Will just 4 volt loss in wall power hurt tone?

[SlyStrat]

I have a Furman Power Factor Pro with voltage indicator. My Marshall amp tone gets harsh and thin sometimes. The other night I noticed the voltage was about 116 and fluctuating. My tone was trebly and thinner.
Yesterday it read a consistent 120, and my tone was good.
My imagination?
Suggestions?

[Old Tele man]

...changes in line (wall) voltage are MAGNIFIED in the power transformer (PT), so a 4V(rms) change at the PT-primary can become ±20-50Vdc(peak) change at the PT-secondary...changing both the B+ and bias voltages!

...the order of "affected" votlages:

bias voltage......causes most! amp change
screen voltage..second most amps change
plate voltage....causes least amp change

[ryhlick]

It is not your imagination, that happens to me sometimes. I will use my meter to check the outlet and have had it as low as 114. Big difference in tone from the normal 119 that it general puts out.

[John Phillips]

I have a Furman Power Factor Pro with voltage indicator. My Marshall amp tone gets harsh and thin sometimes. The other night I noticed the voltage was about 116 and fluctuating. My tone was trebly and thinner.
Yesterday it read a consistent 120, and my tone was good.
My imagination?
Like Old Tele man said, although the internal voltages change by the same amount proportionately (since they are all related to the supply voltage by the same fixed turns ratios in the PT), they have disproportionate effects - especially because the tube current is very sensitive to small changes in bias voltage. (This is actually why amps were fitted with bias trimmers originally BTW - because the current is more sensitive than the typical resistor tolerances used to set the bias voltage could be relied upon to achieve - not to fine tune the tube current for tone.)

Running a fixed-bias amp at lower supply voltage usually runs the tubes hotter at idle - because the bias voltage is reduced, which increases the tube idle current proportionately more than the plate voltage is reduced - though not at full power, because the reduction in plate voltage reduces maximum dissipation, as the maximum tube current is not increased and may also decrease.

I have to say I haven't really studied how much the tone changes from relatively small supply voltage variances, but it certainly does for large ones, so there must be an effect for smaller changes too.