Here is the 830 minus its covers
One of the replacement 6146Bs
The two valves in Situ
Pa compartment cover screwed back on!
The good news is the 6146bs I had work. The neutralisation process I am not sure about however. As the handbook suggests I tuned the rig up on 28.500, turned the SG switch on the rear off and using my R1000 receiver adjusted TC1 for a minimum "s" meter reading on the R1000. The only problem was that I didn't get a sharp reduction in the S meter reading, in fact it didn't vary much irrespective of where TC1 was. So I left TC1 more or less where it was with the old PA valves.
At present the rig now has healthy RF output with a pa current well within the specifications. Everything seems stable with no fuses blown so far. I may get a local amateur friend to look at the PA neutralisation issue again before I use the rig in anger though!