Mister Jalopy writes- Cool site that highlights some common tube amplifiers. Virtually guaranteed to name every issue that your amplifer DOESN’T have. This page is designed to lead you through debugging a problem with a tube-type guitar amp. Just click on the appropriate links, making the appropriate tests as needed. This page fits older Fender and Marshall amplifiers best, especially in the sections relating to amplifier specific things like preamps, reverb and tremolo. Good companion for volume 2! Link.
Tube amp debugging