You are right that there were lots of JARMANs in Wales and Welsh families may be the ancestors of some American JARMANs especially those who settled in Chester Co. PA before 1700. But there were also great numbers of JARMANs and GERMANs all across lower England - Devonshire possibly has more than Wales. The name seems to derive from naming boys after St Germain and passing the name and variatious spellings to their sons as surnames. Consequently I would expect that JARMANs do not have a single common ancestor. This suspician is supported by modern surname distribution studies which fail to find a central concentration of the name in UK.