I just read through the SLI-80 manual and they state that the bias should be readjusted for 100 ma. AFTER A TEN MINUTE WARMUP. It's not clear that you followed that recommendation.
It's also possible that a poor connection in the 1/4" plug you're using to connect the ammeter is causing the problem you're seeing. You might also try wiggling or twisting the ammeter plug when the cathode current is seen to be very low and observe if the meter reading jumps up.
By the way, solid state amp owners who have read the TNT document mentioned above should take the info to heart. Too high a DC offset at the speaker terminals will have a greater effect than just generating heat in the voice coils. The speaker cones will be physically offset by the DC current, compromising the sound quality and dynamic range of the drivers involved. And, you may damage the voice coils. The info about replacing the very poor quality trim pots typically used to set the DC offset and bias voltages is spot on. In many cases, with the original pots,it's simply impossible to properly set the offset and bias even when the pots are new.
It's also possible that a poor connection in the 1/4" plug you're using to connect the ammeter is causing the problem you're seeing. You might also try wiggling or twisting the ammeter plug when the cathode current is seen to be very low and observe if the meter reading jumps up.
By the way, solid state amp owners who have read the TNT document mentioned above should take the info to heart. Too high a DC offset at the speaker terminals will have a greater effect than just generating heat in the voice coils. The speaker cones will be physically offset by the DC current, compromising the sound quality and dynamic range of the drivers involved. And, you may damage the voice coils. The info about replacing the very poor quality trim pots typically used to set the DC offset and bias voltages is spot on. In many cases, with the original pots,it's simply impossible to properly set the offset and bias even when the pots are new.