It probably won't measure much. Consider how much less wire that winding has than the HT winding. Then consider you have a bunch of filaments in parallel with it.
Edit: It doesn't track with, "only does it with the el84s in", but those amps are notorious for problems with BR1. It is for the DC filament supply of the V1-3. They are usually intermittent or open, but a shorted one wouldn't surprise me. They run very hot. The rectifier is supplied by the filament winding, so it does track with, "If i pull the heater fuse F4 , for the el84s and PI , it goes away.". It's worth checking anyway as it's a common problem. If you end up needing to replace it, stand the new one off the board a bit to keep heat away from the solder joints and board.