Bias Glow on DBT Bulb?

I biased a Sansui AU-5500 30W a channel amplifier with a 72W bulb but didn't get a slight bias glow on the bulb that is usually there when biasing bigger wattage amps? The bias setting in the service manual is 25ma give or take 10ma. I changed to a 40W bulb and still didn't get a bias glow, is it that there is just not enough current to make the bulb glow slightly? I'm just being careful before I put it on the mains power.:D
A low power amplifier such as a 30 watt unit probably has rails around 20 volts. That would put the bias power around a half watt. Not really enough to give an indication on a light bulb rated above 10-15 watts.

You might try playing music at a very low volume and when the bias is too low the distortion should stand out. Then adjust until it just barely goes away.

No glow at 40 watts it is safe to go to mains power directly.


Paid Member
2003-06-12 7:04 pm
Maine USA
The idle power of a 2*30W amp at 25mA is about 3 Watts. I would not expect a 40W incandescent to come into "glow" at that power level.

Yes, bigger/hotter amps may glow even a 72W lamp.

The point of the DBT is not to make fine adjustments but to delay burn-up when something goes wrong (bias runs-away to 25 Amps, etc).