Biasing a Marshall M1989

[patlaw]

This Marshall M1989 (discussed in another thread) needs to have its 6550s properly biased. What is the recommended plate dissipation? Is 70% the best idea?

[pdf64]

Based on a 25 watt plate, 70% seems a reasonable guideline; I don't see why 6550 would need a wider conduction angle than EL34, or 6550A wider still?
Generally, most of the AB amps using the bigger octals seem to work and sound fine with 30-35mA cathode current.
The point of the 'idle at 70%' guideline was that with signal, the average max plate dissipation shouldn't then exceed 100%.
Whereas I suspect that the point of swapping from EL34 to 6550 for the North American market was that they're a higher capacity, more robust tube, that should never get anywhere near 100%, so should be way more reliable; then the internet and the 'idle at 70%' thing came along.