By Steve Herschbach
I have a 1990s era Fisher Gold Bug, the original 19 kHz analog version, long since replaced by newer models. The old 19 kHz model was not particularly hot, but it did have an ability to ground balance out some hot rocks that newer and hotter VLF machines struggle with.
The detector came with a 10" elliptical search coil. I have a couple very rare 3.5" round coils that Fisher made for these models before the little 4" x 6" elliptical replaced it. It's the smallest search coil I've ever owned for a commercially sold metal detector, making the unit into more of a probe than a normal detector, it is so small.
The threshold was cutting out if the detector was bumped, something loose, so it's been sitting idle for a very long time. I decided it was doing no good as is, and by chance had an opportunity to call Felix at Fisher recently. Felix is another old-timer in the industry like myself, who I have not talked to since I left my old dealership in Alaska over eight years ago. Anyway, since the detector is still actually functional Felix figured they could give it a good refurbish and fix whatever is loose.
The main thing I wanted, however, was to get the detector tuned for the 3.5" coil. Most people do not know it, but analog Gold Bugs are hand tuned for every coil. The coil they ship with is tuned for the detector, or should I say the detector is tuned for the coil. Accessory coils may or may not be a perfect match if the original coil is swapped out for something else. Long story short is I am asking them to match the detector to the 3.5" coil as well as is possible.
The original Gold Bug is a genuine classic. Prior to it, detectors were large square boxes with extremely poor ergonomics. The Bug features a very compact control box that is removeable from a S rod, enabling easy chest or hip mounting. The design seems normal now, but you have to have used what came before to see the genius of what Fisher did with the Gold Bug. 19 kHz was also a radically high frequency at the time, as people were just catching on to the idea that higher frequencies are better for small gold nuggets than the lower frequencies that were the norm of the time. The 19 kHz Gold Bug replaced the 4.5 kHz VLF-660 Mother Lode as Fishers top nugget detector. Frankly, Fisher was not seriously in the game prior to this point, with Garrett probably the leader in VLF gold detectors at the time. A new company named Minelab was just starting to sell detectors in Australia, and had yet to really make a name for themselves.
Anyway, control box and coil boxed up and on the way to El Paso. I'll let you all know how long it takes and what the result is.
Here is a picture of one of the coils. The Gold Bug coils first used a gray dual lead cable, which was replaced very early on with the stouter black cable used to this day. This is the older coil of the two, though it also has less wear. The one I sent in has the heavier cable but is in poor condition. I need to reinforce the coil ears when I get it back since one is cracking. But I decided I wanted to use the coil with the heavier cable, with this older one serving as backup until I sort this all out. And in case anyone is wondering, the old 19 kHz coils do not work on the newer digital versions of the 19 kHz Gold Bug.
3.5" round search coil for original analog Gold Bug
To reiterate what a radical design advancement the Gold Bug was at the time, here is a picture of the detector it replaced, the 4.5 kHz Mother Lode...
Fisher VLF-660 Mother Lode metal detector
And the new 19 kHz Gold Bug...
I finally got out to test out my new 8" X-Coil, I've been wanting a size like this for a long time. I thought it was going to be impossible to fit the GPZ Super-D design into such a small coil however X-coils after some time have achieved it and it works exceptionally well. I was running my GPZ in HY Normal with gain of 20, manual ground balance all day.
My first pellet recovery 🙂
These were my weapons of choice for the day, the little 8" hardly looks much bigger than the little 6x3" sniper on the GB2. It made the GPZ feel reasonably light too, I ran it all day without my harness which is handy as I was in rocky hilly terrain always putting my detector down so it was great having the extra freedom.
I was comparing targets with my GB2 all day, seeing it's widely regarded as the most sensitive VLF I wanted to see the difference in performance to the GPZ with a small coil. The first thing I noticed almost immediately on little shotgun pellets which I found a lot of by the way was the GPZ was noticeably deeper than the GB2. There were pellets that the GB2 didn't pick up at all where the GPZ had a reasonable signal on them. I also found the Discrimination on the GB2 was near pointless on a lot of targets as it had no signal at all on them only giving a signal in it's all metal mode. The pellets had to be much closer to the coil than they were in the soils for the Discrimination to work, I was quite disappointed in this as I was hoping it would be a good gold discriminating pin pointer.
This area has a lot of hot rocks, by a lot I mean they're everywhere and they're often green, you can't do a swing of a VLF without hitting them on every swing, I've been to this area a lot lately, its where I recently found 9 grams. I've moved on from that exact area I found the gold last time as it dried up but the same general area. The GB2 was of course having a lot of trouble with them, with the Nox I just notch out -8 and -9 and all is pretty good and the Nox doesn't appear to lose depth when discriminating like the GB2 and Gold Monster do. The 8" X-Coil was handling the hot rocks well, most of the smaller ones it was blind to, the big ones, some the size of a football or bigger it would get a signal on, so I tried the quick track button and waving it over the top of one, it took about 10 or so sweeps but it was able to give minimal reaction on one afterwards so I had my GPZ in manual balanced over a big green hot rock for the day so instead of using my Yellow ferrite ring I used a hot rock, it made more sense to me seeing they're the areas problem, correct me if I'm wrong.
My first bit of gold was downstream in the old timer wash channel from my 9 gram spot. I did my usual pellet scrape, the signal lasted longer than a few scrapes, so I did some bigger scrapes and the target was getting better so I took a short video as I was starting to get pretty confident.
It turned out to be a nugget in the gravelly layer below the top soil, pretty small nugget too.
The GB2 did not pick this one up at all until I was closer down to it, that was in maximum settings with audio boost.
Little longish bit.
0.177 of a gram, my biggest of the day 🙂
I walked across to the next wash as that was my plan for the day and was finding my usual pellets and comparing the two detectors, I was finding the GB2 was handy as a pin pointer and it's 71kHz wasn't interfering with my GPZ at all. I was also tipping my GPZ on it's side and using the edge of the 8" coil as a pinpointer and that was working pretty well for me. Seeing I was finding so many pellets, way more than usual in this location I was getting plenty of target recovery practice 🙂
Off to the edge of this bedrock I got another bit.
Almost a little ball, when it was dirty I thought it was just another pellet.
After cleaning the dirt out of it it's not as much like a little ball as it looked. It's a ball with a tumor.
I heard a couple of bikes coming along the nearby track, turned out it was JW and his wife Robyn, they knew I was going to be there today so popped in for a visit, JW also has unfinished business in the area where he's been attacking a rocky wall of the wash for months and still getting gold out of it, he had his 15x10" X-Coil on which was a surprise, he put his 10" on and I thought he'd forgotten how to remote it and it rarely left since, he just loves that little 10". He was impressed with my little 8"', especially the size knowing It can get into places the 10" can't. We started detecting again and Robyn found comfy spot to read her book.
It wasn't long and I could hear JW's usual TAP TAP TAP as he's smashing away at the rock 🙂 I kept checking out the lower areas of the wash and it wasn't too much longer after 5 or so more pellets I found another bit of gold.
It was down in that bit of a gap in the rock.
It was starting to cool down a lot now, well the day was never warm, I guess a maximum of about 5 degrees Celsuis but once the sun starts to go behind the mountains it cools down quickly so I went back to a bit that still had some sun 🙂
At this point I'd seen the bit of gold and knew where it was sitting down in the gravels but I wanted to show the crazy prickle bush I was dealing with, I'd already broken some branches off at this point, look at them thorns! This video also shows the sideways method I was using to pinpoint. The center of the coil is definitely it's hottest deepest spot but the edges are still good.
This was my last bit of the day
And just some scenery shots of the area, gold can be anywhere around here, even in the most unlikely of places.
In between the bedrock is has been the most productive for me, which is why I like little coils.
Everywhere you look are old timer rock piles, they're gradually getting buried by plant life.
Once those giant prickle bushes grow in them, they're no longer detectable.
Piles everywhere though, it goes for a few miles
It's starting to get very overgrown in places, every year it's getting worse, gradually disappearing.
Some of these cliff edges would be great to detect and would hold quite a bit of gold I'd imagine but they're just too steep for me.
This is the sort of stuff JW is chunking away at all day getting gold out of the cracks, he said he's now bought a little battery powered jack hammer type thing to help with the job as it's hard work with a pick, hammer, cold chisel and screw driver but there is a fair bit of gold trapped in the rock.
It's all in layers and you can smash them out slowly with your pick.
And my junk for the day, well the junk I didn't lose from my pocket. So overall I'm very impressed with the 8". It seemed to me like it was exceeding the GB2 in performance on small pellets at depth. I'm still better at pinpointing with a small coil like on the GB2 but with practice I'll get better.
As I was about to leave I went over to see how JW was going, right as I got there he had a target, I took over my GB2 as I know he often uses his one as pin pointer too, especially when smashing out rock as it can save a lot of time. He was using the nose of the 15x10" coil as his pin pointer and said it was working really well, he found the tip to be pretty sensitive but still too big to get down into the spot well so we used the Gb2 to get narrow down where to smash out... after a fair time of hitting he had the signal out, and it was a bit of gold smaller than my smallest bit and the 15x10" was sounding on it loudly. He had another signal in the rock to recover so I left him to it as they can take half an hour or more each to get out.
I said I'd never do it, buy a detector that seems about as old as I am for active use, however I did always want to own an old classic. I was always aiming to get an old Whites from the 80's or so. The older the better, just to have it in my collection, they're very hard to come by in New Zealand and the only decent one I've found the seller was on the North Island and didn't want to ship it, pickup only. I wasn't going to fly up to get it.
I got my first metal detector when I was about 8 years old as a Christmas present, loved the thing. It's a Radio Shack, in NZ/Australia Radio Shack was called Dick Smith after it's owner so it was rebranded as a Dick Smith detector on the box, but inside was a Radio Shack 🙂
It still works fine, I'm surprised I still have it, it's the only toy from my childhood that I still have. For some reason I kept it.
It took me about 30 years to decide I need a replacement metal detector, I actually think it all started when my daughter lost her favourite Thomas the Tank Engine train in a sand pit and this little Radio Shack found it, I then found this forum and the rest everyone knows 🙂
An old model Gold Bug 2 came up for sale locally, at a good price, the owner barely used it. Barely a scratch on the coil.
The foam handle had a split it in from age I guess, I've since replaced that. I broke the headphone plug the first time I used it, I've fixed that by adding a new rubber one onto it rather than the plastic one. It has the wrong coil but I've ordered the little 6", should arrive today as I used overnight express although this coils pretty impressive too.
I've tried it out, sensitive sucker, even with that big coil. #9 pellets are easy for it. The manual ground balance is simple enough to work out, after using the QED I was used to that. And one bonus, I was reading Steve's review here about the old model GB2s where he talked about some had a bug/feature where you can supercharge them in disc mode. It turns out after my testing, this one has the ability to super charge!
To quote Steve, "There is an undocumented trick that may or may not work on any particular Gold Bug 2 in iron disc mode. The threshold control usually has no effect when the unit is in iron disc mode. However, some units display a distinct difference in iron disc performance between the threshold being set low or being set high. This ability to "supercharge" a silent search disc mode by turning the threshold up is not unheard of in other detectors and it appears some Gold Bug 2 models have this ability. Several of us used this ability to good effect at Ganes Creek. The detector pops and clicks a lot when supercharged in this fashion but adds considerable depth on large gold nuggets. After awhile the popping and clicking is mentally tuned out as nuggets have a distinctly clearer beep.
This ability may have been an accident on some units, as more recent Gold Bug 2 models display no change in the disc mode when the threshold control is manipulated. A simple air test between low and high threshold settings while in iron disc mode will reveal if your Gold Bug 2 has this ability to be supercharged."
So, I've now got an old classic and to me it really feels like an Antique.
So I found this Gold Bug detector for sale. The owner says it is the GB2 which it appears to be by the housing, but I think it may be an earlier version (?). Id appreciate if anyone could give me some info on this unit. In particular, my questions are:
What frequency does it run on?
Is it as sensitive as the GB2?
Is this maybe something similar to the CZ3D where the older models are more desirable/better?
With the official announcement on May 15 of the (hopefully) mid-summer release of the Garrett Apex there was considerable discussion of the (only) stock coil planned for release and its affects on depth. Within that discussion Chase gave me incentive to do some testing. This post is a result of that, but since I think my testing is applicable to more than just the Garrett Apex I'm creating this post in the general DetectorProspector forum.
The gist of the topic there was how much compromise the 6" (wide) X 11" (tall/high - my choice of word) Apex stock would have on depth. As I mentioned I have quite a few coils for each of my detectors, but subsequently I realized there was one detector (Fisher Gold Bug Pro) and coil combinations (5" DD round and 5" X 10" DD elliptical) which would best address this issue. (I also have some other options -- White's TDI SPP and Minelab X-Terra 705 -- but those are a bit less ideal as will be discussed later. Since I have two other coils for the GB Pro I decided to include those for completeness although they add more variables/concerns and thus don't fit quite as neatly as the other two.
I initally started with my variable depth test stand which allows me to vary the depth of small targets in 1/2 inch increments from ~ 1" down to 12" depth in the ground. However, in the midst of that part of the study I realized that I have some (likely iron) trash targets in the field-of-view which compromise the tones/measurements. Fortunately I also have two cleanly placed buried coins -- a copper alloy Lincoln Memorial USA penny buried at 5" depth and a Jefferson nickel alloy 5 cent piece at 6 inch depth. Neither of these currently suffers from nearby trash targets. I subsequently altered my study to use those targets for the coil performance tests. Unfortunately these also aren't ideal since under the conditions of testing they are too shallow to determine in-ground depth limits. What I did as a hybrid compromise is to carefully (i.e. measurably, with shims) raise the coil above the ground until the signal disappeared. For a second (more/less confirmation) test, and one that should be easily repeatable by anyone with the same/similar detector and coils, was to then perform a standard air test.
Let's start with the conditions of the tests:
1) Ground conditions -- moist ground (we've had a typical wet Spring season), Fe3O4 mineralization measure of 2.5 bars on both the Fisher Gold Bug and Fisher F75 (2.5 meaning that about half the time I see 2 bars and half the time 3 bars).
2) Gold Bug Pro running in "all metal" ("motion all metal" in USA terminology which I like to call minimally filtered), max gain, threshold at 11 (which is about where Kevin Hoagland calls "mosquito buzzing in your ear"), no headphones (so detector's speaker).
3) My precision for "depth" is 1/2 inch. That coincidentally was the height of the shims I used in the hybrid test and also my ability to control the hand-held coin distance in the air test.
4) My determination of (maximum) depth limit was simple. I increased the depth until I thought I could barely detect an audio signal. I then decreased the target-->detector distance by 1/2 inch and required that I subsequently heard a clear signal. If not I reduced the depth/distance and repeated.
Here are the raw data results. I'll explain the meaning of the columns shortly.
You can see the four coils I tested. The first three are all Fisher manufactured and the last is the NEL Tornado. Rather than to use the nominal product quoted dimensions ('dim' short for 'dimension' in the column headings) I actually measured the coils and interpolated to account for the fact that a coil doesn't typically have a single extent but rather is a bundle, and further that the bundle obviously fits inside the housing. For the closed coils this is obviously more vague but in those cases I just used half an inch less than the housing dimension. An addition oddity is that DD coils aren't really simple ellipses but some overlap of two independent elliptical coils. 'geom mu' is the geometric mean of the two just determined transverse dimensions -- more specifically the square root of their product. Hopefully you'll see later why I calculated that quantity. It's not really relevant for the main conclusions I draw. The last two columns are the actual distances between the target and coil for the limiting distance (see item 4 above). In the case of the air test that is obvious. In the 'part ground' test that is the sum of the depth of the coin in the ground and the height of the coil above the ground for both coins.
At this point I think it's worth discussing some caveats/assumptions/limitations of this test. Then if you've stayed with me I'll go a bit farther and hypothesize on how to use these data to draw conclusions for other coils.
1) Although I chose a detector/coils combination that was as consistent as I could be (same manufacturer and same 'width' coil), it has been discussed on this forum previously (sorry, no link) that the quality control of coil manufacture is a difficult task. It's certainly possible, although not necessarily likely, that my 5" x 10" elliptical coil is a high end tail performer among its peers and/or my 5" round is a low end performer.
2) With any measurement, there are in particular systematic errors and biases. I can't "double blind" my method. That is, I do know which coil I'm testing at a given time and if I have a prejudice for or against a certain coil that could show up in the results. Also, statistical uncertainties (more succinctly, how repeatable are my data) can contribute to errors.
It is worth pointing out that swinging the heavy NEL 15" coil effectivly makes taking in-ground measurements with it difficult. As a result I was unable to confidently get a max depth reading for the penny using that coil, which is why that cell is blank. No problem with air tests because there, as is standard (?) I mounted the detector in a stationary horizontal position and just 'swung' the targets to determine the (max) limit distances.
Again, it's really the 5" round DD and 5" x 10" elliptical DD that are most relevant. The others are include for information purposes but also to add to the plots I show later. As you can see, in these tests there is a clear and significant advantage for the 5" x 10" elliptical over the 5" round in both the hybrid test and in the air test.
OK, I now go a bit deeper. Is there a mathematical relationship which can predict coil depth performance if I know the coil dimensions? Compared to above this is another leap into the unknown with additional uncertainties. However, here are a couple plots which seem to indicate relationships between the potential maximum detectable depth and the geometric mean of the coils width and height dimensions.
(Sorry for the confusion but the Blue dots in both plots are for the 1 cent piece and the red dots are for the 5 cent piece.) It's better to look first at the 2nd plot -- air test. There appears to be nearly linear relationship between max depth and the geometric mean of the coil's dimensions, although it appears to trail off with the large (NEL) coil. Superimpose upon that the effects of ground noise and you see a further deterioration both in absolute depth and also in the trend which is shown in the first plot. Simply put, it is well known that mineralized ground, even moderately mineralized as in my back yard, negatively affects attainable depth. The larger the coil, the more ground it "sees", and thus the more ground interferes with performance.
I'll finish by pointing out that this isn't the first study I've made. Back 3 years ago when DetectorProspector member Karelian made detailed measurements of a large collection of mono coils on a White's TDI in both ground and air, I noticed the depth vs. geometric mean relationship. However, without a theoretical (physics/engineering) reason to expect this relationship, at this point it's merely a convenient correlation. Karelian's data are further muddied by the fact that the coils studied have many manufacturers: Coiltek, White's, Miner John, Nugget Finder, Minelab,... I could show those results but I think I'll await the reactions to the above. I can also do more tests (e.g. with the X-Terra although there is not clean comparison of round vs. eliptical coils with the same width, at least in my collection) or repeat these. I await your posted reactions (including yawns 😁).