As a rule, a 5751 is less microphonic and quieter than a 12AX7, especially when you are comparing a NOS 5751 to current production 12AX7's. It has a higher gain factor than a 12AT7 and I would always suggest trying the 5751 first. If the amp still has noise, then it is probably coming from a non-tube source and you would have to let a tech have a go at it. If you are going to use a 5751, they have NOS one's for sale on "The Tube Store" or "The Tube Depot" and the prices are reasonable. Take care, Marty
