The 66tr was my first machine as a pimple faced jr. high kid in those days. Re.: Frankn's 6 inch answer. Yes, perhaps it could get a coin that deep, but ......... that was the max limit. Realistically 4 or 5" on a good day. But if you had it dialed in tight, and were listening for whispers, yes up to 6". But it has no form of disc. (other than the inherent ability to null over small iron and individual nails), and would do poorly in mineralized and/or un-even ground.
For vintage, sure, they're a cool-looking "buck rogers" type machine to have in a collection. But for actual usage, they're a dinasour. And for $100, you probably could have got any number of modern used units that would do better. JMHO. Oh, and as an all-metal TR, they also had the innate ability to see through small iron. So technically speaking, that would make them a good ghost town machine (assuming the coins weren't too deep). But to be honest with you, there were other TR all-metal machines of that era (namely the Compass 77b) that would do the same thing, and do it much better and smoother, in my opinion. Both were susceptible to larger iron (RR spikes, cast iron fragments, etc..) yet would null and see through small iron, like individual nails. Other than that, they're complete dinasours.
