The Model #551-3 Power Units is specifically designed for those jobs that might not warrant a more expensive heavy duty unit. This unit will give you all the advantages of Luma?s resistance soldering method with its speed, flexibility and economy. The #551-3 is built to Luma?s exacting standards and is capable of handling all but the largest tasks. Like the 551, it is a cheaper model with fewer heat stages, although it is just as powerful.