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.
Aloha.
