I would need way more info. Mullard used all kinds of tubes, some were their own, and some weren't. At the bottom of the tube, there should be a code etched in the glass. This will tell us the year, place, and typre of tube you have. Please note that there is a HUGE difference between an XF1 Mullard and an XF4...that is why we need the codes. Post the code, exactly as it appears on the tube, and I will tell you what you have. Price is going to be all over the map if you don't have a tube tester.