Biasing amplifiers

Bias drift occurs for many reasons, the tubes themselves can cause drift, the condition of the amp, and even its design can cause drift, and the time in use can cause drift.

It also depends when you measure - many amps will have bias drift during warm up continuing on 'til everything is warm and stabilized. Some amps will drift downward with excessive continuous use.

1ma is not(!)a big drift unless you are extremely anal. :-)

1mA drift is only 1.5% off. I wouldn't worry about it - just enjoy the music!

all the above is right on, but if the tubes get to a point where the bias is all over the place (i.e. it won't hold), that's a sign that your tubes are getting old.

Bias also drifts with variations on the AC powerline. It sounds like you've got SET's, in which case there's no pair-matching of output tubes to worry about either. Relax and forget about this unless one of the tubes stops being able to bias close to the recommended value within the range provided by the set-point adjustment, in which case you should suspect the tube's gone bad and replace it.

Thanks very much for the replies. The amps are the latest version Cary CAD300SE Sigs, but with manual biasing as it would give me more flexibilty to tune the sound a bit as per preference.

I generally start biasing after the amps have been warmedup about 1/2 hour. But ofcourse i have to switch them-off again to connect the meter & to remove the speaker leads etc. as per instructions. Then on powerup I will leave the meter connected & run the amps about 15 minutes letting the bias settle. Hope this is correct.

Speaking of a 300B tube or more specifically WE300B's , is there a minimum & maximum bias level i should stay within?

Also, if any of you have prior experience with the Cary's+WE300B what bias level were you having at the time.