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It is quite likely that with most amps 40% would be low enough to cause crossover distortion though. That’s the critical factor - *not* the percentage of maximum dissipation. The correct bias point is just above where the crossover distortion disappears, regardless of what percentage or idle current that is - that’s why Mike Soldano was biasing an amp using a scope.
With a particular type of amp the two are of course related, because they all behave in roughly the same way and so the crossover distortion tends to disappear at about the same idle current, so you can do it that way as a shortcut. For the standard Marshall 50/100W power stage it’s usually around 27-28mA - 13.5W or 55% of maximum dissipation. Hotter than that tends to just increase the risk of valve failure when the power stage is overdriven.
This is also one of the many reasons the DSL/TSL series is unreliable, they were biased up to 40mA - nearly 19W or 75% - from the factory. If you set them at 30mA they tend to be more reliable.
If you do it by ear, you need to be aware of the trap of the old hi-fi salesman's trick that slightly louder always sounds better, as well as listening for the crossover distortion. It’s easy to just turn up the bias a bit and hear what sounds like an improvement, but you don’t know if that’s due to tone or volume unless you measure the output voltage.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Ive owned most of the current range from Jet City and they’re bloody good amps, they destroy every other brands budget offerings. Currently getting to grips with the Amelia, which is pretty cool!
Then I can get rid of my Fender HRDiii