By Steve Herschbach
I have now put enough time on all these units to at least reach a basic conclusion in my own mind. And that is that they are far more alike than different. Trying to get clear differences to appear in actual field use in highly mineralized ground is a true exercise in hair splitting.
A couple detectors that can be added to the title list are the Teknetics T2 and G2 models. First Texas owns Fisher and Teknetics. The T2 is the predecessor of the F75. They are not exactly the same detector (they do not share coils) but almost identical in performance. The G2 really is just a Gold Bug Pro in different clothes.
13 kHz - Fisher F75 and Teknetics T2
15 kHz - Nokta FORS Gold and FORS CoRe
19 kHz - Fisher Gold Bug Pro, F19, Teknetics G2
In actual use the frequency just about says it all. The lower frequency F75 and T2 are just a tad less sensitive to very small low conductors, like a small gold nugget. The 15 kHz FORS is almost an exact match to the Gold Bug Pro/F19/G2 for sensitivity to small low conductors and so despite the bigger frequency gap I would say the FORS models come closer to the higher 19 kHz models than the lower 13 kHz models.
I have to say it all just boils very much down to the feature list, and again, they line up pretty well. The less expensive Gold Bug Pro and G2 have a more limited feature set than the F19. The F75 has the most options for tones and settings at the highest price on the list. The Nokta units at their new lower price are a real good value.
For me when it came down to actual performance the Gold Bug Pro/ F19 were so close to the FORS models I let the two Fishers go and kept the Nokta. Basically just to get the automatic ground tracking which can be very useful in variable ground but also the three tone option, which is nice for coin detecting. I also like the way the Nokta units balance better with larger coils. All I can tell anyone at this point if you want a detector to use for nugget detecting and also for other purposes, the Gold Bug Pro/F19/G2/FORS Gold and FORS CoRe are so close in actual field use that it will all come down to the operator and ground variations. I think the machines are a toss up from a performance perspective and so just line up the feature list and go with whatever floats your boat. I think for sheer value at this time the Nokta FORS models are tough to beat.
The T2 and F75 give up a slight edge on small low conductors. What this means is that all the previously mentioned models are better for smaller gold nuggets. The trade off is the T2 and F75 are better all around detectors for general purpose use, gaining in coin and other high end conductors some slight advantage simply because the machines are not quite so sparky on tiny non-ferrous trash. In moderate to low mineral ground conditions the T2 and F75 have a clear depth advantage on high conductive coins but in very mineralized ground the advantage is nearly non-existent.
In my case at least I feel like there is a 90% overlap between my latest version F75 and the two FORS models. If I head out the door right this second to go hunt coins I am more likely to grab the F75 as I like the extra tone schemes. There is the 3H mode that gives a high tone beep on all normal coins but also takes US nickels, which usually reads as a mid tone, and puts it up in the high tone range also. This is a great cherry picking mode. The standard 4 tone mode is great for cherry picking jewelry digging the low mid tones. I like the big screen and the backlight, etc. So I am also keeping my F75.
But if I was heading out the door chasing gold nuggets right now in a really trashy location and not wanting to use a PI, I would grab the FORS instead. It pulls low conductors like small gold nuggets out of the ground better than the F75. Not by a huge margin, but enough to matter to me.
And that is where it will stay for now. I am waiting to get my hands on the new Makro Racer models this summer, and using the F75 and FORS plus Racer units all summer. Then proceeding to phase two of the weeding process. I am trying very hard to get my detector collection down to just a couple PI detectors and a couple VLF detectors. It is down to that stage of the game however where it just needs a lot more in field use to let things sort out for me.
What I can leave you with for sure right now however is that these are all very good detectors that are ridiculously close in performance. You really just can't go wrong with any of them. Mid frequency VLF technology has matured to the point where it is almost impossible for anyone to really stand out from a performance standpoint. Nearly all the performance debates I see on the internet about these models boils down to differences in ground mineralization more than the machines themselves. Just find one that feels right on your arm and sounds good to your ear and get to work!
This is very much a work in progress and so as I get a chance to use the large coils or hunt under different ground conditions if I come up with anything if interest I will add it here. There is a related thread on VDI numbers and tones at https://www.detectorprospector.com/forums/topic/526-fors-gold-f75-v3i-tone-and-vdi-tidbits/. For detailed information on each model plus the latest prices visit Steve's Guide to Gold Nugget Detectors
By Jim Hemmingway
Benchtesting Rocks & Minerals with an F75 Metal Detector
From the earliest time when we were aware of our surroundings, most of us looked for pretty rocks. We wondered what interesting or valuable minerals might possibly comprise them. Now as adult hobbyists, I doubt if any of us hasn’t benchtested an interesting rock from curiosity, and wondered what actually produced the signal.
Although a sensitive benchtest usually has little in common with how marginally conductive rocks and minerals respond to metal detectors in the field due to ground effects, we can learn and become familiar with how rocks and minerals in our respective areas respond to metal detectors in a benchtest. A sensitive metal detector’s electromagnetic field penetrates rocks, usually generating either a positive or a negative signal in response to whatever material is in the rock. We can sometimes determine whether such signals should be investigated further, or whether worthless iron minerals produced them.
I’d generally describe my benchtest results as worthwhile and informative, but that notwithstanding, I look forward to doing a benchtest because I think it is an intriguing study on its own merit. That said, how do you conduct a benchtest? I’ll describe my methods and hopefully we’ll see what you think about it.
Benchtest Requirements and Techniques
Benchtesting ideally requires a visually displayed, fully calibrated, manually adjustable ground balance that covers the entire (soil) mineral range from salt to ferrite. As a minimum, the detector should feature a threshold-based true motion all-metal mode, and preferably an additional true non-motion all-metal mode for significantly improved sensitivity to borderline samples. Visual displays in either of the true all-metal modes are essential for target ID, Fe3O4 magnetic susceptibility and GB readouts.
I prefer a small (concentric) coil to promote detector stability and improve sensitivity to the rock sample, to ensure uniform sample exposure to the coil, and to minimize EMI (electromagnetic interference) especially if benchtesting at home. Elevate the sensitivity control as high as possible while maintaining reasonable detector stability such that you can clearly hear changes to the threshold.
To check for a target ID, move the sample back and forth across the coil at a distance that produces the best signal but does not overload the coil. To determine ground balance and Fe3O4 readouts, advance the sample toward the coil, back and forth to within an inch or two (depending on sample size and signal strength) of the coil’s electrical sweetspot. Ensure your hand does not come within detection range of the coil to avoid creating false signals. If you extend your fingers to hold the sample, this is not an issue when testing larger samples. If necessary use a plastic or wood food holder that can firmly grasp small samples.
Benchtests should be conducted utilizing a minimum of two widely diverse GB control adjustments. Initially I prefer the same GB control adjustment that is typically required to keep my detector ground-balanced to the substrates in my prospecting areas. It’s a personal preference that works for me. That particular GB control point (F75 / GB86) is more likely to improve any rock or mineral sample’s signal strength compared to using a more reduced (more conductive) GB compensation point.
The next step is to use a dramatically reduced GB control adjustment (F75 / GB45) as suggested by Fisher Research Engineering. This setting ensures that (obviously weathered) oxidized samples do not generate a positive signal from any type of non-conductive iron mineral inclusions, particularly maghemite mineralization that may be present within such rocks. It follows that this second benchtest will, if anything, slightly subtract from the sample signal strength, particularly with low grade and otherwise marginally conductive samples, compared to the first step of the benchtest at GB86.
As a general rule, I do not recommend the F75 / GB45 compensation point for benchtesting (non-oxidized) mafic samples that are dominated by constituents such as common magnetite or other black minerals that normally support highly (non-conductive) elevated GB readouts. Such samples can produce strong negative threshold responses at the reduced GB compensation point. It will be difficult or impossible for the signal from a marginally conductive substance to successfully compete with those negative threshold signals. For non-oxidized samples Fisher Research Engineering suggests using F75 / GB65 rather than the F75 / GB45 compensation point, since obvious iron mineral oxidation should visually be absent from such samples.
With the above discussion in mind, extremely fine-grained, unweathered magnetite that occurs in pyroclastic material (for example volcanic ash) can drop into the GB45 range, but it is extremely rare. Unweathered volcanics do frequently drop into the GB70's due to submicron magnetite, but the recommended F75 / GB65 compensation point will eliminate those positive signals.
The arsenopyrite sample depicted above is a good example of a commonplace mineral that we encounter in the silverfields of northeastern Ontario. Generally field examples could be described as marginally conductive and many are low-grade. A good many react with only a mild positive signal, and sometimes not at all to a benchtest depending on which GB compensation point is used.
The high-grade, solidly structured sample above produces a strong positive signal in either zero discrimination or true motion all-metal mode with the ground balance control adjusted to the GB compensation point required for our moderately high mineralized soils. As noted, that’s approximately F75 / GB86, although in the field, of course, it varies somewhat depending on location and coil type / size employed.
The response is not as strong as a similar size and shape metalliferous sample would produce, but it does generate a surprisingly strong benchtest signal that would be readily detectable in the field. Even with the GB control dramatically reduced to more conductive values (F75 / GB45), to ensure that any positive signals produced by non-conductive iron mineral inclusions should now only produce a negative threshold signal, it is no surprise that this (non-oxidized) specimen continues to generate a strong signal.
For those readers unfamiliar with detector responses to such minerals, the same general response scenario described above with arsenopyrite applies to other marginally conductive minerals such as galena, pyrrhotite and to a lesser extent even iron pyrites. Ordinary iron pyrites is generally innocuous, but maghemitized pyrite, pyrrhotite, and the copper sulfide ores, particularly bornite and chalcocite, can be a real nuisance in the field due to magnetic susceptibility, magnetic viscosity, and / or electrical conductivity, just depending on what minerals are involved.
Such variable responses from arsenopyrite and many other mineral and metalliferous examples clearly infer that signal strength and potential target ID depends on a sample’s physical and chemical characteristics, including the quantity of material within a given rock. These factors include structure, size, shape, purity (overall grade), and magnetic susceptible strength of iron mineral inclusions. Moreover, the VLF detector’s sensitivity, the GB compensation points employed, the coil type and size, and the sample profile presented to the coil further influence benchtest target signal strength and / or potential target ID readouts.
Incidentally, neither of my PI units will respond to the arsenopyrite sample depicted above, even with a TDI Pro equipped with a small round 5” mono coil, the GB control turned off, and a 10 usec pulse delay to deliver its most sensitive detection capability. That result is typical of most, but certainly not all sulfides and arsenides that occur in my areas. Higher grade and solidly structured pyrrhotite, an unwelcome nuisance iron sulfide, and collectible niccolite, a nickel arsenide, are commonplace mineral occurrences here that do respond strongly to PI units, although their respective VLF target ID ranges are quite different.
As a related but slight diversion, the photo below depicts a handsome example of the widely occurring mineral sphalerite. It forms in both sedimentary beds, and in low temperature ore veins. It is interesting to collectors because it possesses a dodecahedral cleavage which means that it breaks smoothly in twelve directions, and it is usually triboluminescent, meaning that it gives off a flash of light when struck sharply. Like many desirable minerals lurking in prospecting country, unfortunately sphalerite doesn’t react to metal detectors.
A Final Word
The foregoing is intended to illustrate that sensitive metal detectors can be utilized as a supplementary tool to assist with evaluating rocks and minerals. There is no question that the benchtest has serious limitations, particularly if trying to distinguish positive signals produced by some types of iron mineral inclusions from weak conductive signals.
That notwithstanding, a positive signal that persists below the F75 / GB45 compensation point cannot be confused with iron mineral negative threshold signals produced at that same compensation point. Therefore a positive signal merits further investigation. Such signals are almost certain to be generated by a marginally conductive mineral or a metalliferous substance.
On the more interpretive side of a benchtest, we need to point out that weak positive signals from lower-grade samples of minerals such as arsenopyrite, galena, pyrrhotite, chalcopyrite, and doubtless a few others, may disappear well before the GB control is reduced to the F75 / GB45 compensation point. We learn early that benchtests are frequently equivocal and require interpretation based on any further evidence that might support the benchtest result. Look for iron oxidation in addition to structural or other physical evidence as described above that could explain why a sample reacts as it does to a metal detector.
By Steve Herschbach
I am a big fan of the Fisher F75 from a different perspective than most. I am a prospector and have done very well finding gold nuggets with the F75. The very powerful all metal mode combined with the simultaneous on screen target id numbers have allowed me to quickly and efficiently hunt trashy tailing piles in search of large gold nuggets. The light weight and superb balance make the F75 a pleasure to use for long hours in rough terrain. It also was my detector of choice for my one and only trip to the UK that I have done so far, and it served me well there.
I spent a month in 2013 metal detecting on Jack Wade Creek near Chicken, Alaska. I kept my great results there quiet pending a return trip there in 2014. That trip has now been made but that is another story already told in detail on my website. Now I can finally reveal the details of the 2013 expedition.
I started out early one morning with my big gun pulse induction metal detector, but got onto a tailing pile that had ferrous trash scattered down one side, and I was just not in the mood for it that morning. I went back to my truck and got out my trusty F75. I run the F75 in all metal because it has instant target response; there are no worries about recovery times in all metal. The coil picks up every variation not only in targets but in the ground allowing me to monitor what is going on at all times. Knowing what the ground is doing is important in keeping the ground balance properly adjusted for maximum results.
The key thing I like about the F75 in all metal however is that the meter always runs in discrimination mode and places a nice, large target number on screen while in all metal. The audio alerts me to a potential target, which I then analyze more carefully while watching the target numbers. All metal goes deeper than discrimination modes, so no on screen number means a very deep target beyond discrimination range. This alone makes running in all metal desired when prospecting because running in discrimination mode would miss all those extra deep signals.
In all metal I dig them until a target number shows up. Deep targets or small targets in mineralized ground will often read ferrous, so I watch the numbers and if they even once jump to non-ferrous, I dig. Only targets that give a 100% strong ferrous reading over multiple sweeps can be safely passed. Though I will throw in my caveat that no discrimination system is 100% accurate and there is always a risk of passing a good target. When in doubt, dig it out!
I do often employ pulse induction detectors and do very often just dig everything. I advocate that when time and conditions allow. The reality is this is not always practical for many reasons. Maybe it is just limited time and overwhelming amounts of junk. Better to increase the odds by using discrimination than bogging down digging 100 nails in a small area. In my case it often boils down to fatigue or flat out not being in the mood to dig junk.
So it was on this particular morning, and therefore my F75 came out and I got to work sorting through the trash working my way up the side of the tailing pile. I crested the top and got a strong reading and looked down. There was a shallow dig hole with leaves in it, obviously from some hunter there in prior years. I figured the guy had recovered a trash item and kicked it back in the hole so I cussed him quietly under my breath. I hate it when people do that!
Then the target numbers caught my eye. They were all over the place. A crumpled piece of flat steel might give numbers like that though. Still, I was curious and figured I would retrieve the trash this person left in the field. I gave the old dig hole a big scoop, and out pops a big gold nugget!!
I seem to have a talent for finding ugly gold nuggets, and this one was perhaps the ugliest I have ever found. It looked more like a rock burnt in a fire than a gold nugget when I dug it up, though the glint of gold is unmistakable. This gold however was very pale and in fact later analysis revealed it to be roughly half gold and half silver and other metals.
It is a little known fact that gold alloys tend to have very poor conductivity ratings. Gold is very conductive, and silver is a superb conductor. You would think adding silver to gold would improve the conductivity, but in fact just the opposite happens, and the conductivity lowers dramatically. Gold/silver alloys are closer to lead in conductivity than that of the pure component metals, explaining why bullets read identically to most gold nuggets.
This ugly nugget is a detectorists worst nightmare, because the 50-50 alloy mix and rock content give it a much lower conductivity reading than would be the norm. I surmise what happened is this earlier operator got a poor signal and gave a dig to get the coil closer to the target. The signal did not improve, as would be expected with most gold nuggets, so the operator decided it was trash and moved on. The rest of the hill being covered with junk no doubt contributed to this decision.
It was my insistence on investigating everything except 100% ferrous readings that made the difference. The readings on this target were not solid as one would expect from a pretty strong signal but all over the place. Most people would say that indicates a trash target but I have seen many gold nuggets do the same thing in mineralized ground. The result is I dug a shallow 2.33 ounce gold nugget that somebody else walked away from. Sadly for them one more scoop would have revealed the nugget for what it was. Hopefully this is a reminder to the reader that far too often detectorists look for excuses not to dig. How many good finds get left behind because we do not want to take that extra minute or two to dig a target?
This nugget is far from a premium find, but I have already sold it for over twice the cost of a new Fisher F75. That detector was a real money maker for me as that was far from the only gold I ever found with it.
Unfortunately I say was. I made a huge change in my life in 2013 and moved from Alaska to Reno, Nevada. The move resulted in a desire for me to weed down my detector collection. I was pretty excited to do some coin detecting in Nevada where the potential finds were much better than those possible around Anchorage, Alaska.
Almost all my detecting with the F75 had previously taken place in rural locations far from possible electrical magnetic interference. In Reno, EMI raised its ugly head. I found much to my dismay that the F75 did not like my new location, and in fact when turned on to hunt the yard at my new home I could not get it to settle down at all. No matter what I did the machine chirped and beeped and numbers flew all over the screen. Unfortunately I experienced what many urban hunters have found out – the F75 is a very sensitive high gain detector that does not get along well with electrical interference. I ended up selling my F75 in 2013 for this sole reason.
Fast forward to the fall of 2014. I am contacted by the good folks at Fisher wanting to know if I am interested in trying out a new version of the F75 they are preparing for market. I of course say sure as I am always game to go metal detecting with different units. A new F75 is sent my way along with a list of the possible improvements. One immediately gets my attention – improved resistance to electrical interference.
All the focus was on a new mode or “process”, as Fisher likes to call them. The new FA process is intended to better pull non-ferrous items out of trashy or mineralized ground. It does indeed work as advertised as I found out in an accidental situation I came across.
I went to a local park and did a simple hunt for non-ferrous targets, comparing the DE default mode to the new FA fast mode. I did not really care what I found as long as it was non-ferrous. I should note the ground here is very difficult, reading 1 on the Fe meter, the second highest reading you can obtain. Hunting in this park is very much like nugget detecting, and the best detectors get very limited depth and highly inaccurate target numbers as a result of the high mineralization.
One spot really summed it all up for me. I found three targets I could cover in a single wide swing that all read as ferrous in DE mode, but when I switched to FA mode all three switched to non-ferrous. FA mode is very fast with short, machine gun type reports in the audio. I was running in two tone mode, with ferrous giving low tones and non-ferrous high tones. In DE mode I could sweep and get three low tones in a row. Simply switch to FA mode and now there were three high tone reports in a row. This was an extremely dramatic result seen in person. In this case all three targets proved to be nothing more than aluminum targets, but they could just as well have been small hammered coins in the UK or small gold nuggets in Alaska.
I hate to oversell things and I have to note that the difference in going to FA mode is not going to be earth shaking. Most targets read the same in DE and FA modes. But FA provides a tipping point, a little push that takes targets previously ignored and lights them up. By shortening the audio response on targets it also attenuates responses to a degree and so depth and signals on the tiniest targets may be impacted. Depth however is not useful if a target is misidentified or ignored completely due to target masking from nearby objects. FA mode is another tool in the toolbox that can help produce targets in specific situations previously overlooked by others.
The new F75 also expands on the available audio options in ways many people will appreciate. These additions and the new FA mode will tend to get all the attention, but for me they pale in comparison to the new ability of the F75 to engage and disengage the new Digital Shielding Technology (DST). The version of the F75 I received had DST engaged at all times, and the difference in my ability to use the F75 at my home was as dramatic as it gets. My previous F75 was basically non-functional. My new F75 ran just fine, with only minimal EMI discernible at higher gain levels.
I noted no downside to this. Given the situation, how could there be? Other field testers however were concerned that in low EMI situations perhaps there was an edge lost by having DST engaged, and so Fisher decided to add the ability to engage or disengage the feature as desired. It does not get any better than that. Use it if you need it; leave it off if you do not.
All I know is this. What difference is there between a detector you can use and one you cannot use? All the difference in the world, and in my opinion I struck gold a second time with the F75 seeing it run with the new Digital Shielding Technology. That one feature alone means I can use the F75 in urban areas where I could not use it before, and vastly improves the reasons for my owning the detector once again. I am very confident a great many people will agree with me when they get a chance to try out the new, improved F75. Everything else in my opinion is just icing on the cake.
I have a feeling it FT is about to rock the detector world, they have been too quiet for too long. They have some of if not the best detector engineers in the world, they have lowered prices on the F75/T2 platform to a level that is bringing them into the mid-level machine price range, even though they they are still at the top performance wise. FT is behind in the all terrain business but that wont last long and I think you will see a repackaged waterproof F75/T2 machine very soon and a true game changer shortly there after. I had an Equinox on pre-order but other than being waterproof (which I really don't need), my F75 LTD SE has more depth and is plenty fast for my hunting, thus I canceled my order. I am going to wait and see what FT has up their sleeve, until then I will keep going behind the latest and greatest machines and dig what they can't hear.
An acquaintance has asked if I'd help him find some shotguns he buried several years ago. They are wrapped in oilcloths, sealed in PVC pipes (~3 in = 7.5 cm diameter) and buried about 20 inches (half meter) deep according to him. He says he can show me the approximate location within about 10 m. If all this is accurate it seems like an easy task.... Then again, he also said someone in his family (without him being present) tried to find them with a detector and couldn't. That could be due to a lot of reasons as I'm sure you are already thinking, but my concern is that they may be buried more deeply than he remembers.
Which of the following would be your first choice?
1) TDI/SPP with 12 in round mono and 16 V battery pack.
2) X-Terra 705 w/15 in Coiltek 3kHz.
3) Gold Bug Pro (19 kHz) w/15 in Nel Attack.
4) F75 black (13 kHz) w/11x7 in^2 coil operating in cache process.
Assuming he has the time and patience I'm going to have all four with me to do a comparison, but I'd like to start with the one that gives me the best chance. Your advice is appreciated.