Some pointers on the new mods on the GPX detectors.
The manual frequency (tune) control on the back lcd panel is now made into a frequency control that can adjust the transmitter pulse to smaller or larger values, massively greater than the adjustment range in a factory standard detector.
Setting the manual tune to low numbers is perfect for really big coils as longer pulses can energise bigger gold at depth to get a larger return signal, lower numbers also allow you to use more gain as the extra long pulse on transmit is also matched by longer off time before the receiver turns on allowing most ground noise and hot rocks to fall out of the detection window, longer off times are detrimental to finding small gold.
When setting the manual tuning higher there are a number of factors to take into account, the manual tuning has a limit that is governened by how good the coil that you are using is made, it needs low capacitance to decay faster than the receiver turns on, a slow decaying coil will allow back emf into the receiver in the detector and kill the performance. I always say test the setting on a test target to make sure that you are not cranking everthing up but killing the performance, it is a bit like driving on wet roads, gentle on the accelerator.
When using high frequency numbers on the manual tuning using too much gain ( either the one we add or the gain in the back menu) can allow too much ground noise into the detector and jam the electronics in such a way that it will not respond to metal targets. It "ALL" depends on the type of ground and type of coil. On non mineralised soil the main problem is the coil, on very hot ground the main problem is ground noise.
In normal timings the detector is far more forgiving when you crank up the gain and frequency but beware some hot rocks will also be more detectable but you can lower the frequency to reduce the respone or if there are many just ground balance on a known hot rock and lock the detector into fixed tracking, this will eliminate all hot rocks with the same characteristics but also gold with similar decay as the hot rocks, you would need to be careful doing this but it can make undetectable areas detectable.
The higher numbers are pushing the limits of the GPX and GP detector technologies, the higher frequencies do not work the same in all modes , especial fine gold on a GPX 5000 as fine gold already seems to switch the receiver on faster and thus you cannot crank up the gain and frequency with high capacitance coils as the detector will go deaf.
Minelab make these detectors in such a way that the operator cannot get into trouble by putting fixed limits on the gain and fixing the frequeny. By knowing what you are doing and by changing these fixtures greater performance can be obtained. My own tests indicate that using high numbers in the manual tuning small gold sensitivity is greatly enhanced, specimen gold can be detected that normally would be ignored, greater ground control can be obtained by matching the front end gain to the ground type and adjusting to just remove excessive ground growning while swinging the coil.
So these are the things that need to be considered, ground type, coil type, gain settings both front and back, the front one we add is far more important than the one in the back menu, i always say put the back one on say 5 or around that number but it is not hard science, it is only a suggestion to get you started , i have seen operators use it at 18 and turn the front one down to work non mineral soils in high emi areas, i have seen professional prospectors turn the back gain to 1 and the front nearly all the way up using fine gold but using a lower frequency.
The correct way to understand the moddifications is to test the detector with the coils and ground type, there are so many variables that it is impossible to write up settings due to all variables. As an example i use a 14" mono coil with back gain on 6 and front gain about half in normal mode and frequency of around 160 around Ballarat, when I go to Dunolly i use back gain on 8 and lower the front gain to 1/3 and the frequency to 120 but this is based on the ground type. At the end of the day you need to buy a test target in the ground and test the settings to see what is the best, after a while you will master the controls and get more gold.
