Welch Allyn 2.5V Halogen Bulb (03400) We supply a wide range of replacment bulbs. Deciding on which replacment bulb you require is easy: 1. Firstly remove the old bulb from your Welch allyn instrument head 2. Make a note of the product code that will either be printed on the side of the bulb or moulded into the plastic base on the bulb 3. Cross reference the product code with the bulb code.