AFAIK, they are autobiased. This is from memory but I'll check the manual tonight.
10 responses Add your response
They are not autobiased. If you are using single eneded RCA outputs the bias trimpot position doesn't matter, but for XLR balanced outputs you should adjust the bias after replacing any of the tubes. The easiest way is as follows:
1. Disconnect cable from preamp to power amp.
2. With the preamp warmed up for 20 minutes, play a 1 kHz sine wave from a test CD or signal generator.
3. Use a voltmeter set to AC, and measure voltage from pin 1 to pin 2 on left channel XLR output.
4. Adjust volume on preamp so voltage output is approximately 1V.
5. Measure voltage from pin 1 to pin 3.
6. Adjust left phase balance pot until pin 1-2 voltage equals pin 1-3 voltage - check 1-2 after adjusting 1-3 and adjust again as necessary.
7. Repeat for right channel.
The bias tool that comes with SF Power series amps
is perfect for adjusting the pots, otherwise you'll need a long shank small blade screwdriver.
Well, I checked the schematic for the Line2 and what has been suggested balances the bias between the two sides of the 2nd 6922 (voltage amplifier stage) in order to help insure balance between the two phases of the balanced output. There is no way to adjust bias directly nor any adjustment for either the input 6922 or the driver 6922.