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My bias readings tend to wander all over the place. Sometimes a given tube will be plus or minus ten percent of the 70 mA it ought to be running.

Is this normal? Do the output tubes "settle down" after they've run a couple hundred hours?

Are you measuring the bias with no signal ?

I mean are the readings "wiggling around"
or are you taking them weeks apart ?

The tubes age, temperature affects things, as
does line voltage, how long the amp has been
on when you measure them, and even the
resistance of your test points.

10% is nothing to worry about.

... tom

TomMcNally wrote:Are you measuring the bias with no signal ?

Of course. Dead short on the inputs.

or are you taking them weeks apart?

Maybe checking every fifteen minutes or so.

The tubes age, temperature affects things, as
does line voltage, how long the amp has been
on when you measure them, and even the
resistance of your test points.

Amp has been on for at least and hour or two to let everything "warm up". I've got a couple of meters, and I'm just leaving the probes stuck in the test points.

10% is nothing to worry about.

Good! I'll stop worrying! :)

While tubes should be pretty steady after a warm-up period - heck, ten minutes even - I would guess that your AC mains is the culprit. It can definitely drift around thanks to a lot of variables. I bet it's your neighbor's blinking Christmas lights. :)

You're right about the line voltage. I guarantee it wanders all over the place. I wasn't so concerned about keeping the bias dead nuts on 70 mA, but rather one tube goes up while the other goes down. Sometimes they're 10 mA apart from each other. I'll go and fiddle the balance pot, and an hour later they're 10 mA apart the other direction.

edit: I woke up this morning and said "Gee, I'm stupid." They're not drifting 10 mA; they're drifting 10 mV. That only 1 mA apart. How could I forget the resistor is 10 ohms, not one? Guess I can stop fiddling now.

Thanks for the smile Ty ... it's been a crappy day.

... tom

Whew! I was kinda worried for you. Yellow_Light_Colorz_PDT_04