The bulb is situated in the centre of the roof of the oven, it is covered with a glass cover which unscrews, (must be that " bulgy protuberance" you described) it is usually quite tight to remove. There is a special plastic tool to make it easier if not you can try a jam jar lid remover.Wear thick leather gardening type gloves as sometimes it cracks all way round middle leaving a sharp edge. cleaning the roof you will have to work around the element as it does not come out .cheers
Kenjo, January 2015