By BT (GMT Goldmaster - discontinued)
At first glance the White’s GMT may seem like just another VLF machine but when fully understood it is anything but, in fact, it has now become my favourite detector to use. One of the main reasons for this is the inclusion of the mineralisation meters that have increased my ground reading skills tenfold. They have given me another dimension, so to speak, as far as what is going on under my feet, thus allowing me to properly identify the type of ground I am working and take advantage of the fact. I can now electronically sample the areas I am in and believe this has to be a first for the detector prospector. Following are some tips to get the best out of this little rocket.
Getting Started:

For our Aussie conditions I have settled on starting points of 4 to 5 for the gain and 7 for the V-sat. Think of the gain like the headlights of your car. In heavy fog the last thing you do is turn on your high beams because it just makes things worse.  Mineralisation can be likened to the fog, use too much gain (light) and things just get worse, increasing ground noise and masking target response. This should be simple enough but many people insist on using high gain settings and complain of too much ground noise and noisy operation.

Sure, if you can run a high gain setting without false or erratic signals, by all means do so. But if everything is going off you are wasting your time. Signs of a gain setting that is too high are: false signals, erratic behaviour and hyper-sensitive coil.

V-sat Control:
This controls the recovery speed of the threshold after it detects pockets of mineralisation or rapid changes in mineralisation. If it is set too low for the given area, you will get dropouts in threshold sound. If this happens as you go over a target it will not register. By increasing the sat speed the threshold recovers faster and the dropouts disappear. The target in the dropout zone is now clearly audible.
All other detectors have a set average sat speed which in some cases works fine but in others fails miserably, causing target losses whenever their sat can’t keep up. Some may advise to merely slow right down causes the target response to meld into the background, only leaving the very obvious sounds to be heard. The peripheral sounds will not exist. Having total control over sat speed is a big plus.
Every ground has its optimum sweep speed for maximum target recovery. Sweep too fast and targets are lost or not heard and the same rule applied if you go too slowly. By using a test nugget the optimum gain, sat and sweep speed can be determined giving the user a greater chance at coming up with the goods.
Coil Height:
While many people, professional and novice alike, believe ‘low and slow’ it the way to go, this simply does not work in all conditions. I have already mentioned sweep speed but now let’s look at coil height above the ground. I believe what follows applies to all detectors.
In heavy mineralised ground, by actually running the coil on the ground, you will miss gold. The coil and detector become swamped with too much info and you actually get a reduction in depth and target response. By raising the coil it lessens the effect of bad ground giving better depth and penetration.
Next time you’re told “low and slow is the only the way to go” don’t believe it. In some cases it may be fine where the mineralisation is low but often, in gold-bearing ground, it is not. Again, a test nugget will help to determine the right settings, height and speed.
Iron ID System:
The Iron ID system on this machine has to be one of the best I have used. In quiet grounds run it in the trigger-away position. This adds audio grunt on iron targets in conjunction with the LCD readout. There is no depth loss. If it pushes past 75 per cent and grunts, forget it. In hot ground, use it in the central position which only uses the visual ID on the meter.
I dig everything above 50 per cent to determine whether or not it’s worth looking at. Analyse by pulling the trigger and holding it, passing the coil back and forth about half a dozen times. If the LCD jumps to 100 per cent and grunts, then forget it. The reason I don’t use it in the ‘away’ or constant ID position in hot ground is that if a bad patch of iron mineralisation is hit, it will give a small, less than half-hearted grunt which to me is a little too distracting.
When you come across a target, wait for the LCD readout to clear before analysing because if you don’t, it may give a diminished target response. It only takes a second or two to clear and it analyses much more accurately by doing this. It does not take long to get the hang of and when you do, it is very reliable.
Not though that some types of iron, depending on shape, will not ID, this sounds bad, but believe me it’s not. The designers and manufacturers have erred on the side of caution here, the reason being that they don’t want you to miss any gold. Many discriminators, in fact most, will often mistake iron-coated gold for junk. Believe me; I have seen it happen too often.
The Meters:
The meters speak for themselves regarding what they can do and how to use them. Watch them for large or unusual changes. For example, you’re on a hill and the left meter reads 60 to 70 and the right on 30 to 40 when suddenly the left changes to 80 to 90 and the right to 70 plus. This is where you need to have a good look. It stands out like a beacon from the rest of the area. Believe me they work. I’ve seen it first hand, experienced it and found gold by using them.
The Grab Pad:
The Grab Pad is quite unique and a very clever inclusion. It works like this. The digital processor in the detector is constantly analysing and storing ground info as you detect. If for instance you track onto some strange pocket or composition of mineral that causes some erratic noises, simply push the pas once and instantly the tracking adjusts to normal.
The Grab can also be used in manual mode where you can adjust ground balance yourself. In manual you can use the ‘+’ or ‘-‘ pads to balance. Raise and lower the coil a couple of times then press Grab for instant balance. As you scan and the balance changes, now and again press Grab. That’s all you need to do.
If you like to run your machine slightly positively balanced for a sharper target response, simply raise and lower the coil, press Grab and press the ‘+’ pad a couple of time. As you detect all you need to do is occasionally press Grab and the ‘+’ pad. Three or four touches now and then are all that is needed for a constant positive balance.
Signal boost:
With the GMT there is an inbuilt signal boost switch that can really make small soft sounds jump out at you. It works very well but be aware that in noisy ground it will also enhance mineral sounds which can confuse things.
Coil and coil lead:
Allow enough movement of the coil without strain on the lead then tape it to the stem. Tape it ‘straight’ on the fibre shaft and only coil it on the upper metal one. Seal the coil cover with clear silastic – the non-insulating type – to stop dirt, water and small metal fragments from getting in between it and the coil.
To Pick up those faint, edgy signals you need headphones. Though the unit has a speaker built in, the best performance comes from the use of headphones.
Now, just because Joe Blow reckons his are the best for him does not necessarily mean they will be for you. We are all different, so try a few to see which are best suited to your hearing. Look for a set that firstly has good audio tone and secondly, has a lead capable of standing up to the rigours of prospecting. If the pair you settle on sounds great but has a flimsy cord, then get someone to change it to a better, stronger one. The extra buck spent will pay off in the long run.
The headphones should also have a volume control otherwise you may end up damaging your hearing.
Use alkaline batteries because they last longer and recover better than ordinary ones. I prefer to use the same brand all the time because after awhile you will know what to expect from them and how long they will last. Don’t go for any named or unheard of brands as they are known to leak at times and are also inconsistent in performance. I change mine when the battery test shows just above 50 per cent. Below this the audio sounds aren’t quite right, although detection depth is unaltered. A good set of alkaline will last up to 50-plus hours of use. This depends though on how many targets you go over, whether you use headphones and how long your sessions are.
Now some of you may have heard that this machine can get gold others can’t. Is this true? Yes, absolutely, I have found specimens with more than an ounce of gold in them and no other machine has been able to detect them. A couple did in the air but on the ground, let alone under it, there was no response.
Take your time to learn about this detector and good hunting.