Biasing & Matching Tubes

To fully understand these subjects, a background in electronics is more then welcome. But I will try to explain in short what matching and biasing means and how they influence your tubes and sound. Biasing means that you set the current which is available for a tube. Matching means that tubes are selected to have the same technical specs.

Preamp tubes
Preamp tubes are self biasing, so you never have to adjust the bias, current, when changing preamp tubes. In fact many types of preamp tubes can be used as substitution for each other, without having to change anything to your amp. Any of the following tubes can be used without problems: 12AX7, 12AT7, 12AU7, 12AY7, 5751, 7025, ECC81, ECC82, ECC83, E83CC, ECC803, CV4004, CV4003, etc. You will notice that one tube will have more volume and then another, depending on the type. 12AX7 (aka ECC83) tubes have the most gain, while 12AU7 (aka ECC82) have the least gain.

Preamp tubes don't need to be biased but can they be matched? Yes they can, but this is not always necessary. A preamp tube has two separate gain stage, lets call them side A and B. Matching preamp tubes in most cases means that side A and B have the same values. Or if it applies to a set of preamp tubes, this would ideally mean that all tubes have the same values.

Same values for both sides of the tube is only functional for phase inverters, the last tube before your power tubes, or for stereo applications. A phase inverter divides a signal into two. One signal is the positive side of a sinus wave, the other is the negative side of sinus wave. This is done because most guitar amps run in a Class A/B mode. This means that one (set of) power tube(s) is responsible for the amplification of positive (up) side of the sinus wave and one (set of) power  tube(s) amplifies the negative (down) side of the sinus wave. If you would have an unmatched phase inverter tube,you'll get a deformed sinus wave. Which gives you a harsh sound.


a nice sinus wave

a deformed sinus wave

The problem of an unmatched phase inverter can be solved a bit when biasing the power tubes, but it's not an ideal situation because it requires a qualified technician with proper equipment to undo the deformation. Because phase inverters have a big influence on the sinus wave it's always import to change the phase inverter at the same time you change your power tubes.


Power tubes

Just like a phase inverter needs to have matched (balanced) sides, it's important for power tubes too. Above I explained that most guitar amps run in a Class A/B mode, which means that the signal is divided between the positive and negative sides of a sinus wave. With matched power tubes it's much easier to get a nice sinus wave. If the tubes are not matched, the person who sets the bias will need to do a lot more work and in some cases it's impossible to get a nice sinus wave.

But how close should they be matched? Well there are various factors involved in matching tubes, but if they are within 10% it's ok. Within 5% is good and within 1-2% is extremely good. Only a few companies can deliver such quality, one of them is www.watfordvalves.com.

There is a lot to say about the bias of a power tube. I'll try to keep in short and simple. Each power tube has it's own technical specs, even if they came from the same factory made on the same day with the same tools and parts. Because each tube is (a bit) different from the other, it's important to control and the amount of current available for a tube. This is what's called biasing.

There are many ways of setting the bias. Most people use the following method as a starting point. A tube has maximum ratings at which it can operate and it's "normal" to set the bias at which the tube operates around 70% of each maximum rating. For example the maximum rating of a JJ 6L6 is 30 watts. Watts = Volts * Amperes. Let's assume your amp supplies your power tubes with 450 volts then the maximum amount of amperes would be 30 = 450 * x. Or 30/450  = x. This gives us 0,067, which is 67mA. 70% of this is 47mA. This would be a good starting position of setting the bias. The amount of mA set for a tube depends on the maximum ratings for the tube and the amount of volts which are applied on the tube. < /FONT>

What happens if you set the bias lower then 70%. Then your tubes will run 'cold', they don't get enough power to do their work good enough. This will result in crossover distortion, which is a deformed sinus wave. The amp will sound harsh when overdrive, sterile sound, less volume, etc. But your tubes will also live longer. But if you really think your Peavey or Mesa amp is producing 120 watts, you're far off. They run really cold and will in most cases not produce more then 70 watts! But there is some good news for you who are shocked now, watts say not so much about volume, but that's another story.

Not all amps have an adjustable bias but a fixed bias, such as Mesa/Boogie amps. They run their amps quite cold (aprox 60% of max ratings). Although there are several advantages of a fixed bias, if you want to get the best sound possible, installing a variable bias pot is more then recommended!

If you set the bias higher then 70% your amp will run "hot". If it runs too hot, then the tubes won't live very long, so this is not really wanted, but running up to 85% should be possible in many amps with many tubes. An amp that runs "hot" will give you a warm (brown) sound, lots of dynamics, nice rich harmonics, etc. And the sinus wave will in most cases be a nice and even.

For more information on matched phase inverters, check out the articles by Myles Rose at guitaramplifierblueprinting. Other interesting links on matching of tubes and setting the bias: