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HOWTO Bias ASL Tube Amps

I’ve wanted to get into tube audio amplifiers for my stereo systems for a long time, so years ago I bought my first ones, a pair of Antique Sound Lab AV-25 monoblocks. One thing that was a bit tricky was to bias the amps, and the units didn’t come with directions, so I eventually got info directly from the company, but could’t find it anywhere online. I’m transcribing them here for my reference and for others, in case they have the same, or similar, amps from Antique Sound Labs.


AV-20 DT / AV-25 DT Tube Mono Power Amplifiers

Bias adjustment instructions:

  • A small slot-tip screwdriver and a voltmeter are required.
  • Install tubes and connect amplifier to loudspeaker.
  • Voltmeter set at mV, connect (-) probe of the meter to chassis or loudspeaker ground.
  • For out put tube 1 (VB-1) connect (+) probe of the meter to TP-1 and adjust VR-1 until meter shows 330 mV.
  • For out put tube 2 (VB-2), connect (+) probe to TP-2 and adjust VR-2 until meter reads 330 mV.
  • Allow amplifier to warm up for 20 minutes and adjust bias for both tubes again. 5% variations between each tubes bias readings is acceptable.

Bias should be checked each time output tubes are replaced or a defective tube is suspected.