18vdc vs 30vdc

[mountianjustice]

ive built a few valvecasters and would like to up the plate voltage on one of them will i notice any difference between 18vdc and 30vdc using a 12au7 or 12ax7 tube?

[Transmogrifox]

You will notice it takes a lot hotter drive on the input to distort with 30V plate voltage.  As to whether you want more clean headroom will determine whether you prefer this.

You will notice the output is significantly louder with 30V.

At 30V it will begin to behave more linearly, so the clean headroom will be more "open" sounding.