Why does it take so many hours to brea in arc preamps and amps?

>> "I have yet to hear a tube amp that sounds as good after 5 minutes than it does after 1 hour let alone 4 hours." <<<

I think you are confusing 'warm up time' with the process of 'burning in - breaking in.'  Warm up time may take for you 5 minutes and is how long it takes for a component to stabilize after turning on.  Burn in time different, and is a transformation that takes place that causes a component to reach its design potential. Its like breaking in a new car engine that can take hundreds of miles.  Its not to be confused with how long it takes an engine to warm up before running.

My passive pres work right away. Just kidding, I have many others which need break in!

My passive pres work right away. Just kidding, I have many others which need break in!

"Let’s play with the "burn in" crews heads.

If you think about it, all the optimum adjustments are made at the factory after the amps been on for a while, maybe an hr if your lucky.
If thing change after 600hrs then those optimum factory adjustments have just gone out the window and need to be redone again.

So it’s either out of adjustment because of aging, and therefore not optimum, or the factory needs to have them on for 600hrs before they are adjusted and sent out to the shops. 
Either way if things change they would be out of the original optimum adjustment. "

Cheers George 
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In response to this, "burn in" and "being out of adjustment" are not the same. When a unit reaches burn in doesn't make it out of adjustment and requiring new setup from a technician. The components may be aging but this is a good thing. Everything reaches its optimal state of age and for most electronics it takes a long time to consider them not good. Generally I think under 100 hours is the most you should expect to break in a large variety of components. The amount of "current" can play a role as well. 

In fact how a unit sounds after break in is a planned part of prototyping to final design. All electrical parameters are set to reach this "break in sound". 

During break in an electrical effect happens, the flow of the electrons across all of the components causes micro surface arcing and tends to smooth and create a shortest route pathway. There is also a small magnetic field that develops around all paths which affects signal flow and interaction between materials.

To give you a visual idea, all this happens over time like water flowing through a pipe that had very fine sand in it. The effect smooths and polishes the edges and surface of the pipe, to electrons at an atomic level it also aids in flow. The flow of electrons can be a violent one, if they "carve" themselves the optimal pathway noise decreases and more of the original signal is heard. 

In response to this, "burn in" and "being out of adjustment" are not the same.

If you know better convince Nelson Pass of this below, in case you missed it.

Nelson Pass:

"We burn products in for two reasons -

1. We want to see if anything fails.

2. We want to readjust the amplifier against any drift in performance that comes with a burn-in."

Nelson Pass on "Burn In"
"It's pretty clear that any such long term behavior is going to be
obscured by the burn-in of the listener. People come to new
audio components carrying the experience of the previous
equipment, and may experience some dissonance with the new
sonic character, even if they like it overall. Over time they often
get used to it and grow to like it.
There are plenty of cases where they initially like it, but the sound
becomes irritating over time. That is called burn-out."

Cheers George