What is the 'Burn-In' process - Generally speaking; For decades tube manufacturers 'burned-in' tubes. This process was performed at the end of the manufacturing process. Tubes were placed on large 'burn-in' racks for where the tubes were brought up to full temperature. This process was designed to 'cure' the metals on the plates, develops key coating on certain parts of the tubes, etc. If a tube failed on the burn-in rack it was discarded.
What are the tonal differences - A tube that has not been properly burned-in tends to sound thinner, harsher & is less inspiring. By comparison a properly burned-in tube sounds more three dimensional, harmonics are richer and is easier to play. IE: 10ga strings feel like 9ga strings.
When did tube manufacturers stop burning in tubes - Circa the later part of 2008 into 2009, which corresponds to the beginning of the recession in the USA. We first noticed this as we began to experience an 85% increase in tube failures. We immediately began researching the problem and discovered manufacturers were no longer burning in tubes. IE: this is technically a labor step at the end of the manufacturing process. Removing this step thereby reduces labor rates and reduces the electric utility bill saving the manufacturer money. Unfortunately this produced a lower quality product.
This led us to designing and building our own burn-in racks. Tube failures and warranty claims were reduced by by 98% compared to tubes that were not properly burned-in.
Can power tubes still be matched if they are not properly burned-in - 'Yes' tube companies can and do sell matched power tubes. Technically they are matched when they are installed into your amp. Unfortunately as you play on the amp the tubes will spread apart sooner than later. Tubes that are properly burned-in and then matched will stay matched for much longer as you play on them.
Does burning in tubes improve their reliability - Yes it does, which is why tube manufacturers burned-in tubes for decades.
How long do you burn-in your tubes for - After exhaustive A / B comparisons over a 6 month term we have settled on 72 hours. Going beyond 72 hours did not yield appreciable results. It is worth noting that In the old days 40 hours was sufficient however the quality of metals was higher compared to today's standards.
