Sometimes we learn to do something without understanding why it is important. For example, these days most players with tube amps know that we have to get our power tubes biased when we replace them. But the reasons why always seem just a bit mysterious because the related folklore among musicians clouds the issue. You could play a tube amp all your life without knowing or caring about the reasons, especially if you have a great tech to maintain it. Still, having a grasp of the general concept can give you more options in fine-tuning your amp's performance. It could also provide just enough information to get through a tube failure in a pinch.
What Does Biasing Mean?
Electronic components like tubes in circuits need specific voltages at their terminals in order to work properly. Most components in a guitar amplifier don't need bias adjustment; the circuit is designed to provide the correct voltage in the right places. But since power tubes are a consumable part that needs to be replaced regularly, we need a way to adjust the bias voltage that governs how much current the tube draws. Setting that bias voltage is what amp technicians commonly refer to as 'biasing your amp.'
Why Does it Matter?
Biasing is important because there is a lot of variation from one tube to the next. Each set of tubes you put into your amp responds differently to the bias voltage. If the bias is incorrect your new tubes might draw too much current and burn out. If the bias is too far the other way they won't draw enough to operate efficiently and they won't sound right. You wouldn't enter a stock car race without getting your motor tuned up properly for similar reasons.
What about these tube companies that say if you use their tubes you won't have to re-bias your amp?
If you buy regular tubes and take them to your technician to get the amp biased, they put them in the circuit and adjust the amp until the tubes draw the proper current. When they're finished you have to pay them for their time. The boutique tube companies take a different approach. They take all the stock tubes and put them in a test circuit to measure the amount of current they draw. Next the tubes are sorted into bins of tubes that draw the same current. Usually they are labeled into categories rated 'hard' or 'soft,' or sometimes identified by current ratings. The first time you buy these current-matched tubes you still have to get your amp biased by a tech. But the theory is that the next time you need tubes you can order the same set from the tube company and they will bias properly in your amp without changing the adjustment. This can be an advantage if you're on the road or at a critical show and experience an unexpected power tube failure. Just keep an extra set of matched tubes on hand for quick replacement without re-biasing. Of course just like your technician, the boutique tube companies charge you for matching the tubes. So it is up to you to decide the best approach for your needs. Do your research: some wholesale tube importers sell tubes that are 'matched' within the set but not from one set to the next. Make sure your supplier allows you to request the same 'hardness' or current-rating as the last set you purchased.
It is worth noting that preamp tubes don't need to be re-biased; the circuit is self-regulating. There are also a small number of amps with a different power-amp design that are self-regulating, called "cathode-biased" amps. The manufacturer should tell you if your amp needs to be re-biased in the operator's manual, or you can ask your technician. So that tech-savvy users can adjust their own bias, Carvin Audio provides detailed biasing information on our current models here.
As always with electronics, don't ever open the amp chassis unless you have the training to do it safely. Amp techs are less expensive than doctor bills! And now that your amp is dialed in to perfection, get out there and make it sing!