Minelab makes 8 coils that will work with your 505. There are currently no aftermarket coils available. The 9-inch concentric comes in 3 kHz, 7.5 kHz and 18.75 kHz. The 6-inch comes in a DD at 18.75 kHz and a concentric at 7.5 khz. The 10.5 inch DD comes in 7.5 kHz or 18.75 kHz. And the 6 X 10 elliptical comes in 18.75 kHz.
In a nutshell, lower frequencies provide a better response on higher conductive targets such as silver and copper coins. Higher frequencies provide a better target response on lower conductive targets such as gold coins and jewelry. Lower frequency coils will find larger targets, deeper. Higher frequency coils are more sensitive to small targets. All three frequencies will detect all metals. But the audio response is different between them, depending on the target composition. Concentric coils will detect deeper than a comparibly sized DD, in moderate soil conditions. However a DD coil will detect deeper in highly mineralized soil, due to the DD design acting as a neutralizer for the mineralization. DD coils separate targets better as well.
After setting your noise cancel channel, set your ground phase setting (GB). Some folks find it useful to lower the sensitivity while setting the GB and raising it after reaching the proper GB. I run mine so that it is just on the brink of chatter. Set the Threshold to a level that is just barely audible. Setting it too high can result in a target not producing enough signal to overcome the sound of the threshold. Setting it too low can result in a target not produting enough tone to overcome the silence. Chose the number of tones you want to listen for and start detecting. Check your GB occassionally as soil conditions can change dramatically. The difference between a yard a the flower bed can be 20 or 30 points. HH Randy
