Generally speaking when we talk about biasing an amp we’re specifically referring to the output/power tubes. However there are some applications where specific pre amp stages may have adjustment or calibration procedures. A great example of this is the original Ampeg SVT amps. These typically have two bias adjustments (one of each half of the poweramp section) and an output balance adjustment for the driver stage. With that said the vast majority of guitar amps do not have any pre amp section adjustments so the pre amp tubes are totally plug-and-play.
Cathode Biased amps are considered plug-and-play. Most cathode biased amps are designed to bias the tubes right at their maximum dissipation limits (see FAQ #6 for more info on this). In some cases this limit is purposely exceeded. Because of this we generally recommend using moderate to cooler grades of power tubes in these amps. Using cooler grades of power tubes will provide a little more tube life and a slightly less compressed sound when driven hard. A cooler grade can be requested via our comments section during checkout or it will be done by default with our Full Retube Kits.
Fixed/Grid Bias amps that do not have an adjustment trimmer need the correct grade of power tubes to keep the bias point in its optimum range. Below is a short list of amps that fall into this category:
- Mesa Boogie (virtually all models)
- Peavey 5150 series
- Peavey Classic series
- Fender Pro and Blues Jr amps
- Fender Princeton (original/vintage)
- Traynor YCV series (pre 2008)
- Marshall JTM30 and some of Artist amps
We have an extensive knowledge base on both vintage and current production guitar amplifiers. In most cases we can select the correct grade of tube for your fixed bias non-adjustable amp. We also have data and firsthand info on virtually all things Mesa Boogie. If you order tubes through our online store please leave us a note in the comments section about what model of amp the tubes are going into so we can select the correct power tube grade. If you’re ordering one of our Full Retube Kits we will automatically select the correct grade range for the power tubes.
Adjustable Bias amps must have the bias checked and adjusted as necessary anytime the power tubes are swapped. With that said once the bias has been set for a given grade of power tube from us, we can usually supply exact matching sets that will be plug-and-play, maintaining the original bias setting. We cannot match grades from other vendors as there is no industry standard for grading power tubes.