Mains voltage fluctuation and setting bias

[blackstone]

Recently biasing my amp and have noticed fluctuations in B+ voltage. On further research I've found that it's not uncommon for mains voltage to fluctuate 10-20V.

This got me thinking that if I set the bias when mains voltage is low, it could mean the output tubes will exceed maximum dissipation when the voltage is high.
So, say I set static dissipation to 70% when the voltage is low I estimate if it shoots up 20V static dissipation will be about 76% - not good.

Does anyone take this into account when biasing an amp?

Further when calculating bias; if you're measuring voltages across the OT primary to ground and the voltage fluctuates, means you get a wrong reading - since you can't measure all voltages simultaneously. Better (but maybe not safer) to measure directly across the coil taps.

[vibratoking]

You're on the right track with your thinking. I don't think the mains fluctuates so fast that you can't reliably set the bias. But the mains can fluctuate +/-5% over the period of several hours IME. Some guys want a very stable voltage supply so that their equipment operates the same regardless of the supply variation. Some use a regulated supply for this reason. I use a Furman AR1215 for this purpose. There are other options as well. On the flip side, many guys don't worry about it and do nothing.

[matt h]

(deleted)

[Merlinb]

This isn't really a big issue. No one biases their amp so precisely close to redplating that a measly 10% rise is mains will suddenly transform a happy amp into meltdown city. Is it biased to 70%? or 76%? what about 70.001%? They're only vague numbers- don't sweat the small stuff.

In any case, cathode biased amps are self adjusting and, as Matt noted, fixed bias voltage will also usually increase (more negative) with increases in mains, so it too is self adjusting in this sense.

[xtian]

Always happy to see you here, Merlin. Every couple of months I pick up your Preamp book and reread, slapping my forehead as new pieces of the puzzle snap into place.

[Smokebreak]

This isn't really a big issue. No one biases their amp so precisely close to redplating that a measly 10% rise is mains will suddenly transform a happy amp into meltdown city. Is it biased to 70%? or 76%? what about 70.001%? They're only vague numbers- don't sweat the small stuff.

In any case, cathode biased amps are self adjusting and, as Matt noted, fixed bias voltage will also usually increase (more negative) with increases in mains, so it too is self adjusting in this sense.

Of course biasing is forgiving 10% one way or the other, and could even be chaulked up to taste, but the fact remains that sometimes the same amp sounds different in various rooms, and sometimes rarely similar in the same room. Nonetheless I can always feel a difference , and have to work with that.
Is that merely a voltage difference or something else?

[JazzGuitarGimp]

This isn't really a big issue. No one biases their amp so precisely close to redplating that a measly 10% rise is mains will suddenly transform a happy amp into meltdown city. Is it biased to 70%? or 76%? what about 70.001%? They're only vague numbers- don't sweat the small stuff.

In any case, cathode biased amps are self adjusting and, as Matt noted, fixed bias voltage will also usually increase (more negative) with increases in mains, so it too is self adjusting in this sense.

Of course biasing is forgiving 10% one way or the other, and could even be chaulked up to taste, but the fact remains that sometimes the same amp sounds different in various rooms, and sometimes rarely similar in the same room. Nonetheless I can always feel a difference , and have to work with that.
Is that merely a voltage difference or something else?

More likely chalked up to room acoustics. Temperature and humidity can also impact the sounds we perceive within a given room, as well as placement of the speaker and our position in the room.

[cbass]

Your mains is fluctuating by 20 volts? That is a bunch I've seen 5or 6 volts but not 20

Or did you mean b+ voltage

[matt h]

(deleted)

[pdf64]

Temperature and humidity can also impact the sounds we perceive within a given room

I started tracking temperature and humidity of my band room at each session, alongside our perception of the tonal balance.
I can't get any correlation between them, however, atmospheric pressure seems like it may be a more significant factor with how sound seems to propagate in the room, eg lower pressures seem to reduce the feedback threshold.
Pete