Getting to know the Minelab GPX 4000
pulse induction metal detector.

By Gary's detecting

Minelab GPX metal detector information

Garys detecting home page
Minelab GPX.....Raising a few pulses
I recently purchased a Minelab GPX 4000, my original plan was to look for a 4500 or a 5000 but used prices were simply out of my price range, as a rule pulse induction metal detectors are not the ideal choice for searching in land so it's not something I would consider using all the time simply due to it's limited iron rejection, which is active up to around 6" deep.
After using my machine for an hour I realised that it has a fault, the audio threshold had an intermittent broken sound, originally I thought it was the headphones but on further inspection I found it was the patch lead what linked the harness mounted battery to the machine, the headphone socket is inside the battery, sadly the GPX has no internal speaker.
I managed to make a replacement lead but ordered a screamer pack from Staffordshire detectors. The screamer pack is a scaled down battery and audio booster kit that fits onto the side of the control box, eliminating the need for the harness and the large battery, my unit was made by Doc's detecting in the USA it is supplied with 2 batteries, a good idea however so far I can only get 3 to 4 hours detecting per battery, I am beginning to think these small battery packs may be failing and not producing enough constant power for these hungry machines, if anyone has further information I would be grateful.
The GPX controls
minelab gpx control panel minelab gpx 4000 gold detector

It may be worth noting that the GPX 4000 does not have a loud audio sound, most people fit an after market booster amp, the gpx 4500,4800 and 5000 has a built in amplifier within the factory battery.

Battery and signal patch lead
Minelab GPX patch lead
Aftermarket screamer pack
Minelab GPX screamer pack

Why do I want a pulse machine, I never search for nuggets or hunt beaches ?
The GPX range is something a little different, for a start it is a massive challenge to use inland for relic hunting, iron identification takes a keen ear and a fair bit of skill, so I guess deep down I wanted to give myself a challenge.
The advantages of using a pulse machine such as the gpx can be massive, providing you know when and where to use them, my first outings were on pasture sites which I had detected before and for some reason had it in my mind the goodies were all now out of detecting range. This was quite possibly true but what I had failed to realise is that these sites were also heavily iron contaminated, on one site my find rate was 99% iron. Don't get me wrong the GPX 4000 dealt with a lot of the iron with ease, but it's a certain size and shape that gave a text book diggable signal you just could not afford to walk away from, the problems were made even worse because there were virtually no non ferrous signals to compare against the iron. Just as an experiment I took the CTX 3030  for a spin and it took nearly 45 minutes before I got my first decent signal, the iron contamination was awful, I wanted a challenge but this was no place for a pulse machine.

Pulse metal detectors the advantages
Pulse machines are very different to VLF, they omit thousands of pulses per second into the soil, these pulses can be changed by adjusting something called "soil timings" making the machine very adaptable to different types of terrain. Conventional detectors do not perform well on bad soil, a pulse machine when set up correctly goes through this soil like it is non existent. On average soil the pulse machine quite often has minimal or no depth advantages over a conventional machine...however when it comes down to finding small to medium sized targets at depth in mineralised soil nothing can touch it, wet sand is another good example, be prepared to dig and remember pulse machines love small Gold !!
Minelab gpx finds small gold celtic coin

Pulse metal detectors disadvantages
Due to the nature of the pulse it can be badly effected by EMI (electrical magnetic interference) which can be picked up from a great distance, thankfully the gpx has over 250 channels to choose from, the whole range can be automatically scanned with the push of a button, combined with other software features EMI can normally be overcome in the field.
Battery consumption can be quite high and also the weight may be an issue to some users, most people use a harness with the minelab, especially when using large coils. And naturally the limited discrimination can be classed as a disadvantage to some.
minelab gpx iron rejection

Why does the GPX only have limited discrimination ?
Those of you who know about pulse machines will realise, a pulse machine with any form of iron rejection is a colossal breakthrough in technology, some still say it is impossible to make a discriminating pulse work, but minelab have been using this technology for some years, so I guess we must be grateful for some iron rejection, after all this machine has been designed for finding tiny nuggets in the gold fields and not for relic hunting.
Two coil choices are available with the Minelab GP range in a wide variety of sizes, Mono coils or Double D, Mono coils can be slightly deeper but offer no discrimination, Double D coils can be used to reject iron, bigger coils tend to be better discriminators than smaller coils.

So how can I use a pulse machine to my advantage ?
Now I have become more familiar with the GPX 4000 I know it's limitations for inland hunting, I am quite happy to have this machine gathering dust in the corner knowing it will earn it's keep when called upon, I would use the GPX for sites that have a productive area or previous hoard spots, highly mineralised soils where vlf metal detectors can only penetrate a few inches, area's of medium to low iron contamination such as woodland and meadows, archaeological digs where all metal is required.
So from a obsessive metal detectorists point of view this is something that you should have in your armoury, as it only takes one find !

Minelab GPX 4000 in the field
Here is a short video on how the GPX performs, you can see the iron rejection capabilities for UK searching, personally I think the GPX 4000 will a good machine for hunting specific yellow targets providing soil is not to badly contaminated with iron.....more to follow so keep visiting


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