V1 drives everything. A 5751 is similar to a 12AX7 but in V1 it will lower the gain stage. You won’t get much more headroom because it’s too similar. You need a 12AT7 before you will really notice a noticeable difference in headroom. But overall ratio of preamp to power amp dirt is lower.
Changing the phase inverter tube type will change the percentage of how much preamp signal is passed to the output stage. So it has a more dramatic effect in taming an amp. You get less power tube section distortion at higher preamp gain, so that gain is more useful. Usually on a 5150 anything past 7 just gets muddy and feeds back. This allows usable gain to 10. And it’s less distorted on the power tube side so it’s more articulate sounding (like a German amp?). A 5751 there lowers how hard the power tube section is hit by 10-30% and a 12AT7 by 20-40% (based on contrasted brands gain factors) IIRC.
Yeah, I have piles of tubes of every kind...CP & NOS.
Got some nice old Mullard X2f's el34's I've been saving for a future build.
My memory is fuzzy on the 5150/6505 schematic,
I though people were looking to lower the amount of gain JUST for the the drive channel and not to globally affect the amps sensitivity.
I do remember about ten years ago many 5150 owners
were hunting down old Black Plate 5751's for either v2 or v3.
Again, strictly to have an effect on the drive channel...soley.