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Guitar Amp Bias

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A Biased Look At Guitar Power Amps

The power amp section of a guitar amp is comprised of the amp’s ‘big’ tubes and the output transformer. By big tubes I mean those that are typically referred to when describing an amp such as EL84s, 6V6s, EL34s and so on.

This month we’ll look at how they are typically configured, how they are “biased” and, most importantly, how those characteristics affect an amp’s tone and feel.

Push – Pull

Most tube amp power sections feature multiples of two power tubes: dual EL34s in a Marshall 50 watt; quad 34s in a 100 watt. The reason they are in pairs is because in this style of power amp one half of the power tubes are conducting current while the other is not and they rapidly alternate between being ‘on’ and ‘off’– one half pushes while the other pulls, which is the origin of the term “push-pull”.

The other common guitar power amp configuration is called single-ended. This uses a single tube or multiple tubes in parallel that conduct current all the time. Single-ended amps can have a unique and very appealing sound. We’ll tackle them in a future article.

Bias

Most electric guitarists have come across the term “bias” in reference to tube amps. Much has been written about its definition, the different methods of biasing an amp, which is ‘better’ and how to do it. Don’t worry; I’m not going to dive into an esoteric discussion of biasing details. We’ll focus on the basics of what bias is, look at the two most prevalent methods of biasing tubes in a push-pull amp and discuss how they affect an amp’s tone and feel.

Here’s the simple scoop:

When your tube amp is on and no signal is applied to its input (you haven’t plugged in your guitar or you aren’t playing) the tubes are operating, but obviously not amplifying a signal. They are in their no-signal, resting or idle state. The tube’s electrical operating condition at idle determines how much current is flowing through the tube with no signal. The idle state is determined by selecting the values of various components (resistors) associated with the power tubes. The tube’s idle state has to be carefully set up so that the tube will not be damaged and so it will properly amplify the signal.

So, biasing refers to establishing the idle state of the power tubes. More specifically, it refers to applying a particular sized voltage to the tube. Changing the value of a resistor (or resistors as the case may be) changes the bias voltage thereby adjusting the tube’s idle state.

That’s as far as we need to dig into the details of biasing!

Cathode Bias

The first bias method we’ll look at is called “cathode bias”. You’ve probably heard the term, but might not know what it means. Here goes!

Every tube has a cathode. A cathode is one of the components within a tube that is always connected to somewhere within the amp’s circuit.

In the case of a cathode biased power amp, the cathodes of the amp’s tubes are connected to a resistor and usually a capacitor (a cap is often used along with the resistor to increase gain). that are in turn connected to the circuit’s “ground”. The type of tube, the amp’s power supply voltage and how the designer wants the signal amplified all help determine the cathode resistor’s value thereby determining the bias voltage and the tube`s idle condition.

An important characteristic of a cathode biased amp is that the operating condition of the tubes changes as the amplified signal changes – as you play your guitar. Simply put the bias voltage does not remain constant. This characteristic helps define the sound of a cathode biased amp. More later!

The below diagram shows the layout of a push-pull amp that features cathode bias. Note that there are a few extra resistors shown. They are not relevant to our cathode bias discussion, but they will be when we look at fixed bias.



cathode_bias.png

Fixed Bias

The term fixed bias refers to the fact that with this set up the bias voltage remains fixed no matter what the power tubes are doing. A simple reason why this bias method is called “fixed”!

As you can see in the below diagram, unlike cathode biasing, the tube cathodes are connected directly to ground. The bias voltage is applied to the power tube circuit directly from the amp’s power supply – components are added to the power supply to provide exactly the correct bias voltage to set the power tube idle condition.



fixed_bias.pngWho cares?

If you’ve made it this far you might be wondering “why do I need to know the difference between cathode and fixed bias?” Well, in your never ending search for tone, it might be helpful to understand that an amp’s tone can be affected by the biasing method.

We learned above that the bias voltage in a cathode biased power amp varies. This actually affects the sound and feel of the amp as compared to a fixed bias rig. The change in bias voltage creates natural compression of the amplified signal. This results in what is often referred to as a “squishy” feel to the amp.

Conversely, with a fixed bias amp this natural compression does not occur and the amp will tend to have a “stiffer” feel and what some might refer to as a “barking” overdriven tone.

As always, I have to point out that there are awesome sounding amps of both types and that I do not believe one bias method is ‘better’ than the other. However, when you’re amp shopping take note of the type of bias method that each amp uses and listen for the difference.

As always I’d like to hear from you regarding this article or any amp related questions or comments. Just send me an email!

Don Mackrill

Mack Music Systems Limited