Hey get2it1 - the SP3 is not an autobiasing amp, you need to re-set the bias whenever you replace the 5881s. Technically the 6l6gc is not a direct replacement for the 5881 and needs to be biased higher than the 5881. The 5881 bias should be set a 1.15V. Some fellow sp3 owners on the av123 forum calculated the 6l6gc tube bias as 1.60V for the sp3. When I put in my 6l6gc I just biased it at 1.15V with the plan of increasing up to 1.60 but I never got around to it because it sounds good at 1.15 and I figure I'm stressing the tube less.
As for actually biasing the unit you need to get a voltmeter if you don't own one already. You can pick up a digital voltmeter at Radioshack for under $20. Turn your system on and let the amp warm up for 5 minutes or so. Don't play any music while you are biasing the amp, this is important. On each side of the amp you will see 5 holes. Turn your meter on and insert the black lead into the center hole. Insert the red lead into the hole on the far right and check the meter for a reading. Insert a small screw driver in the hole between the leads and turn the screw until the bias reads 1.15V. Once this is done remove the red lead and insert into the far left hole and then adjust the screw between the leads until it also reads 1.15V. You've just biased the two tubes on the side of the amp. Repeat the procedure on the other side and you're done!
One thing to remember is that these tubes take about 24hours of play time to break in and during that time the bias will often drift so you may need to reset occasionaly during this break in period. After this breakin period you should only need to check the bias every 6 months or so.
Good luck!
As for actually biasing the unit you need to get a voltmeter if you don't own one already. You can pick up a digital voltmeter at Radioshack for under $20. Turn your system on and let the amp warm up for 5 minutes or so. Don't play any music while you are biasing the amp, this is important. On each side of the amp you will see 5 holes. Turn your meter on and insert the black lead into the center hole. Insert the red lead into the hole on the far right and check the meter for a reading. Insert a small screw driver in the hole between the leads and turn the screw until the bias reads 1.15V. Once this is done remove the red lead and insert into the far left hole and then adjust the screw between the leads until it also reads 1.15V. You've just biased the two tubes on the side of the amp. Repeat the procedure on the other side and you're done!
One thing to remember is that these tubes take about 24hours of play time to break in and during that time the bias will often drift so you may need to reset occasionaly during this break in period. After this breakin period you should only need to check the bias every 6 months or so.
Good luck!