Know Your Amp Bias

Know Your Bias

Different Types of Amp Bias

Just like the idle of your car may need adjusting when a major change is done to the engine, new power tubes need their idle (the “bias”) adjusted to make sure the amp sounds as good as possible. Some amps do this automatically, but many do not – particularly in the world of guitar amps. Many tube amp owners are unaware of this simple regular service for their amp that will keep it sounding its absolute best. The bias methods of amplifiers fall into three basic categories:

    • Non-Adjustable Fixed Bias amplifiers (no bias adjustment potentiometer)
    • Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the “fixed” bias point)
    • Cathode Bias amplifiers (also commonly referrred to as self-biasing)Each of these types is unique.

Adjustable “Fixed” Bias

Adjustable Fixed-Bias

For adjustable fixed bias amplifiers, the amp should be biased by a tech with experience with this procedure. It’s not difficult and it doesn’t take long but if you don’t know what you’re doing you may hurt yourself (amps can give lethal electric shocks even when unplugged, and NO, we’re not just saying that to scare you). Also, an unbiased amp may sound horrible or cause undue wear and tear on the tubes or power supply. Until you learn how to do it correctly by yourself, it’s best to pay a technician a few dollars to do it for you.

There can be some exceptions. If you already have tubes with our Perfect Pair rating, or rated tubes from another seller such as Groove Tubes, you can replace them with a new set that has the same rating and safely avoid rebiasing. When ordering you should mention the rating you require in the “Comments” box on the checkout page.

Non-adjustable “Fixed” Bias

Non-adjustable Fixed-Bias

Non-adjustable fixed bias amplifiers without an adjustment pot cannot be adjusted. These amps (for example, all Mesa Boogie models) use a fixed resistor to bias the tubes, not a pot, and are designed to run with tubes that have a specific current draw range and should only use tubes that fall within that certain range. Fortunately, these amps are very forgiving and will work well with a pretty wide range of tubes.

For the best results, when you order be sure to specify the make and model of your amp in the Comments box on the checkout page (or simply order a Shop by Amp tube package) and we will make sure you get the best tubes for your amp.

One more thing: some fixed/non-adjustable bias amps have a “bias switch” to select for a specific tube type. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune.

Cathode Bias (Self Adjusting)

Cathode Bias (Self Adjusting)

Cathode bias amplifiers shouldn’t require any adjustments and will work with a wide range of tube plate currents, as the circuit is “self adjusting”. Generally speaking these amps are considered plug’n play when it comes to replacing the tubes.

Bias exceptions

Bias exceptions

As with anything, there are always weird exceptions. In the rare case where your amp is an “exception” we suggest you consult your manual or a tech to learn if any special service is required to change the tubes.

5 Responses

  1. dave
    dave at |

    i thought i would replace the power tubes(6l6gc) in my vintage fender princeton silverface amp.the present tubes only say 6l6gc china. what tubes do you recommend, and does the amp need re-biasing?would also like to replace preamp tubes(12ax7). all tubes are 16yrs old. thank you.

    Reply
  2. Jon @ thetubestore
    Jon @ thetubestore at |

    A silverface Princeton should not use 6L6GC tubes. It is designed to use 6V6GT tubes and these are very different from 6L6GC. Unless the amp was modified to operate with 6L6GC tubes the transformers will be over-worked and the tubes likely very under biased. Assuming you have a Princeton that is not modified we suggest you rather use Tung-sol 6V6GT tubes for a good authentic vintage tone at a reasonable price. This amp does not have adjustable bias.

    Regarding preamp tubes there are a ton of great options. Our Preferred Series 7025 tubes will be very tough to beat for both tone and quality. And they have the longest warranty (6 months). Other nice preamp tubes are Tung-Sol, Svetlana, and the JJ’s if you need something very inexpensive.

    Look at our Premium Package for a recommended tube set:
    http://www.thetubestore.com/Shop-by-Amp/Fender-Princeton-Amps/Fender-Princeton-Premium-Package

    Reply
  3. Mark
    Mark at |

    I have been coming up with a lot of shorted output tubes from groove tubes in the fender amps so far el34 and el84 maybe from shipping dropping but fender always sends new ones. pretty obvious when you turn an amp on and the speaker is making a loud popping sound before tubes have time to warm up.

    Reply
  4. rene
    rene at |

    I have a Marshall 9000 slave amp with 2 channels that are working ok but both are putting out some hum from the speakers. Does this mean the valves might need adjusting or replacing? By the way, I have never changed the valves in all this time at least 30 years.

    Reply

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