Phil Allison wrote:
"jasee"
Phil Allison wrote:

Actually I was rewinding the voice coil. (I've done this
succesfully before, but not with this tweeter). It seems that the
enamelled copper wire that was used was not standard enamelled
copper wire as the total length of the dual layer coil was about
2.84metres which in 40SWG gives a dc resistance of about 3.7ohms.
However the original which appears to be the same gauge measures
about 7ohms. So that is that, unfortunately, as far as I can see.
:-(


** KEF's data on the T27 says that the voice coil is rated for
continuous operation at 130 C.
It also says that it can withstand 220C for 5 seconds.
Suggests that the enamelled wire is not low temp ( self fluxing)
grade.

Thanks for your reply.
The standard stuff I've got has a 'rating of 155C'.
I wasn't aware that there were any other grades!
Having had a quick look around, there are loads of types! Colour is
no guide (you can have any colour you want!) I must have been just
lucky the first time!
SWG in standard enamelled wire seems to define an overall diameter
and a depth of insulation. The measured diameter was the same. If
it is standard copper then it would have to be significantly
thinnner to make the dc resistance (which it doesn't appear to be).
So, still a bit puzzled.


** My calces give 0.1 mm dia *copper* as having the appropriate
resistance value for you given length.
That could translate to 0.2mm overall and maybe = 40 SWG.

0.1mm gives about the specified dc resistance of 6.25ohms, maybe this could
translate to 40swg in special cases (I thought swg referred to the _overall_
diameter (incl insulation)) This seems to be borne out using a micrometer on
the wire I have in stock. Obviously with the thinner wires the effect of the
insulation layer becomes more significant

155 C grade enamel is in the middle of the available temp range - 220 C is
the highest IIRC.
It could be that KEF actually used 220 C enamel, but the adhesive to
hold it on the former is only 130C safe.

Can't actually find a good comparison chart with detailed results compared
enamelled wires.

Surely there are standards? According to one 40swg enamelled wire has a
diameter of 0.7988 and a resistance of 3.35ohms per metre which is way too
high.

I suppose if I get the number of turns right over the same area, with the
same dc resistance, it shouldn't actually matter (much!) if the wire is
thinner! Though I suppose I will theoretiically have to add mass to get the
resonance the same!

Rewinding tweeter voice coils must make brain surgery look easy.

It's not _that_ difficult if you have a binocular microscope, good lighting
and patience. The original wire has left grooves in the varnish (as before).
I supect that the original may have been done by hand as there are slight
traces of uneveness.
























...... Phil