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How To Bias Guitar Tube and Valve Amplifiers

How To Bias Tube Amplifiers.

Do you own a tube guitar amplifier? Yes? Then it's good to know how to bias tube amplifiers, or we need to keep paying a technician to bias tube amplifiers for us. Just understanding the principles of how your amp works is well worth the time and effort.

We also sell a range of bias manuals written for specific amplifiers that you may be interested in.

This short article is an introduction on how to bias tube amplifiers (aimed at guitar amplifiers). I aim to explain what ‘bias’ is, and how to find out how to do it.

WARNING

There's a lot of free information on the internet. It's written by well-meaning people but a little bit of knowledge is a very dangerous thing. The manuals that we sell on how to bias a specific amplifier are carefully written by a senior electronics engineer with 40 years experience. (That's me!) They are available as a direct download to your computer and are written for specific amplifiers with colour photos of the full procedure.

If you want to bias tube amplifiers just be aware that there are lethal voltages inside the chassis. E.g. 500V DC- which is an instant killer.

So don’t mess inside your guitar amplifier unless you know what you are doing. This article is NOT an incitement for you to bias tube amplifiers or start taking your guitar amplifier to bits! A little knowledge is a dangerous thing. Don’t touch anything inside with your bare hands even if the amp is switched off. High voltage capacitors retain their charge, sometimes for days, so always aim to bias tube amplifiers safely.

How to Bias Tube Amplifiers: Why is a Bias Adjustment Needed?

You (or someone else) needs to bias tube amplifiers if:

  • If you want to change the tone (i.e. you’re not happy with the sound or you want to play around with the sound).
  • You have changed the output tubes and the sound is still not as good as when the amp was new.

Those are the only two reasons to mess with the bias. But here’s the really key point:

You don't ALWAYS need to bias tube amplifiers after a tube change.

In fact many guitarists never bias their amps. Some amps are more forgiving than others and it's true that in many cases you can get away without biasing. If you have a good ear you can hear whether the valves are running too cold or too hot. The important thing is to ensure that when you replace the power valves you always order "matched" valves. This means both valves are sharing the workload equally.

You know that amplifier valves don’t last forever. They need changing if you want to keep that great valve tone. So how long do a set of amplifier tubes last? Well, there are quite a few variables, like how hard you play, but 1,000 hours is about right. Think car tyres. A set will last longer for a careful driver than for a boy racer!

You can buy new tubes here, by the way, before you try to bias tube amplifiers.

How to Bias Tube Amplifiers After a Tube Change.

First off, when learning to bias tube amplifiers we’re talking OUTPUT TUBES only. The big ones. The power valves. So this has nothing to do with the small preamp tubes (like 12AX7, ECC83 etc.)

Typical output tubes are EL34, EL84, 6V6, and 6L6GC. In fact, that covers 90% of the amps out there.

Next, your amp will usually have either 2, 4 or 6 output tubes. Putting it simply, with 2 tubes, one ‘pushes’ the speaker cone and the other ‘pulls’ it. You may have heard the term ‘push-pull’ amplifier.

So if your amp has 4 output tubes, they are simply doubled up to give more power. So TWO tubes are pushing the speaker and TWO are pulling it.

A 2-tube amplifier will be about 50W and a 4 valve about 100W.

As mentioned above, when changing your output tubes ALWAYS buy a matched pair or matched quad. That’s non-negotiable. It means that for a given bias setting, each tube will draw the same bias current.

What The Heck IS ‘Bias’ Anyway? What is Tube Bias? Why Bias Tube Amplifiers?

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Anyone wanting to bias tube amplifiers for themselves or others needs to understand what ‘bias’ is.

Setting amplifier bias or adjusting the bias on a tube amp is a confusing subject for the non-technical. Putting it simply, bias is the ‘idling current’ the tube draws in the absence of an input signal. You can think of it like the tick-over rate of a car. All tubes in the amp are biased, but we mean the output tubes when we talk about ‘bias tube amplifiers’.

Bias Tube Amplifiers ‘Hot’

If the bias current is set too high, the valves will glow hot and will have a shorter life.

Bias Tube Amplifiers  ‘Cold’

Too low a bias current and the tubes will last longer but the sound tone will be affected. So we aim to adjust tube bias somewhere in the middle - a compromise between long life and good tone.

How to Bias Tube Amplifiers For Different Tones.

Before talking about how to bias tube amplifiers, I will briefly explain the difference between ‘hot’ and ‘cold’ bias.

‘Hot’ bias means when you bias tube amplifiers with MORE idle current than the optimum.

Result? A ‘fatter’ and warmer tone and shorter valve life.

‘Cold’ bias means when you bias tube amplifiers with LESS idle current than the optimum.

Result? A thinner tone and longer tube life.

So I always bias tube amplifiers smack in the middle of where they should be unless specifically requested otherwise.

so how do I bias my amplifier? We have created the definitive "How to Bias Amplifier" Manuals for some of our most popular guitar amplifier valve kits. Click on any of the links below to purchase that bias manual or scroll down to find out more about amplifier valve biasing.

Biasing amplifiers: How to Bias Tube Amplifiers to Get the Best Possible Sound.
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Okay, so you’ve fitted your new matched pair or matched quad of output tubes. What next?

Answer – set the amplifier bias adjustment for that set of tubes in that particular amplifier. This means adjusting a preset control INSIDE the amp in the amplifier bias circuit. This is a variable potentiometer.  In some amps, there is no 'pot' so you need to swap out the fixed resistor to one with a different value to bias the amp correctly.

You do this whilst measuring the actual current going through one of the output tubes.

You only need to measure ONE tube. That's because you will always buy a matched set so they are all the same.

Learning to bias tube amplifiers can be tricky depending on the amp. But it's not rocket science. It simply needs some knowledge and some basic equipment like a voltmeter.

Also, many/most amplifier designers have not made it easy for the user to bias tube amplifiers. The reason is obvious. If they put a handy ‘bias adjustment knob’ at the back of the amp, people wouldn’t be able to resist messing with it! The result would be burned out valves. There isn’t even an adjustment pot buried inside the chassis some of the time. Quite often the technician has to replace a fixed resistor value with the correct one if he/she wants to bias tube amplifiers.

Here’s what a technician does to bias tube amplifiers.

  1. Remove the chassis from the case.
  2. Fit the new set of tubes.
  3. Unplug ONE of the tubes and plug in a ‘bias probe’ into the tube socket (pictured above).
  4. Plug the tube into the top of the bias meter socket. (Now the bias meter can read the pins of the tube concerned.)

The bias meter shows two things:

  1. The Anode (plate) voltage on that tube. E.g. 450V
  2. The bias current (‘tick-over current’) through that tube. E.g. 35mA.

If there is an internal ‘bias pot’ (which there is on about 75% or amps) the technician will adjust this to bias tube amplifiers to the correct bias current. If there is no pot, the technician will locate the fixed resistor which achieves the same result, and ‘adjust on test’ its value until he/she gets the correct bias reading in mA.

Now you may say "I want to bias tube amplifiers but I don’t have a fancy bias meter!” Well, that’s easily solved. Buy one! They are about £100.

How to Bias Tube Amplifiers to The Correct Current

Okay, what’s the correct bias current? How the heck do you find it out when trying to bias tube amplifiers?

Here’s the problem (for you). There are about 1450 different amplifiers and the bias on each one is adjusted in a different way. The principle is always the same, it’s just a matter of where they’ve hidden the adjustment pot, or which fixed resistor to adjust.

I will be adding material to this article for specific amplifiers but I hope this brief introduction on how to bias tube amplifiers has been useful for you.

We stock a superb range of valves/tubes for all amplifiers so you can buy replacement matched tubes here.