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just to clarify...all coils made for a Minelab Pi are interchangable...however, each Pi had coils that were made for it. The commander coils are no doubt the best for 3500 to 5000's.

Also, any coil made for a Minelab Pi can be used with a TDI...you just have to find the correct settings.


Just with the Minelab PI coils being used on the TDI's. I've heard that once you get over about 14" with some coils the TDI's struggle to run them regardless of pulse delay settings etc? Was wondering if anyone has had any success with a larger coil on a TDI? Most, in Australia at least, seem to have success with smaller coils on the TDI series like NF Sadie & 12" x 7", 14" x 7/9" etc. Only ever heard of one being used with a 16" Coiltek but never seen or heard back any results with it, although I see one bloke said he tried one & then sold it going back to a Coiltek Blitz 14" x 9" which runs fine at 10uS. They seem to do well on small gold with the smaller coils here @ 10uS.

Was wondering if this is why Miner John/Razorback are making coils specific to the TDI & Minelabs I.e. they have a 14" round for the TDI but it's not compatible with the Minelab PI's. They do however have other coils specifically for the Minelabs.

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The Minelab coils and TDI coils are not the same. The TDI is very forgiving and can run most Minelab compatible coils. The Minelabs however are more demanding and will not run coils made specifically for the TDI.

Some larger Minelab coils overload when run on the TDI at lower pulse delay settings. I tried quite a few briefly to test this and found that the pulse delay had to be run from 10 uS up to 17 uS or more to just get the coils to work at all in the air. No telling what exposing them to bad ground would do.

In my opinion the coils that are going to works best on the TDI are those made specifically for it. I have no real evidence to back that up however, just a feeling based on the fact that the Minelab coils are "out of spec" as far as the TDI is concerned. In general it does appear from the evidence that the larger the Minelab coil is the less likely it is to run properly on the TDI, or at least without having to increase the pulse delay setting.

I would also be interested in hearing of any success with oversized coils on the TDI. The largest I actually used for any real hours was the stock 12" round coil. This was my best find with the TDI, a 1.93 oz specimen at Moore Creek, Alaska. http://www.detectorprospector.com/steves-mining-journal/whites-tdi-moore-creek-alaska.htm

More on subject here


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On 6/20/2015 at 6:41 AM, fredmason said:


if Reg is around he could best answer your question...i had the GS5 which was the TDI's daddy...and I am certainly not an expert.


Hi Fred - I actually found a post by Reg from a year or so back where he explains that the TDI was never designed to use very large coils but went on to say that with some testing & higher pulse delay settings it should run larger coils that can be used with a Minelab + that it is extremely forgiving with coil sizes.

Link deleted since Findmall Forum update broke all old links

I tend to agree with Steve that coils made specifically for the TDI would be best considering they will still work at 10uS. Jason from Razorback Coils had also made this comment in regards to larger coils " The biggest one would go on the TDI would be the 16" round mono. The TDI does great at that size for larger items and bigger gold. After that size the TDI really does not gain any depth with the bigger coils there is just not enough power there to push any deeper." This would be why they have their available coil sizing no doubt.

I think between Steve & what I've found that I've got an answer but it would still be interesting to hear/see any success with larger coils on the TDI.

Steve - that is a nice lump of a specci!

Sorry for getting off track on the other GP/GPX thread - I have big coils & deeper gold on my mind lately. A by-product of too many hours on the SDC?

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I've swung most every coil made for the TDI and those that work best for small gold in nasty soil are the 6x10 elliptical DF, 6x10 folded-over Miner John or the 7.5" round Aussi mono. Bigger loops and the machine just struggles because it  doesn't have the power for consistent depth and stability. 

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I agree that a coil made specifically for a given machine is usually best.  Same with size...the manufacturer generally knows what their machine can tolerate well...

good luck


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  • 4 months later...
On 6/20/2015 at 9:19 AM, Steve Herschbach said:

The Minelab coils and TDI coils are not the same. The TDI is very forgiving and can run most Minelab compatible coils. The Minelabs however are more demanding and will not run coils made specifically for the TDI.

Some larger Minelab coils overload when run on the TDI at lower pulse delay settings. I tried quite a few briefly and found that the pulse delay had to be run from 10 uS up to 17 uS or more to just get the coils to work at all in the air. No telling what exposing them to bad ground would do.

In my opinion the coils that are going to works best on the TDI are those made specifically for it. I have no real evidence to back that up however, just a feeling based on the fact that the Minelab coils are "out of spec" as far as the TDI is concerned. In general it does appear from the evidence that the larger the Minelab coil is the less likely it is to run properly on the TDI.

It appears the reasonable argument is weighted in favour of coils that a made for specific brands of detectors.  

I'd just love to pay $170 Oz dollars (used) for a 14" NF, but the RazorBack will end up being the one I buy, even though it's nearly $300 Oz money (new).  And of course it is a Blue Moon if you see a used RB available used.  There's a message in that.  At least I would think so.

Thanks for this one, Steve. 

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I have used an 11x17 NF Mono on my TDI coin hunting and it ran great.

Nobody has mentioned the use of a Minelab DD coil on the TDI. I think it was back about 2010 I saw a blurb in one of Reg's post about discriminating with a TDI/DD combo. How it worked(as near as I can remember) was as the center of the two windings(blade effect) passed a ferrous target the tone would swap. On non-ferrous targets or really small ferrous targets the tone would stay the same. Well, I gave the technique a go and it actually worked o.k., but my problem was the ferrous had to create a pretty good signal, ergo a nail etc....  

I was searching an area with lots of disintegrated can slaw that was tiny, too small to make the tone swap so I gave up and went back to digging it all with my GP3500.

I always meant to give it another try so about a year ago I threw a DD on the TDI. Dang it if I could duplicate the results. I don't know what I was doing wrong if anything or if that particular DD did not like the TDI. I should have spent more time on the problem because if there is one thing I learned from Reg is some results may arise that will puzzle the heck out of you. Brain cells need to be expended then the light bulb may click on.

Some day I should do some further testing as this may be the ticket for hunting the outer edges of old cabin sites in hot ground or areas of disintegrated sluices, or?

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Exchange rates are what they are John and nothing White's can do about a strengthening currency for the U.S. and a weakening currency in Australia. White's has always been at a disadvantage because of their stupid marketing policies - witness your difficulty in finding a TDI Pro to buy in the UK. White"s by design made the TDI hard to buy from day one for reasons I may go into some day.

Back to the subject at hand, even Reg would tell you that the TDI was never designed to run large coils and it is just not what it does best. Sure, they can be made to work on the TDI but the circuitry is not intended to be running large coils effectively and that is why you do not see many large coils made specifically for the TDI.

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There are different TDI's even though the gain of each is basically the same.  First there was the through the hole TDI which is basically a GS 5 with a couple of mods.  Then came the standard TDI which was followed by the TDI Pro and finally came the SL series.  

Each is a little different in filtering and/or sampling timing.  The result of the minor changes could be seen in testing a series of targets of different size and material types and noting the differences.  In most cases, one would see little or no difference, but in other cases there would be subtle changes.  

Now, the differences show up on coil responses also.  For example, a ML compatible DD coil may work reasonably well on the GS 5 but not on the later TDI's well if at all.  The problem lies in the fact how the different TDI's react to the heavy current of the ML coils and the delay of the detector.  

As for the delay, the early TDI's and the later surface mount do not have the same minimum delay.  But, but but, all models say 10usec is the minimal delay.  Well, it depends upon just how you interpret when and how the delay is measured.  In other words, if you use the original TDI procedure, then the original has a 10 usec delay and the later units have a 13 usec delay.  This small difference affects both small gold detection but also how well a ML DD coil works on that detector.  

Now, add filter differences and other subtle changes show up.  Take the fact the TDI has both low pass filters and high pass filters and if they overlap, certain things can occur.  The TDI Pro corrects some of those problems by altering a key low pass filter setup.  This filter change helps in the detection of small gold but also helps in sweep speed detection.  This helps in using larger coils providing a faster sweep speed is used.  

Take a large coil and place it on a bench and then take a small coin and pass it over the coil at different speeds.  As you increase the speed, you will notice the signal get louder if the coin is held at or near maximum depth of detection until it reaches a point where the signal will begin to fade again.  Do this test with a TDI and the Pro and you will see a noticeable difference.  

In simple terms, the assumption that a TDI doesn't have enough power to power a large coil is totally false.  Instead, the interaction of the filtering inside the detector is the culprit.  So, what does this mean?  Simple, modify the filtering and you could use larger coils much easier.  you may have to put up with more noise if you don't try to reduce the noise at its sources though.  

Now, when it comes to coils, the TDI is forgiving but works best with coils made for the TDI and not ones made for the ML's.  As an example, ML DD coils don't work well on the later TDI's or earlier TDI's if the delay is turned up a little. However, DD coils made to work on the TDI should work fine.  In fact, some are being tested and used in OZ right now. 

One final note, the SL has extra pads where extra parts can be added in certain filter locations.  This would make modifying some filtering easier.  


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