The rectifier tube in this circuit makes a significant difference in the sound of the preamp—
more so then most. Since each rectifier listed has a different efficiency
(voltage drop) it effects the voltage feeding the signal tubes. The lush and forgiving
sound of this preamp is because of the lower voltages it was designed around.
Raising the voltages will result in a tighter signature with a more forward sound and
less lushness. The 5Y3GT shipped with your preamp falls in the middle. Changing
to a 5AR4 will typically add to the lushness, while changing to a 5U4 will typically
have the opposite effect. This can be a valuable tool in helping you dial the signature
of the preamp in to best compliment the amplifier and speaker combinations
you might through at it.