https://static1.squarespace.com/sta...7197843e46b/1510591099401/Judah_TubeChart.pdf From Bad Cat's site. Should be the one right in front of the 6v6s. Or, if the amp has reverb - could be the driver tube. If that's the case, the PI is the one to the left of the EF86 in the drawing. Try swapping both and see what your ears hear.