Bias measurments- warm amp vs cold amp

[62vauxhall]

The service manual for an amp I recently acquired said bias should be set for 4mv after it warmed up for 5 minutes under load. I checked it right off the bat without warm up and it was about 2.5mv. Not reading the "fine print" first, I set it for 4mv but after being on and not hooked up to speakers, it climbed to about 10.5mv.

I intend to follow the manual to dial it in but is it "normal" for bias to increase 2 1/2 times between cold start and operating temperature?

 

[laatsch55]

Don't know if that ratio is the same with all outfits , but yes, it's normal to have bias rise as the temp rises, that's what is NEEDED to happen...

 

[8991XJ]

On one amp, just taking the cover off increasing airflow and hence cooling made the bias completely different. Hard to dial in unless it was turned all the way down and turned up in steps to reach the value because the temp of the heat sink had such an effect on the bias. Yes, your numbers are not out of line.

 

[Gepetto]

The service manual for an amp I recently acquired said bias should be set for 4mv after it warmed up for 5 minutes under load. I checked it right off the bat without warm up and it was about 2.5mv. Not reading the "fine print" first, I set it for 4mv but after being on and not hooked up to speakers, it climbed to about 10.5mv.

I intend to follow the manual to dial it in but is it "normal" for bias to increase 2 1/2 times between cold start and operating temperature?

Yes bias is a servo control loop intended to change with temperature.