Difference Between Cathode and Fixed Biased Tube Amps

Tube amps are the gold standard, even in our time when digital amp modeling is gaining a lot of popularity. The warmth and thickness of a tube circuit is not easy to replicate, which is why most of the pros still use high-end tube amps.

In those amps, all power tubes must achieve bias by some means, where bias is the amount of power dispersed based on the amount of voltage applied.

I'll cover the two types of bias, and the technical differences between them.

• Cathode Biased
• Fixed Biased

I'll be honest, I did not understand the distinction before digging into this topic. There are actually some significant implications worth considering if you're looking to buy a tube amp.

We'll cover the technical and style distinctions between the two.

What is bias?

As I mentioned, bias refers to the amount of power a tube dissipates, based on the voltage applied. 

Without bias, tubes would burn up pretty quickly. 

And if bias isn't effectively dialed in, you can end up with a sound that's just too cold and boring, or a bias that is way too hot that can destroy the tube. 

Note that this terminology applies only to power amp tubes. Preamp tubes are almost always cathode biased. So don't worry about preamp tubes for this discussion.

Cathode Biased Tube Amps

A cathode biased tube amp uses a resistor, connected to the cathode and then grounded. In this system the cathode resistor self-regulates, allowing the tube to achieve bias automatically, depending on the level of signal it's receiving.

That's a bit confusing, but the implication is that these tubes run cooler and have a longer life-span than fixed bias tube amps.

Tone characteristics

There are also some tone characteristics to consider:

• Smoother/softer
• More compressed
• More mellow
• Lots of sustain

Overall, cathode biased tube amps are subtle, and good for styles like jazz and blues. Some would also say they have a more forgiving playing feel, though I would argue that's pretty hard to detect. This also makes them less ideal for heavier tones and high gain settings.

Disadvantages

There are also some disadvantages to cathode biased tube amps.

Primarily, they tend to have lower head room, meaning they can't handle as much volume before giving you unwanted distortion. This can make it difficult to play in environments where a lot of volume is needed.

This also makes them a less ideal companion for heavier distortion sounds. It's just not tight enough to make high levels of gain sound good.

A couple examples of popular cathode biased tube amps include:

• Fender Blues Junior
• VOX AC15

On to fixed biased.

Fixed Biased Tube Amps

Fixed biased tube amps use a separate circuit to provide negative voltage to the power tubes, while the cathode is just grounded. This means that the voltage applied to the grid doesn't change, which is why it's called a fixed bias.

This is why we get a much tighter and somewhat more aggressive tone from these amps.

Advantages

Fixed biased amps have higher headroom than cathode biased amps, which means they'll put up with a lot more volume before distorting. They also have a much tighter and more dynamic tone, which can better handle high levels of distortion and aggressive styles like metal and hard rock.

For this reason they run fairly hot, and the tubes tend to have a shorter lifespan than their cathode biased counterparts.

A couple examples of popular fixed biased tube amps include:

• Mesa/Boogie Dual Rectifier
• Marshall JCM800

Summarizing the differences

Here's a quick summary of the differences between cathode biased and fixed biased tube amps:

• Difference in how they bias the power tubes
• Cathode biased achieves bias dynamically while fixed biased relies on a separate circuit with the cathode grounded
• Cathode biased amp tones are smoother and more compressed, while fixed biased amps are tighter and more dynamic
• Cathode is more subtle, biased is more aggressive

You can now see why all of this technical jargon has some implications, depending on the style of music you play. Blues and jazz guitar players who work in softer and more subtle playing styles will be better served by a cathode biased tube amp.

On the other side, guitar players who focus on metal, hard rock, and high levels of distortion will want to go for the fixed biased tube amp with tighter tone and higher headroom.

Wrap it up

I'm glad I took the time to figure this out, because there are some really practical implications for an otherwise technically complex topic. If nothing else, I now know why amps like the JCM800 and Dual Rectifier are used in heavier styles.

Same with VOX and Fender tube amps, in more subtle styles.

So if you have questions, or you notice something I missed or got wrong in this article, jump in the comments section below and share your thoughts.

I've been wrong before.

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