What Is Tube Amp Bias?

If you’ve ever owned a tube amp you’ll have heard the term amp bias bandied about. But whether that meant anything to you is another question.



What is amp bias?

Tube Amp Bias is an electronic process that ensures the power amp tubes in your valve amp run at their optimimum capacity so that you can get the best possible guitar tone out of them. It ensures the tubes are fed the correct voltage according the valves resistance rating.

What will I learn?

  • The meaning of the term ‘Amp Bias’
  • How tube amp bias will affect your guitar amp tone
  • When you need to have a tube amp biased

The guitar community seems to be shrouded in the mystery that is tube amp bias and in a way, it’s probably for the best. This is because adjusting the bias on a tube amp can be very dangerous if not done by a professional. But, it’s worth having a basic understanding so that the next time you pop into the store and your sales assistant explains that the Mesa Boogie Mark V is a fixed bias amp and the Fender Hot Rod Deluxe isn’t, you won’t be completely in the dark.

It’s also worth noting that when talking about tube amp bias, only the power amp valves are in question. Typical power amp valves are EL34, EL84, 6V6 and 6L6GC valves.

Please note: We do NOT recommend that you attempt to bias your own amp. This should only be done by a trained amp tech. There’s a lot of charge stored in a tube amp and just touching the wrong part of the amp CAN KILL YOU ?. Even if the amp is switched off or has been off for a few days. Voltage capacitors retain their charge which can be very dangerous!

So why is Tube Amp Bias important?

In absolute layman’s terms, having the correct tube amp bias will ensure that your power amp valves run at their optimum. This means you can get the best possible tone out of them because the tubes are being fed the correct voltage according to the valves resistance.

Correctly biasing your amp will mean one set of valves can have incredibly long life because they’re running at the perfect ‘temperature’.

So if you don’t bias your amp correctly you’ll face the following scenarios:

  • Run the valves too cold (Over-biased) – This means that there isn’t enough voltage being provided to the power amp valve which will give you a thin, sterile sound without any of that lovely warm tube saturation. Valves will probably have a far longer life because they aren’t being overworked but the amp will never sound the best that it can.
  • Run the valves too hot (Under-biased) – This means you’ll cook and saturate the valves very quickly. This can cause massive problems for your amp because you’re allowing far too much power through the power amp circuit. At best you’ll get an overly saturated tone with a much shorter valve life than normal. At worst you’ll melt other components in the amp further down the line or even cause an electrical fire.
  • Run the valves at optimum – This is somewhere between hot and cool where the valves can perfectly resist the power coming at them. Happy valves means happy tone and a very happy player. This is the balance between tube life, and ultimately, the best tone.

Tube Amp Bias is like water through a tap

If your perfect bias is a steady, smooth stream of water flowing from a tap, then having the amp incorrectly biased can drastically alter that flow.

You can turn your perfect, steady stream of water into a slow drip (By running it cold) or an uncontrollable overflow that’ll eventually flood your kitchen (Running the amp too hot and letting too much power run through).

mullard valve amp tubes

When does a tube amp need to be biased?

On 95% of all tube amplifiers, you’ll need to bias the amp whenever you change tubes.

Some amps will never need to be biased because they come out of the factory at a fixed bias like Mesa/Boogie amps. So this means you can change the valves out for other Mesa/Boogie valves without worrying about bias because they test all of their valves before leaving the factory. You can then use the correct tube spec in your Mesa/Boogie amp.

Most companies do not test valves before shipping them which is why they can vary in terms of resistance.

Example of incorrect tube spec:

Imagine your amp is currently set to run quite ‘hot’ but has high resistance tubes which match perfectly.

But you now want to change tube brand or spec…

If you changed to tubes with a lower resistance without biasing the amp you would quickly burn through those tubes and potentially damage the amp and yourself in the process. This is because the amp is still pushing out current at a higher level than the valves are able to take. Until it’s been correctly re-biased.

Conclusion: You’ll need to bias most tube amps if you change the tubes. This is because you cannot buy tubes with a similarly matched resistance. You should always buy a matched pair or a matched quad of tubes, but the chances of them matching your current pair are very small. So, get the amp to a tech and get it biased!

Tube Amp Bias

This is one of those topics that can easily go over your head if you aren’t too tech savvy. But as with all things guitar, if it sounds bad, change it. If it sounds good then play it until you can’t anymore.

If you have any further questions about your amp, tubes or similar please contact us.

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Jed has worked on our shop floor, handled guitar content on the site and now leads the digital content team. He's equal parts rock frontman/guitarist and wannabe folk singer-songwriter. Jed's a PRS, Tele and Orange Amps lover with an unhealthy obsession with fuzz, octave and ambient effects.

Responses & Questions

One response to “What Is Tube Amp Bias?”

  1. Steve Colley says:

    Brilliant piece of writing Jed I run 3 messa amps a road king Head a 25watt rectoverb and a MKV head have changed valves in them over the last few years without any problems and like you say the new valves are matched pairs . Ive just purchased from andertons a New Victory Sherrif 44 head and they have a Bias adjustment fitted on he back that can be done outside of the amp with a mutimeter and an adjustment screw. Have you ever had dealings with this product. Martin Being a great designer has allowed a great facility here

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