What Do You Hear When Your Amp Needs Biasing?

Hi, Everybody,

I was listening to my rig the other day and noticed that some (but not all) closely miked recordings of female singers (Diana Krall on some of the cuts on "When I Look in Your Eyes" comes to mind) sounded a bit edgy or harsh. I checked the bias on my Manley Mahi amplifiers and found that some of the values on the multimeter, which should have been at 250, were as low as 240. I made the bias adjustments and, lo and behold, the vocal tracks sounded better. So I'm wondering what, in general, is a tipoff that a to be of a fire needs to be biased?


Tube bias can drift as the tubes age. Especially when the tubes are new. As a rule I like to check the bias every few months. Much more often when the tubes are new.

The higher the bias the better the amp can sound to a point. But that will also have an impact on tube life. The manufacturers recommended bias is usually the sweet spot for both sound and tube life.

Thanks for the post. What I was wondering is what the audible signs of a tube out of bias typically are.

You really don't want to wait until you can hear an audible difference. The bias can also drift upward beyond tube tolerances and if left unchecked, lead to a tube failure. It is best to check the tube bias often and make any adjustment that is necessary. If you start to hear an audible difference you are not checking the bias often enough. Think of a tube amp as a fine sports car. You want to keep the engine finely tuned and running well. You don't want to wait until it starts running ragged before you consider having it serviced.
On my Ayon Spirit II I audibly hear certain piano transients as having a ringing to them. Also, harshness in clarinet, some vibes transients, and some sibilance in female voices. What rectifies this situation is biasing to ensure that all 4 tubes are at the ideal setting, and moreover, that they are set identical to one another. When the adjustment is done, these distortions/harshness issues go (almost completely) away --- being reduced drastically.
I find it hard to believe you could hear the difference between a setting of 240 and 250. Actually with a slightly lower bias setting the vocals could sound a little smoother unless it was only one tube here and there that was low. In that case you could have one or more low biased tubes in a push pull pair that is low which would cause a rise in distortion which would explain hearing edgy or harsh close miked female vocals.
Your amp need bias if it is sounding too neutral and uncolored. I recommend playing Rush's show to increase the bias on the right channel but since Keith is off the air I don't know what to use on the left channel.
+1 Stan
More often than not I'll think I hear something off, check the bias and it's still dead on. There was one time however that a certain piano concerto I'm very familiar with sounded a bit zingy, so checked the bias and one of the power tubes was way off.

Most of the time though I think it's pretty hard to hear the small wanderings that are common.
What does the manufacturer recommend as to when you are suppose to recheck bias on the amp? That is the criteria I would use.


All Manley says in the Mahi manual is that the number should be 250. On average I've been checking them about every other month.
Pretty simple, when your tubes need biasing, Kenny G music will make your both your ears bleed.

When perfectly biased, only one of them will.