I finally found a use for the pair of PX4 I had sitting around . They both test very close , but one of them has erratic anode current on the tester . It seems to vary +/- a few mA after a minute or so , never seen this before . I've reflowed the pins no difference . When the valve is tapped no difference . It just does this randomly . There is a slight blue glow around the inside of the glass but no grid current drawn on the gas test . Anyone have any ideas what is going on ? Oven baking job perhaps ? Fingers crossed the thing is not a write-off . Otherwise anyone have a spare they can swap for my single finned anode ST ACO44 ?