I would say Nigel is correct here...
Modern sets, certainly all that I have seen in the last 15+ years, have all used one scheme or another for automatic tube compensation. When the tube can no longer be controlled by automatic means, all sorts of poor picture troubles show up. Probably among the most common would be flaring of one gun or another, usually seen as streaks of colour around edges in the picture, but at the same time a lack of definition of that colour in the actual picture content as a whole.
Loads of fairly modern sets will actually blank the picture completely when the tube wears beyond automatic control. Philips sets seem to be very prone to this in my experience, or maybe I have just been unlucky...also most of the sets in recent years I have ran across using Philips tubes all seem to have had tube issues of some sort or another. About 5 years ago I had two that failed within warranty, the original factory fitted tube, and the replacement sent from the factory several months later, I chased the set to another dealer the last time to fight the warranty claim.
A common trick, to get a last gasp from a dud tube a good few years back, was to reduce the filament resistor in value a little, making the cathodes a little hotter, but to be honest, the mileage of doing that in a modern set is doubtful at best. I think your best bet is to rip out the chassis and donate it to your local repairman for spares, and scrap the cabinet and tube at the local amenity tip. It's unfortunate, but we now live in a throw away world, and the cost of a repair of this nature would be better spent on a new set.