Hi Everyone,
I’d like to jump into the conversation for a few moments if that would be OK with everyone.
First, RetRmy…neither the Garrett Ace 100 or the White’s 5900 DI are suitable for serious nugget hunting anywhere. I’d suggest a dedicated unit if you hope to have the best chance at success. Which unit you choose will depend on what conditions you expect to encounter…iron mineral levels and variability, target size, abundance of small bits of iron trash in the area (tailings) and so on.
If searching in natural landscapes, having a discrimination circuit will offer no advantage unless you encounter hot rock areas or intend to do some relic/coin hunting at any abandoned sites you might locate. On the other hand, if you intend to search iron infested mine tailings (depending on size of nuggets) a discrimination circuit could be useful. It is possible that you may be willing to forgo the smaller nuggets in favor of low level iron discrimination thus allowing you to “cherry-pick” the area more efficiently. Under these search conditions, a unit with only iron target ID will require the user to “check-out” a much higher % of the individual signals that will be mostly small iron. That’s time consuming, and means you will cover much less ground. That decision wrt discrimination is up to you, but it’s nice to have a choice.
Any prospecting unit should have a manual ground balance. Otherwise, as with the MXT for example, have the option of turning off (“locking”) the auto track. Such an option allows the user to “grab” the updated ground balance at any time with a flick of the GB switch and then return to operating without tracking. Keep in mind that this technique is not the same as having manually adjustable ground balance, whereby for example, the user can make adjustments to facilitate ore sampling, or positive offsets for more sensitivity to fringe depth targets.
The best option is to have both types of ground canceling, such as provided on the GMT. The posters above cover the value of having manual ground balancing. In variable highly mineralized ground, the value of having ground tracking comes to the fore by keeping the unit properly balanced. Both the GMT and MXT, despite being produced some years ago now, have extremely fast, accurate and fine resolution tracking
The White’s GMT is the best dedicated VLF nugget hunter that technology affords today in my view. It will rival the GoldbugII on tiny nuggets, but has an edge on larger nuggets (not useless air tests, but in the ground where it matters...air tests on articles such as ear studs/fine gold chains provide no useful depth/sens information wrt how well they will respond in harsh mineral conditions). It also performs better in really tough ground, where the GoldbugII can struggle. Of course, in extreme ground, a suitable PI unit provides a more likely alternative, but lets restrict our comments here to VLF units.
The GMT does not offer a discrimination circuit, but does provide an effective visual iron ID probability, in addition to an audio “grunt” where the unit interprets a signal to indicate the strong likelihood of iron.
The MXT offers a similar, perhaps slightly improved, prospecting platform to the GMT (see the Engineering Report at the White’s website) but utilizes a much reduced yet effective operating frequency. It will not detect the very tiniest nuggets, but will still respond to sub grain particles as Willy points out above. It should (as a result of it’s lower operating frequency at least) go deeper on larger nuggets, and will handle higher mineralization levels better than the dedicated higher frequency units. The MXT also offers two very sensitive discrimination modes that respond especially well to low conductive metals. Depending on the size of gold, for example in tailings, this unit might be a better choice…again…if you don’t mind passing up the very tiniest nuggets.
It could be argued the MXT might actually meet or outperform the higher frequency units in really bad ground. The higher frequency units will lose their edge on tinier nuggets in such ground, while the MXT’s lower frequency will come to the fore in handling such conditions to more favorable advantage.
Arizonaames, you made the statement…“The nice thing about the MXT is that the dual control is a discrimination control in Coin and Relic modes but in Prospecting mode it becomes a variable SAT. What this means is that when a signal is sent from your coil and hits the target, it then comes back to the coil and registers a target in a certain time frame. The speed that this happens (Threshold speed) is controlled by the turn of the SAT (dual control in Prospecting mode).”
This underlined part of this statement describes “phase” which can be defined as the length of time between eddy current generation sustained on a metal target’s surface and the resulting secondary electromagnetic field effect on the coil’s receive winding. SAT keeps the threshold smooth wrt spurious noise resulting from changes in ground mineral, and electrical drift. It provides for greater stability, especially in the all-metal mode, particularly when high sensitivity/gain levels are used. For prospecting, faster SAT speeds are required due to using inherently more sensitive higher frequency units over the variable high iron mineral conditions frequently encountered. Some folks might confuse this with recovery speed..which refers to the time it takes the detector to process one target signal such that it is ready to process a second target signal.
Willy… Regarding the F-70 comparison wrt unit pricing. The F-75 offers real prospecting capability since it provides manually adjustable, calibrated ground balance. This capability allows the user to make field adjustments to enhance the unit’s sensitivity to tiny nuggets (positive GB offset). The scale’s calibration further allows you a precise knowledge of any setting chosen. More, the calibrated scale facilitates the ease with which ore/rock testing can be accommodated. A GB setting can be established whereby iron mineralization (includes all oxides and reduced iron (Fe+2) in combination with elements other than oxides; does not include conductive iron sulphides such as pyrrhotite, pyrite etc…which must be viewed realistically as genuine targets desirable to some collectors) will result in a negative threshold response as contrasted to the positive response from sufficient conductive amounts of metals (including a host of metal sulfides) that may be present within any rock structure. Due to the varying amount of iron mineral “bias” (especially at the magnetic end of the ground balance scale) affecting threshold responses, this is not a perfect technique, but generally very effective. The F-70 does not provide manually adjustable ground balance, only the autograb. Yes, it could be used by and large, especially in a pinch, for general searching, but I tend to draw the line on units that neither provides auto tracking nor manual ground balance.
For prospecting purposes, the DD coil is certainly a must have item, and you would be required to purchase it as an accessory for the F-70. I have not checked on the 11” DD stock coil price, but I imagine it would run somewhere close to $200 dollars…since the small 3”X 6” coil runs at $150 dollars.
The F-75 provides a static all-metal mode. I consider this mode of limited value, but then again it does accommodate ore sampling.
The F-75 provides the backlit target meter, which has value to many users for nighttime coin hunting pursuits.
Finally, my understanding is that the F-70 power settings are directly related to both the Threshold and Sensitivity controls. I have no first hand knowledge of precisely how these controls interface, but I assure you that I would prefer to set my threshold to a faint hum, and not have to consider any resulting impact to my overall power settings.
Regarding the MXT/F-75 comparison. I own both of these software driven units and regard them as distinctly different tools. It should be expected that one will demonstrate advantages over the other, depending on the task or search circumstances.
Some examples. The MXT does not have manually adjustable ground balance making it more difficult (impossible to maintain a positive offset) to perform the prospecting tasks as outlined above. It does have the autotracking.. offering more suitability over variable ground. The F-75 is a more powerful deeperseeking unit with even faster target response and recovery, and it is more sensitive to tiny pieces while offering significantly better depth on larger pieces. On the other hand, the MXT is certainly no slouch in these categories, and does offer the manually adjustable SAT control. The F-75 “retune” is fixed, but seems quite adequate for most situations.
It seems to me that in the event the ground requires more retune speed than available on the F-75, then perhaps it’s time to reduce the sensitivity a bit. The operator can afford to turn down the sensitivity and still get very good response on both small and large low conductive pieces that the MXT with the 6’X 10” DD will not see here in my ground (ground phase 83 to 85ish, magnetite 0.1%).
Stability: Up in prospecting country, the F-75 was completely stable and quiet as a mouse at max sens/gain levels. The MXT by contrast, constantly chattered at similar high gain settings. At times I found it annoying, and had to reduce the gain level just to get a break from it.
Weight/Balance: The MXT wore on my arm after awhile, whereas I could swing the 75 all day long and no issue.
So you see, there is no be all, do all unit for every task. I like both these units. BTW Willy, when you play with the F-75 in disc modes, view the discrimination levels as the primary gain control. The gain/disc level interface is quite different in JE from the other modes.
That’s about it, sorry about the length of this post everyone, there was a lot to look at here.
Jim.