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Old 21st January 2008, 11:50 PM   #161
John L is offline John L  United States
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Here is another couple of questions that needs to be answered soon for me. I plan to remove the dust cap of the Pioneer B20, and set up a phase plug, which will have the small PE tweeter mounted on the end.

First, Is there a certain amount of length that the phase plug must have in order to add to the value of the sound, thus causing the tweeter to stick out further? I am ignorant as to how the setup would work out in theory and practice. In other words, never done this before. Also, this will make a big difference as to how I am going to have to place the driver ANd the diffusion lense since it will be working with the B20 and the tweeter together.

And Second, since the B20 does not have a vented pole piece, is it possible that a hole can be drilled in the midle of the magnet structure and use it the tweeter wire. I am pretty sure that the center portion of the magnet assembly is not really the magnet, as the magnet is round and donut shaped, correct?

Let's say a hole was drilled and the wiring for the tweeter was run through the hole, and then the hole was resealed by something like eposy or hot glue, would that work without changing the T/S parameters? Or should I just route the wire through two holes in the cone that have been re-sealed. I would really like to try the former process if there is no real trouble there.

Does anyone have any knowledge they can put forward here?

Thanks.
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Old 22nd January 2008, 12:04 AM   #162
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Quote:
Originally posted by John L
Does anyone have any knowledge they can put forward here?
Drill swarf and VC gaps don't mix well.
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Old 22nd January 2008, 12:16 AM   #163
John L is offline John L  United States
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Quote:
Originally posted by pinkmouse


Drill swarf and VC gaps don't mix well.
You are right, and I have thought of this. I had planned to take plastic, or paper and completely cover the area around the voice coil and then tape it down so only the center of the magnet assembly would be exposed. if I drill from the back of the speaker, all the swarf would come out of the rear, where it could be cleared away.

Then carefully inching my way through to the front would be slow and determined, to insure all the shavings from getting loose and into trouble. Also, a large bit would not be necessary, would it? The less you drill out, the less swarf you would have.

Does this make sense?
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Old 22nd January 2008, 12:22 AM   #164
defect9 is offline defect9  Ireland
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Quote:
Originally posted by pinkmouse


Drill swarf and VC gaps don't mix well.
Isn't that the truth.

However, from a DIY perspective, this can be avoided with precaution. First, do the dustcapectomy (great word). Then mask off the VC gap (all the way from the pole piece (including the entire pole piece itself to be safe) to just past the edge where the dustcap used to be. Masking tape is what i'd use. Drill your hole. Before removing the tape, make sure the entire area including the hole itself is completely flushed of swarf. I would see if using a stronger magnet thats small enough to root around in there would work, but odds are you'll be picking it out manually with tweezers.

or theres the safe route, which is to not do it at all, but that's no fun

-Jared

*edit* you posted as i was writing. guess you already thought of it, John
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Old 22nd January 2008, 12:31 AM   #165
John L is offline John L  United States
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Quote:
Originally posted by defect9


or theres the safe route, which is to not do it at all, but that's no fun

-Jared

*edit* you posted as i was writing. guess you already thought of it, John
You are correct, the safe way is never any fun. Besides, it's not as if the B20 is a king's ransom. They do have more that are for sale.

Also, like minds must think alike too.
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Old 22nd January 2008, 01:25 AM   #166
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If you must drill through the pole ( not my thing either, I'd mount the tweet on a "bridge" across the width of the woofer) use what they do for re-threading a cylinder head spark plug hole without taking the head off - grease. Grease will catch the cuttings. Go slow, clean often.
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Old 22nd January 2008, 02:04 AM   #167
John L is offline John L  United States
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Originally posted by MJL21193
If you must drill through the pole ( not my thing either, I'd mount the tweet on a "bridge" across the width of the woofer) use what they do for re-threading a cylinder head spark plug hole without taking the head off - grease. Grease will catch the cuttings. Go slow, clean often.
That's a great idea.!

Try this. Suppose I deposit a small glob of grease where the drill bit will come out, and cover that grease with some tape that traps the entire grease spot. Then when I drill through the pole piece, I will know when I am out the other end, and then quickly back out again without breaking the tape seal. Then remove the tape, clean away the grease. Then feed the wire through the pole piece, going from front to rear, so as not to pull any possible shavings out the front. And finally sealing the drilled hole with epoxy or hot glue, keeping any more shavings from having a chance to escape and possibly foul up the voice coil.

Is that being careful enough?

I have a problem with mounting the tweeter on a bridge because I fear that I am going to be in a pinch trying to get the diffusion lense to fit nicely over the drivers and look like they naturally fit there. The diffraction lense lower half is going to be exactly 5 inches high, and the upper half will be about 2.5-2.75 inches. If I am forced to set it up too high, it will look out of place.

Also, I had asked earlier today if there would be a problem lowering the B20 down a bit, perhaps mounting it underneath the upper lid, instead of on top of it. Does anyone have any thoughts there? Normally that would cause trouble on a conventioal driver mounted to the rear of the baffle. But in this case, I would not think there to be any problem since the diffuser will be reflecting the sound evenly.
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Old 22nd January 2008, 02:15 AM   #168
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Quote:
Originally posted by John L
Here is another couple of questions that needs to be answered soon for me. I plan to remove the dust cap of the Pioneer B20, and set up a phase plug, which will have the small PE tweeter mounted on the end.
I haven't gotten any more work done on the tweeter on the phase plug. I feel that the top of the tweeter should be at about the top edge of the whizzer cone. My approach is to take 1.25" dowel. Trim to length. Fit the tweeter on the end, glue it in place, then shrink wrap the entire structure, then EnABL that (i believe Kensai has his B20s back and is preparing to comment on them). I figure screws/magnetic attachment won't be sufficient, so the assembly will probably need to be glued in place.

I would forget any notion of drilling a hole thru the polepiece, This is hardened steel and for the cost to get it done you might as well just start with something more suitable. Further the proper way to do it is to remove the charge from the magnet, drill the hole and then recharge it (the heat & vibration might cause you to have to recharge it anyway). This is best done with the cone removed. I have seen it done, but the guy had a recharger and he started with a coneless driver assembly.

GM's suggestion of a pair of twisted magnet wires strung across the front makes way more sense.

dave
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Old 22nd January 2008, 04:35 AM   #169
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Quote:
Originally posted by John L


Suppose I deposit a small glob of grease where the drill bit will come out, and cover that grease with some tape that traps the entire grease spot.

Do you have a drill press? I don't - I do my drilling free hand. I've never tried to drill through a speakers pole before though.

I think a 3/16" bit would be a good size to try. Even if starting at the back, I'd still slather the bit with grease. Lubricates the cutting action. For that matter, they used bacon grease for cutting lube in the old days, so you might give that a try if you have any kicking around.

A thimble (for sewing?) full of grease would stick to the pole magnet on it's own, without tape.
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Old 22nd January 2008, 04:39 AM   #170
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Quote:
Originally posted by John L
And here is what a cap looks like with the hole routed out and a reinforcement underneath the cap.
You need to open that hole up on the back side so the driver can breath. We use a 45 degree champher.

dave
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