Line voltage and bias

I am the last residence on a rural electric line and voltage is usually 125 to 126. I was biasing last night, running between the amp and the microwave where I was heating up pizza left over from the game, and as I was adjusting the bias pot the microwave timer ran out and the oven stopped. The meter jumped enough to require a significant adjustment from where it had been with the oven running so there must have been pretty good draw from the microwave even when they were on separate circuits.

It always amazes me how line voltage and changes, even minor, can affect your bias point. I imagine that the bias would have been set too hot if I had not noticed it and would never have seen this phenomenon visually if the oven had not kicked off just when I was adjusting the pot.

Do yourself a favor and invest in a Kill-A-Watt meter. Takes the guesswork out of the equation as it allows you to collect data on high and low points during a test period. Just plug it in to the wall socket and read it back later.

One you've got your average, high, and low data, you'll be able to make a more educated adjustment. Keep in mind if you adjust your line voltage down using a variac, those will drop the power a consistent amount based on how you set it. Drop it from 125vac to 118vac, and if you end up with 120vac out of the wall at some point, the variac will be putting out 113vac. You probably wouldn't want to go much below 115vac as that would tend to brown out your equipment, which can do just as much damage.

Option B is to build yourself a bucking transformer. If your power is consistently high at 125vac, a common 6v transformer set up properly would take it down to around 119vac ... almost perfect. I've seen some nice ones built out of a common strip outlet surge protector for $25 - that way you even have a built in 5a circuit breaker. Just make sure you meter it out before plugging anything into it to verify it's dropping the power the way you want it to.