I built an amp very similar to a Princeton with the main difference being I used a 6AU6 in a pentode mode as the second gain stage. The pentode stage has local feedback in order for it to have the same stage gain (about 60) as the 12AX7 stage it replaces. The only issue I see in the resulting amp is that the 1 meg audio/algorithmic pot volume control that feeds the second stage doesn't seems as "linear" in the volume control as on a "standard" Princeton clone I have. All the volume seems bunched at the top end of the pot as if it wasn't a logarithmic pot at all. Can't figure out why? Could this be due to the difference in input impedance between a pentode and a 12AX7 triode???