SpeakerTalk

[JimFromSurrey]

can anyone tell me which are the correct B139 drivers for my R50's?

most of the sources on the net say it should be a B139 SP1044 which are these

http://img.usaudiomart.com/uploads/large/916101-kef-b139-sp-1044.jpg

but the ones in mine are a 'smoother' oval and look like these :

http://images.audioasylum.com/usr/y2012/03/58011/B139front.jpg

which seem like they are SP6171 (?)

it seems l;ike the early R50's had this shape ones and later cabs changed to the 'flat sided' ones

so....

i need to replace one of them, which do i need ... is it SP6171 in the earlier R50's?

is the SP1044 better (power handling, excursion, go lower etc)?

looks like the work to the baffle to accomodate them would be easy enough to do ... anyone done this?

I obtained (very cheap), some B139 4 ohm versions (i gather they are TS1212 from a KEF C80 speaker) ... these again are the SP1044 shape, but would anyone know what the effect of using these would be?

keen to get the best that i can from these speakers so willing to do the woodwork to upgrade to the SP1044 if it is in fact an upgrade

finally i 'might' have a chance to obtain some KEF B169's ... apart from the obvious 3 inch difference, would these work better than all the others? any idea if they will physically fit in and if the parameters are good for the cabinets?

thanks folks!

Jim

[speakerguru]

The correct speakers are obviously the ones that look the same as your old ones. That said, there was no design intent to change the bass unit characteristics when the chassis was changed. The original chassis had a large flange so that one dimension would be greater than 12". This put it into the industrial speaker category and made it exempt from purchase tax (precursor of VAT). Purchase tax had been abolished when the diecast chassis tooling came up for replacement. The new chassis incorporated an accurate pin location system which made assembly shim removal much simpler.

Later on there was a general tightening up of voice coil parameter tolerances which coincided with a change from using self bonding wire to high temperature coated wire, "wet wound" with an epoxy adhesive. The magnet assembly also changed to being bonded from being screwed together. Neither of these changes had any significant effect on the driver parameters.

Finally don't use 4 Ohm units in a system designed for 8.

[JimFromSurrey]

thats for the informative reply, i had no idea it was down to the 'tax man' !

so is there just the one version of the 'rounded off' ones? any that look like that (and are 8 ohm) will work?

i've seen some with 6171 numbers and then some others visually the same but with a different ident number ... was getting concerned i'd buy the wrong ones if i just go for some that look the same

the one that needs replacing just 'farts', is this typical of a particular fault? can they be fixed or are they bin fodder?

thanks again

[speakerguru]

The "rounded off " ones were a bit before my time at KEF, so I'm not the best person to answer. However 6171 is a spec number, so, if it's an 8 Ohm then any other impedance would have had a different number.

Depends on what you mean by fart. Is it a scraping noise? Voice coil not centered in the magnet gap? Surround torn and leaking? Something else?

Because of the flat solid diaphragm, with no dust cap, the magnet was removed after build, to get to the centering shim to remove it and then the vc centered acoustically! Very hit and miss! Hence the pin location in the redesigned black chassis.

If you suspect recentering is required, with the unit supported face side down, you could try loosening the 4 screws which hold the square magnet top plate to the chassis and then careful and gently move the magnet up/down/left/right and see if the noise goes away. Use about 2v @ 30 Hz