Since: Apr 08, 2004
Tone, and long tube life
Bias is a level of electricity going into the tube, at idle, so that the tube can amplify the AC signal correctly.
So anytime there amplification going on, the device doing so is directly related to tone.
Running tubes too hot (high bias) can sound very good, but eat tubes at a faster rate.
Running tubes too low, can lengthen tube life, but make the amp sound thin, and weak.
Some people don't bother to bias, and don't have problems. BUT, you could run the tubes outside of their 'happy' range, thereby incurring the a-fore-mentioned problems.
Mostly, if people don't bias, and the amp's working OK, then the bias setting is still in the good operating range of the new tubes. I'd guess most times you'd be in the Ok range, but I don't like to guess.
Setting bias can be done multiple ways, but a few of those are deadly dangerous, while other ways are somewhat expensive. There are a few in between, one of which I use, but it's still dangerous, as you're poking around inside while the amp is running: LETHAL VOLTAGES.
If not sure how to proceed, but want it biased, you can get a bias probe, similar device, and do it yourself, without getting inside the amp.
There's one example, from a good shop.
If you just want to play without worrying, Groove Tubes marks their tubes with a bias number. This system lets you set your tubes bias for a certain level of GT tube - say 6 - then when you retube later, you can buy GT tubes of level 6, and plug them right in. No worry about bias, as the new tubes closely match the old ones.
But you'd be stuck running GT tubes from now on, which ain't the best option, in my humble opinion.
All this bias stuff applies to output tubes, as preamp tubes are cathode biased, meaning you can't change them easily. The bias is set with a resistor. Output tubes are (mostly) fixed bias, meaning you control the DC bias with a potentiometer inside the amp.
Of course, you could get to know your local tube repair guy (if you have one) and have him bias it up everytime you re-tube. Doesn't hurt to network =).