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    Line voltage and bias



    Posts : 187
    Join date : 2012-03-19

    Line voltage and bias

    Post by wildiowa on Tue Feb 04, 2014 2:58 pm

    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.
    Bob Latino

    Posts : 2644
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Line voltage and bias

    Post by Bob Latino on Tue Feb 04, 2014 4:25 pm

    If your line voltage runs in the 125 VAC - 126 VAC range you should either run your amp off a variac set to 117 - 118 VAC or as a less expensive (but not as good an) option > Run the amp using a 5U4 rectifier. The 5U4 rectifier will not change the AC voltages in your amp but will drop the DC voltages. The advantage of the variac is that it will also keep filament voltages down where they belong (about 6.3 VAC) and tube life will be extended.

    Also - if your incoming AC voltages change drastically during the day, your amp's bias voltages will ALSO change drastically during the day and you will always have trouble keeping the bias steady.


    Posts : 1466
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Line voltage and bias

    Post by sKiZo on Tue Feb 04, 2014 8:18 pm

    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.

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    Re: Line voltage and bias

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