{TAIL ~ Voltage converter} with GTO thyristors in {HEAD ~ series connection} with at least a choke with freewheeling circuit 
{ TAIL ~ Voltage converter } with GTO thyristors in { HEAD ~ series connection } with at least a choke with freewheeling circuit
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH