You can fine tune your tube amplifier with one tube substitution. The tube that you want to change is V1 which is the first gain stage preamp tube. You can tailor the gain structure with different tubes making your amp run cleaner or dirtier and also change the dynamics. For example changing a 12AX7 for a 5751 ,which has 30% less gain, your amp will be a little cleaner and be more scooped in the mids along with a little more chimey top end. You can identify V1 on your amp by looking at the back of the amp. The tube closest to the input jack is usually V1. On Fender amps it is to the right and on Marshall’s it is usually to the left. You can also open the amp and look at the chassis tracing the wires from the input jacks to the tube. On amps with two channels like a blackface amp you can experiment with both first gain stage tubes. On channel switching amps you may have to consult your schematic or open the amp up and look at the chassis. You can experiment with many different types to suit your taste and playing style. There is a tube substitution chart in the article “Preamp Gain Factors and Substitution Chart” on the Articles page. Click here to check out the Substitution Chart.