Voice coil bobbins for cutters - Test phase 1

[L.K.R.A]

Yesterday I had a quick bash at making the voice coils bobbins for the cutters...

I managed to make 5 in about an hour...

I still require the correct epoxy resign and varnish sealant to do them properly and then wire them up, but so far so good...

I also plan to get the bobbin bits die cut "instead of scissor and stanley knife"

Check out these...

[markrob]

Hi,

Looks great. Should be useful for anybody thinking about re-winding a moving iron head (Presto Audax, etc.).

[mossboss]

We most likely have a die to cut these things and you are in Melbourne au so swing past. We give you enough to last a life time in about 10 minutes. You can also post them to others.
Also you can go to the local motor rewinder and get all the varnish you wish you will not be wanting any more than a cupful
Than you can spray them stick them in the oven at rec temperature. They be fine
Make sure the roast leg of lamb is not in there at The same time, but they get a bit smelly In the process.
Cheers

[L.K.R.A]

Thanks for the offer, but I've got it covered already...

Had a few cut out yesterday & can get more done up anytime I need

Still have to get a hold of the varnish, and will have to hand paint the parts with it, then assemble and varnish again for best results

I had a play around with wiring one up yesterday - only had 0.7mm solder wire to play with...

I got close to 2 feet of the solder wire onto the bobbin so the 29 gauge wire once suggested here on the forum to make 8 ohms should fit...

Here's a few more pics

[L.K.R.A]

I've found a local store that repairs amplifier transformers, they sell enamelled wire and have 29 gauge on their list so i'm going to check them out tomorrow... will also see if they have any varnish to spare...

I'd like to have a go at winding a 15 ohm coil - could anybody tell me which gauge wire and how many feet to use???

[L.K.R.A]

I didn't manage to get to the store to pick up the copper wire today so it will have to wait until Monday...

I did however get time to conduct a second test...

Turns out I have a heap of 9lb fishing line that is 0.28mm in diameter (The same as 29 gauge wire)

In previous topics about rewinding coils on this forum it's mentioned that 8 feet of 29 gauge wire was used to rewind a presto 1d 500 ohm head to 8 ohms

So using the fishing line as a reference to what I should expect from the 29 gauge wire, I quickly wound a bobbin and put about 11 feet of the fishing line onto it... (my crappy little tape measure only goes to 10 feet)

I believe that I could max out the bobbin at about 12 feet if I wind it clean and fill the bobbin to the brim...






More testing and photos to come...

[masterscott1]

Yesterday I had a quick bash at making the voice coils bobbins for the cutters...

I managed to make 5 in about an hour...

I still require the correct epoxy resign and varnish sealant to do them properly and then wire them up, but so far so good...

I also plan to get the bobbin bits die cut "instead of scissor and stanley knife"

Check out these...

Very cool , good work on hard to find stuff , I merely make the coils and seal them , no need for any bobbins that way.

[L.K.R.A]

Thanks, not having any experience with voice coils before I thought id start out with bobbin type coils - im pretty sure that when I have a go at a coil with no bobbin the first few will look like bird nests... I've also been advised that i dont have to be too fussy when winding a bobbin where as i would have too be a bit more carefull to wind a coil without a bobbin...

I also own a presto 1C which will be next on the rebuild list if the bobbins hold up and pass testing...

well im off to pick up some wire n source the varnish in the morning....

Stay Tuned!

[L.K.R.A]

Picked up the enamelled copper wire today...

when I got back home I quickly wound some onto a bobbin, (about 10 feet) - Once wound I measured with my multimeter

Now I am not sure weather my multimeter reads dc or ac but the reading confussed me...

Nearly 6 ohms - A picture tells a thousand words...



I thought that my multimeter may be off so I checked it with a couple resistors

2x 180 ohm 3 watt & 1 x 180 ohm 2 watt - (All Tested good and the multimeter is accurate)

[markrob]

Hi,

Your meter reads DC resistance. 6 ohms is just about right for an 8 ohm coil. Keep in mind that the stated impedance is really an iffy thing. Its more of a general figure of merit, since it varies over the frequency range. The spec doesn't even tell you how reactive the load is. Many times, with speakers its the value at 1 Khz. In general, the DC resistance will be lower than the stated impedance. You should be good to give the head a try.

Mark

[L.K.R.A]

hi mark, i think that the batteries were gettig low so i've thrown them on the charger for the night... will revisit tomorrow with full chagre...

found 2 suppliers for the varnish... will register an account with one of them over the next couple of days. also ordered some cloth wire- should arrive 2morrow or the day after...

[L.K.R.A]

Yep - Flat Batteries...

Being low the results from the multi when set to read x 1 ohm values were off...

Now charged up - I've popped them back into the multi and set to read x 1 ohm value

Connected the positive and negative and signal strength was much better - calibrated to 0

Connected the coil and results are much more consistent with markrobs post... 0.8 ohms dc

Double checked with the resistors and alls fine now...




My question is now... is this coil still suitable for 8 ohms on its own or would it be 4 ohms and then when in series with another coil (as per presto 1C & 1D heads) would then make the rated 8 ohms

[markrob]

Hi,

It might be a bit on the low side of DC resistance. Did you measure the length of the old coil? Was it a 500 ohm winding? If so, I can help you estimate the required length.

Mark

[L.K.R.A]

the old coil i'm replacing is a 15 ohm - when i bought the wire yesterday i had the guy measure the gauge, he said it was a 37 gauge. he had 37 in stock but it was expensive and i couldnt afford to buy both spools...

havnt measured the length.. would have to soak the coil in solvents to remove the varnish...

ive just been sort of replicating your old post. "8 feet of 29 gauge resulting in a dc resistance of 0.84"

i just peeled the copper off the bobbin i measured and theres 11 feet of 28 1/2 gauge = 0.3mm giving me 0.8 dc

ive got a coil winding machine lined up and when it arrives i'll be able to get a bit more onto a bobbin...

[L.K.R.A]

i also remember seeing a post on here somewhere where it was mentioned that a pierre clement had about 4+ meters of 37 gauge resulting in a 15 ohm head...

im no good at the math, so if you could do the calculations that would be great!

[L.K.R.A]

i also remember seeing a post on here somewhere where it was mentioned that a pierre clement had about 4+ meters of 37 gauge resulting in a 15 ohm head...

im no good at the math, so if you could do the calculations that would be great!

My bad - it was an ar sudgen that had 4+ meters 40 gauge resulting in 15 ohms....

[EmAtChapterV]

Wikipedia is a good reference: http://en.wikipedia.org/wiki/Awg - according to the chart, 4 metres of AWG40 would be 13.76 ohms DC. Your 11 feet of AWG29 ought to be 0.9 ohms. If you bumped it up two gauges to AWG31, you'd be able to get 58.9% longer wire in the same amount of space, so about 17 1/2 feet, and would end up with a DC resistance of 2.28 ohms, which will no doubt make your amplifier a whole lot happier than 0.9 ohms. To get a proper 8 ohms (actually 9.1) at DC, you'd need 35 feet of AWG34.

[L.K.R.A]

Cool, Thanks for the advice...

Been using this chart this arvo to do some math...

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/wirega.html

"If this chart is correct - I'd agree that the 11 feet of 29 gauge should be at around 0.9 and that my analogue multimeter is off by 0.1 ohm (Not too bad for a cheapy)"

Also if I was to replicate the original 15 ohm coil with 37 gauge I would require 28.75 feet to get to a dc value of 15.03 ohms

"I'm thinking about just bighting the bullet and sending this 29 gauge to the scrap yard and getting some 30+ gauges"

"Also thinking that I should bight another bullet and get a proper impedance meter like this"...

http://www.ebay.com.au/itm/Test-Equipment-Audio-Impedance-Meter-MTQ2005-/250924234064?pt=AU_B_I_Electrical_Test_Equipment&hash=item3a6c3ff550

[L.K.R.A]

To get a proper 8 ohms (actually 9.1) at DC, you'd need 35 feet of AWG34.

I just checked the 34 gauge on the chart I've been using... 35 feet = 9.1

So I did the math and...

30.75 feet of 34 gauge should give a DC reading of 8.02

"The 31 & 34 gauges look like a good options - given tests so far on the bobbins using 0.7mm solder wire I got near 2 feet, 29 gauge 0.3mm got near 12 feet so"...

17.5 feet of 31 gauge or 30.75 feet of 34 gauge should fit on the bobbins nicely...

[L.K.R.A]

I've just ordered 31 gauge, 34 gauge & 4 Oz of Varnish

The products are coming from California and living in Australia it will be about a week or two before they arrive....

"Should then be able to produce coils with a dc from 1 to 8"

Next on the list will be that ac impedance meter with 1k test tone...

Stay tuned!