I've just bought my second Photolux bulb after thinking the first was defective. Both have given me a prawn shaped hotspot which is completely unworkable. It's visible with and without the condensers - tried with both 35mm and 6x6. With another bulb however, which has a thicker coating, the light is completely uniform but the exposure times are running into a minute or more.

The enlarger is a Krokus 3.