Hi Dennis,
There has been a good debate over this very topic of what is the best autotune speed. To be honest, it is a tough question to answer. The reason lies in the interaction between the ground and the buried object.
An air test will clearly show a small depth loss as the autotune speed is increased. However, the issue is more comlex than that.
In low mineralized areas where the ground produces little if any signal, then yes, a very slow autotune will generally produce the best results.
However, as the ground mineralization increases, then the autotune speed becomes more critical. This problem is further compounded by the actual physical layout of the ground, meaning is the ground rough or uneven, very flat or level, or something in between.
Ground signals generally are slower responses, while a buried object's response is a much more abrupt signal change. It is this distinction that allows a VLF discriminator to work. In other words, the filters in a VLF discriminating circuit are much like the autotune except they are very fast in comparison to the speed of the autotune.
Now, when the ground signal is strong, it can drive the audio threshold quiet far enough that any small change that might be heard normally, will be lost. By advancing the autotune speed, the audio level change caused by the ground can be minimized, thus making it more likely to hear a target.
I have conducted a whole lot of tests trying to find the ideal autotune speed and have determined, there is no one "best" setting. In other words, it all depends on the ground conditions. I normally run my autotune at about mid position but will deviate from that setting depending upon just how strong the ground signals are. Generally, I will try to run will less autotune speed when possible, but I am not afraid of increasing it even to near maximum in the worst case conditions. Normally, the depth loss due to a faster autotune creates less of a problem than a strongly varying ground signal.
About the best answer I can give is really a recommendation, and that is to try burying different objects down to the maximum level of detection and try different autotune speeds. This should be especially done in area where you plan on nugget hunting, so you can get a feel of what works best for you. I think you will find some areas will allow the use of a slower autotune, but others will require a faster setting to obtain the best results, which, of course, is the reason for the autotune feature in the first place.
Reg