The ins and outs of tube bias....

Over the years, a very common question that comes up in regards to tube amps is bias.  While there are many in depth and thorough explanations out there, I figured we would strip it back to its simplest form and give an idea of why this is such an important part of a tube amplifier's health and tone.  

We will be addressing the two main types of biasing used in guitar tube amplifiers:  Cathode bias and Fixed bias.  These are two different approaches to the same problem.  What is the problem?  Let's start with some basics on how a vacuum tube operates.

A note on how vacuum tubes work...

Vacuum tubes are also called electron tubes for a reason!  Every tube has a few parts:  the cathode, the plate (anode), and the grid.  These parts operate inside of a vacuum.  The cathode is responsible for the emission of electrons when it is heated.  At this point, the hot cathode is producing a bunch of electrons that repel one another in the vacuum.  The plate comes in here.  The plate carries a positive charge and so attracts the electrons.  The electrons go to the plate and become moving current.  Now the grid plays its role:  The grid is literally a small mesh of wire that surrounds the cathode without contacting it.  By applying voltage to the grid, we can control how many electrons make it through to the plate.  This controls the output voltage of our tube and allows us to get a relatively large change in output with a relatively small change in input voltage.

 So....where does bias come into this picture?  The bias is in relation to the grid.  It is a constant negative voltage applied to the grid to control the amount of electrons that may pass.  If there was no control of the amount of electrons passing to the plate (the moving current), the components inside the tube would break down and fail.  When setting the bias, you are literally setting the idle to be right for a particular set of tubes so that they may operate optimally inside of their specification.

Types of bias

Fixed Bias - Fixed bias is where we apply a certain "fixed" negative voltage to the grid, usually by an adjustment inside and dependent on the tube.  This type of bias is stable and efficient -well suited to higher powered amps.  These types of amps require having the bias reset whenever changing the power tubes to ensure proper operation.  Some fixed biased amps include:  Fender Twin, Marshall Plexi, 3rd Power Dream 50 Plexi, Dumble Overdrive Special... 

Cathode Bias - Cathode bias is when we use a component (a cathode bias resistor) to determine the bias voltage.  As current flows through the tube, it also goes through the bias resistor determining the bias voltage.  This method is less efficient (produces less overall power and headroom) and not stable enough to be well suited for higher-powered amplifiers, but does produce a different style of amp.  These amplifiers do not need to have the bias set when changing tubes - they are "self-biasing".  Some cathode biased amps include:  Vox AC30, Fender 5E3 Deluxe, DR. Z Maz 38, Divided by 13 JRT 9/15, Suhr Badger....

Biasing it "HOT"

There is a range to the bias a power tube will operate in safely.  Biasing towards the hotter end of the range will produce earlier breakup and also reduce the life of the tube since it will be heating up more.  Biasing is cooler will increase the headroom but can make an amp sound sterile and lifeless.  Proper biasing will allow the amp to operate as intended by the manufacturer and get good tube life.  Many times when we have a customer with an amp that we know should be amazing but that sounds lackluster, re recommend a retube and bias.  This almost always puts the magic back and resurrects the amp!

A note on preamp tubes

Preamp tubes are almost always cathode biased.  That means you can change these yourself and not worry about the bias.  This can be a fun way to try some new tones in your favorite amp.  Make sure to talk to someone in the know before you go swapping like crazy to make sure you use compatible tubes though

Wrapping it up...

We recommend you retube your amplifier once a year to once every few years depending on the amp and your usage of it.  If your amp is fixed bias, always have your bias set when replacing power tubes.  This will keep it happy and healthy for years to come!

We carry a nice selection of preamp and power tubes here at Righteous Guitars and offer on-site biasing.



November 05, 2020 — Ben Calhoun