My SP-600 is the '51 Contract one by Northern Radio. It is a J-11, NO X. It is a diversity model. It has some of its own particular quirks. Bob - N0DGN On 12/4/2012 1:50 PM, David Wise wrote: > Issue 6 and JX-21A Issue I are 6AC7. > JX-17 is 6AH6. > > Was the JX-17 the only version updated to 6AH6? > Is anyone actually using the FCU today? > > Thanks, > Dave Wise >