Has anyone built a Large DD coil for there Minelab GPX 5000/ 4800/4500?

[adam(Va)]

I have considered building one but have heard that it is easy to "smoke" the detector. Haven't seen to many posts about folks building one. Mine is still under warranty so I don't plan on opening it up to mess with any dampening resistors or take any oscope readings. I do have an lcr meter and can verify that the winding inductances/ ohms are close as compared to the stock DD coil.

[Kev]

Hi Adam,
I've not attempted a DD but I'm working on a second mono. The first didn't work, I used nickel loaded paint for the shield and it beeped randomly, its now in the tip. The problem with nickel is that it displays some magnetic viscosity, superparamagnetic effects and my 4500 didn't like it at all.

Otherwise it was a good learning experience. Minelab PI's operate in the linear region of the coil time constant and require a very low resistance coil. For instance, my Coiltek 14x9 non Blitz mono uses tin plated copper wire, it has a resistance of 0.47 Ohms 0.08 of which will be the coax. It has an inductance of 300uH which gives this coil a Q or Quality factor of 4, and a time constant of 638uS.

My Nugget Finder 14 x 9 Advantage mono uses Litz wire and measures 0.33 Ohms in total, 295uH inductance giving a Q of 5.5 and a tau of 883uS.

The Nugget Finder is much more sensitive than the Coiltek, getting better depth on all sizes of Au, it is even much quieter as far as atmospherics and interference is concerned. The reason being I believe is due to the Nugget finder being much nearer the maximum effective Q of the front end design, and because Litz wire is used.

Some may expect that the Coiltek having the shorter time constant will allow a greater magnetic field to build in the coil before TX off, but I believe this is negated by the extra noise levels the pre-amp has to deal with, not only from the extra current but the fact that more eddys are produced within the tin plated coil wire itself.

Even the coax cable, inner and shield, needs to be made from enamelled copper wire, much like Litz to really get a coil worth the totting around.

I'm presently finishing off a second coil which also appears to be a flop as I can't get it to ground balance correctly.

Making coils for GPXs is fraught with frustrations and you need to be interested in experimentation and have bucket loads of patience and perseverance, if you've been a nugget hunter for a while you'll likely have what it takes.

Do a search for Stefan's posts here, he has a wealth of knowledge about Minelab coils, and he's bent on removing all the compromises, something which drives me also. To be able to maximize a coil for a particular ground type and target gives you a measure of advantage over the other operators.

The new Coiltek Blitz uses Litz wire so will be much more sensitive I expect, besides the coil parameters have been tightened up with the 5000 and it is now more important to get all the details correct.

I must get back to working on this new coil which will be totally waterproof, and even now with its problems is proving much more sensitive than the Platypus.

Cheers
Kev.

[adam(Va)]

Kev,
Thanks for taking the time to make such an informative reply. I probably should have included some more info in my previous post. I don't know whether this makes a difference or not but my primary use of The GPX 5000 is for searching American Civil Relics (mainly non ferrous; bullets, buttons, belt plates/ buckles coins). My biggest obstacles are finding these items in moderate/ to heavy mineralized soil that is littered in small to large iron (nails, barrel bands, ration cans etc...). Finding an item similar in size and characteristics to a small grain/ nugget isn't my main concern. A similar situation that you might be hunting in (and this would only be a guess) would be trying to find a medium sized nugget ( size of a .50 cal bullet) in the middle of a late 1800's mining camp. Also I hunt large open fields that have scattered nonferrous targets at various depths with iron mixed in. By the looks of things I probably need both a Small DD and a Large DD. I have been able to run my GPX in mostly the Sharp and Normal Soil Timings with no issues (just to give you an idea on my soil mineralization). Don't know whether the coils I would build would need to be so precise. I just want the large DD coil for ground coverage w/iron disc and the small for getting between the iron in a littered spot. Would standard enamel coated copper wire be ok in this situation? How about silver plated copper teflon coated wire? How close does the resistance and inductance % of accuracy have to be before you damage the detector? Thanks Adam

[Kev]

Hi Adam,
I've noticed a big demand on ebay and the likes for the old 18" DDs that were supplied with the Extreme and early SDs, I expect it is driven by relic hunters like yourself. I too have thought of constructing a large DD, not for disc, but for using Cancel under and around power lines.

The 5k is very strict about resistance and inductance of the coil especially on the TX side, the Rx side is a lot more forgiving. You would need to get the parameters quite a way out to damage the detector, it is more likely to occur from shorts and things like that, and even then the system monitors coil current and will warn you of an over current condition.

I've blown my Tx transistor twice and it was with one particular coil, I wont mention the brand. It had a resistance that was 30% higher than the standard 11" and both times it happened I was using Sharp, which drives 20% more current into the coil. So If you are really careful about getting the inductance and resistance pretty close for the TX coil you should be OK.

The problem is to get the parameters very close you need to be very fussy about your wire choice for the TX coil, you need to use either Litz or multi-strand tin (not silver) plated wire and it needs to be in the region of 17 AWG all up, no smaller than 18 or larger than 16.
The Rx coil is not as fussy, but it should be teflon of PTFE coated tin-plated or litz for best results. I can't remember what size now as I've been focusing on monos, but by measuring your 11" Rx you'll be able to calculate what is required.

You first calculate how many turns you will need for a given size of coil and inductance, you then know from this how many metres long the wire needs to be. You already know what resistance you need, remember to subtract the coax R first, this then leads to the wire cross sectional diameter.

I find this spread sheet very good for calculating resistances of coils from a given number strand/diameters. There's 2 sheets one with respect to gauge and the other diameter. I'm using Linux and have not experienced any problems with this sheet but if your running a windows box be sure to virus scan it before opening it.

http://www.google.co.nz/url?sa=t&rct...RNcjWw&cad=rja

Another point with DDs is the balancing of the coils, and you will need to get this spot on to get the disc to work correctly. There was one Guy on here a couple of years ago who made coils for his Explorer, CharlesUpstate I think his user name was, by reading all his posts you'll get a good idea of what you're in for.

My wife's gardening and getting a bit irate I'm not helping, so I'd better be a good boy and go and do something "constructive"

HH
Kev.

[adam(Va)]

Kev,
Thanks once again for all the info, You provided exactly what I needed to know. I might hold off a bit before trying to build one now and look into into purchasing one instead. Take Care, Adam

[Kev]

Kev,
Thanks once again for all the info, You provided exactly what I needed to know. I might hold off a bit before trying to build one now and look into into purchasing one instead. Take Care, Adam

Hi Adam,
If you find an 18" DD works as well as you hoped you could then look into building something even bigger like a 24 x 18 this is what I was thinking too. I've bid on a couple on ebay but missed out, as they went for pretty serious prices, perhaps double what they were before the 5k came out.

HH
Kev.

[visio]

Is there a published chart anywhere that lists all of minelabs GPX compatible coils and their min/max depths in light/med/high mineralized soils. I too am looking to run a larger than stock coil with a GPX 5000 for deeper/larger non ferrous targets. Am I correct in assuming that running larger coils shorten battery run time.