Sometimes knowing what you are doing means knowing when you are in over your head. Today's question, while excellent, was one your inSync editor didn't feel he had enough experience with to authoritatively answer. They say it's not what you know, but who you know, and in this case I knew where to get the answer.

First, the question:

"I've been told to do 24 hour burn-ins on all my new tube mics. What does the tube go through in that 24 hour period (if anything really) that makes the tube better? Also, is longer better and can you break the time up into several shorter sessions of burn-in if you need to use the power supply or take some kind of other leave?"

Turns out one of my friends and mentors, Steve Wilson, knows everything and more you'd want to know about tubes and tube amps. In his spare time he does audio for the Kentucky Headhunters, but he's known in some pretty elite circles as a master of tube amp mods (he's done amps for the biggest names in the business). He also has a neat product called Bias King we mentioned in inSync a couple of years ago. (Cheap plug yes, but I wanted to give him something for helping us out here.) Here's his answer unedited.

"A vacuum tube operates on the principle of thermionic emission. When metal is heated, it gives off, or 'emits', the electrons that have been deposited as a coating on the tube's cathode during the manufacturing process. Preheating the tube can extend useful tube life by making the emission more uniform. This keeps the electron stream more even, reducing hot spots, thus promoting a longer useful life."

"I can't really say that I know about 'breaking up the warm-up period', but instinctively I would think that 'all at once' would do a better job. Think, "cake in oven, 350 deg, 4 hours." It just seems it would 'bake' more evenly if it were left on for the full time allowing the temperature to stabilize the coatings, thus avoiding any 'thermal shock' cycling."

It is also widely accepted that most tube-based products tend to sound better (warmer) after they've been on a while. Whether this is solely due to more uniform emissions is debated, but that surely has some impact on the performance of the tube.