simple at first, just replacing like for like and keeping the x-overs internal. I want to keep the speakers as original as possible. Its not easy to do external x-overs on these sealed boxes even while just experimenting. I put the cables through the vent port on bass reflex models, but with the ABR and sealed system it would mean making a hole/holes big enough for the three sets of driver cables, so not ideal.
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Old 31st May 2014, 09:31 AM   #1090
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I only say it because I screwed up some speakers that way . It took me years to admit it .

Big speaker drive unit holes are a blessing aren't they .

For non polars buy high voltage types .
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