Help With Negative Bias Voltage

[trem]

I am restoring an Ampeg V4-B.
Wanted to get the bias current lower, so I am trying to get more negative voltage at the power tube grids.
I replaced the 75k resistor with a 60k pot.
from there I have tried different values in the position of the 56k resistor. Seems like no matter which way I go with that resistor, I can not get the voltage more negative at the grids.
So, with the 60k pot, I have these voltage readings at the power tube grids with these resistors in place of the 56k.
51k... -51 VDC
100k... -46 VDC
150k... -40 VDC
20k... -45 VDC
7k5... -28 VDC
220k... -34 VDC
Do these numbers make sense.? I cannot get the grid voltage more negative than -51 volts.
What am I doing wrong.?
Thank You
v4bpowramp.pdf

[Enzo]

I think you are trying to make a 3 foot piece of string 4 feet long.

Since the 75k is across the supply, lowering its value would tend to increase current draw and thus drag the supply lower. SO a better choice for your experiment would be 100k rather than 60k.

About all you can do it increase the 56k to 100k or something AND increase the 75k as well.

Vut there is a limit to how much voltage can come through that cap, and once that is rectified and filtered, that will be the max voltage available.

[trem]

Per Usual.....Thank You
The tubes bias OK, but are just about at 70% now. So I wanted to get some more negative voltage at the grids.
I will try as you suggested, but like you infer, it may be easier to get tubes that draw differently.
Thanks Again