In a recent thread I recommended using 10 turn pots for bias adjustment. This allows fine adjustment of bias. BUT ..... A month or so back one of the 10 turn bias pots failed >>> redplating! A nice raspberry red. Replaced with a spare I had. Today I hear crackling from the speaker after about 5 minutes warm-up. That's odd I thought. Looked to see what may be causing this >>>redplating! A nice cherry red. Another 10 turn pot failure. So ..... from this experience, I recommend sticking with single turn pots (and the associated coarse bias adjustment). The added mechanics within a 10 turn pot seems to increase risk of failure. (I could have tried some Clarostat multi-turn pots, but $$$ ...... and I'm now shy on the idea.) Ordered some simple pots today - lucky I have a s/s amp to twiddle with.