I have only found the miner john and whites coils GB on 8.5 on my SPP, the whites tdi pro manual states that it should GB around the 8 mark at 10 usec, the SPP is marked 8.5. OK, here is some thing interesting that i found using the SPP and from what i have been told by another operator who owns a tdi pro, it happens on the tdi Pro as well? When i ground balance at my home which is basalt soil it Ground balances exactly on 8.5 and when i take it out to the gold fields, guest what? It balances on 8.5 as well. Now, i don't know about you but that struck me as being a bit odd and lead me to the conclusion that maybe the ground balance circuit on the SPP and maybe the tdi pro are pre-set to the pulse delay and not independent, unless one uses the target conductivity switch in conjunction with the Gb circuit. So that got me thinking if the GB circuit is pre-set to the pulse delay on these machines, than what would the result of having a coil that ground balances say at 9 or so but the machine is set at 10 usec for 8.5. Considering that at GB of 8.5 at 10 usec is designed to give you the best response to small gold, what the effect of having a coil that GB's outside of this mark is anyone's guess, so instead of taking the risk i stuck with coils that i know GB precisely at 8.5.
It is no secret that pulse delay effects ground balance, the higher the pulse delay should see the detector seeing the ground as a lower conductor.
Maybe the ground balance positions of these other coils has no effect to small gold targets, but i was always skeptical if it did and stuck with miner john coils as they ground balance was spot on.
Please try Gbing the TDI pro in hot ground, you know when your in high mineral soils as the ground balance window on these machines gets narrower and narrower until its impossible to pick the middle point (note down your Gb position). Then take the tdi to a different mineralisation area and see what GB reading you get there, I would be very interested in your findings on the TDI pro.
link to tdi manual
https://www.whiteselectronics.com/manual/tdi-pro-manual/
go to page 23 and read Chart A- Mandatory settings if you can not find a Gb point. This is used mainly for benign soils where the ground is not reactive enough to create a ground signal, like a beach. But it it does point out the importance of correct ground balance setting.