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When Do I Have To Bias The Tubes In My Guitar Amp?

I frequently answer this question: "When I change the power tubes in my guitar amp, do I have to have them biased?".

The simple answer is: only if your guitar amp power tubes are "fixed biased".  Conversely, if your amp's power tubes are cathode biased, you don't have to rebias them when they are changed.  

All Mack amps feature cathode biased power tubes so they do NOT have to be biased when they are changed.

What does that mean?  Well, as noted above there are two ways to 'bias' the power tubes of a guitar amp: fixed bias and cathode bias (often referred to as self bias).

What Is Guitar Amp Power Tube Biasing?

Before we get into the differences between the two methods of biasing a guitar amp power tube, lets first figure out what 'bias' means!

For a tube to operate properly there has to be a constant voltage between two of its pins (the 'grid' pin and the 'cathode' pin) called the 'bias voltage'.  The exact value of the voltage is determined by the designer taking into account the tube, the rest of the amp's circuit and how the designer wants the amp to sound.

The act of adjusting the bias voltage is called 'biasing'.

As noted above, there are two ways to set the bias voltage between the grid and cathode: fixed and cathode bias.

Guitar Amp Biasing: Fixed Bias and Cathode Bias

Cathode or self biased tubes are arranged in an amp's circuit such that a single component - a resistor commonly called the cathode bias resistor - placed between the tube's cathode pin and ground determines the bias voltage.  As current flows through the tube it also flows through the cathode bias resistor and a voltage is developed across the resistor - the bias voltage.

If you change tubes the cathode bias resistor remains constant of course.  If the new tube's operating characteristics are slightly different than the old tube's either more or less current will flow through the new tube compared to the old one.  That means more or less current flows through the bias resistor and the bias voltage will automatically change as well.  That happens automatically because as the current flowing through a resistor changes, the voltage across the resistor changes along with it (according to Ohm's Law).

As can be seen above, the bias circuit develops its own bias voltage according to the tube's operating characteristics and that's why it is called 'self bias' - the circuit biases itself when a tube is changed!

Fixed bias refers to a constant (or fixed) voltage that is created in the amp's power supply and wired to the tube's grid pin.  In this case, the bias voltage is completely independent of the tube.  

Therefore, if a tube is changed and the new tube's operating characteristics cause more or less current to flow through it, the bias voltage must be manually altered to maintain the correct voltage difference between the grid and cathode pins.  

That means a tech has to change the fixed bias voltage in the amp to ensure that the new tube is properly biased and operates the way the designer intended.

Conclusion: if your amp features cathode biased power tubes you don't have to worry about biasing.  If your amp features fixed biasing then you have to get your amp re-biased every time you change one or more power tube.

Why Do We Only Worry About Biasing Power Tubes and Not Preamp Tubes?

Preamp tubes are always cathode biased in guitar amps so re-biasing is not required if you swap a preamp tube.

Why Aren't All Tube Guitar Amps Cathode Biased?

You would think that guitar amp designers would always use cathode biasing so their customers wouldn't have the hassle of re-biasing power tubes!  Why are so many amps fixed bias?

There are two reasons.  First, fixed bias and cathode bias amps sound different from one another.  

Cathode biased amps tend to produce more resonant tone particularly when the power tubes are overdriven.  That's why Mack amps are cathode biased: I love the sound of classic overdriven crunch and distortion and the best (and least expensive!) way to get that tone is with cathode biasing.

Fixed bias amps tend to be somewhat less resonant and smoother.  Nothing wrong with that.  MANY great sounding amps are fixed bias.

Second, as the guitar amp's output power increases cathode biasing becomes less efficient electronically.  40 watts is about the limit for cathode biasing.  Above that it almost always makes more sense to use fixed bias.  Then, if the designer wants to capture the resonant crunch of a cathode biased amp they have to implement other design techniques.