Tube Basics with Freddy Demarco


The Tube: A Quick Lesson


If you are choosing a new amp or trying to better understand the amp you already own, it is beneficial to have some basic knowledge of tubes. The tube is a major part of your tone, in fact, I actually play differently when I play through a variety of amps with different tubes, as tubes are critical to the vibe of your sound and feel.Tube 12AX7

A basic understanding of the tubes, and how they make the amp react, will help you to understand what to expect from your amp’s performance, so you can use it to its best potential. Knowing the basics of an amp’s tube structure will be one of the important factors in understanding whether the amp will have a fat dark tone, a bright cutting sound, or a punchy midrange. The tubes work in conjunction with an amp’s gain, tone stack, and its interaction with the transformer. This means the amp’s signal path relies on the tubes.

Pre or Power

Think of an amplifier as composed of two halves. One half of the amp is what is referred to as “front end” of the amp. The “front end” is the part of the amp where the “pre-amp” “gain” “tone stack” (treble, middle, bass) and phase inverter are located. The second half of the amp is often called the “power section” of the amp. This is where the power tubes and transformers work their magic. The “pre-amp” “front end” of the amp relies on “pre-amp” tubes, whereas the “power section” is powered by specific “power tubes”.

The 12ax7 or ECC83

In the United States, the most popular preamp tube is called a 12ax7 tube. In Great Britain, this same tube is called an ecc83 valve. Here in the States, we call them tubes, while in Britain, they are called valves. To be clear, a 12ax7 is exactly the same tube as an ecc83 valve. 12ax7 tubes typically do not need to be changed as often as power tubes, however, experimenting with different brands of pre amp tubes can change and color the gain (front end distortion) and the tone stack slightly. Swapping out pre-amp tubes will not change your tone as drastically as would changing out the power tubes. Some tube brands rate the distortion level of some of their tubes. This means that with a higher distortion rated tube, the amp will break up quicker.

The four most popular power tubes are EL34s, 6L6s, EL84s and 6V6s.Tube EL34

The Mighty EL34 tube

EL34 tubes are often associated with British amps such as Marshall Amps. These power tubes are aggressive sounding and have more headroom than EL84s and also break up quicker. The EL34 is often associated with the phrase, “British Sound”.

The Touch Sensitive EL84 Tube

EL84 power tubes also have an aggressive sound and are usually found in smaller wattage amplifiers. They break up quicker than EL34s and 6L6s. EL84s produce less low end than EL34s, and can be a bit brighter as well, while being the power tubes used in the VOX AC15 and AC30 amplifiers.

Tube 6L6

The Sound of American: The 6L6

6L6 power tubes are used in many amps, including the Fender Twin amp and are great for clean headroom, since they have more headroom than EL84s and EL34s. 6L6 tubes put out a much brighter tone and more top end sparkle. They are often associated with the term, “California Sound”. 6L6 power tubes should not be confused as being tubes for only clean, non-distorted amps, because they also are used in Mesa Boogie Rectifier amps, known for their NuMetal high gain tones. They work great for the heavy distorted de-tuned low power riffs emitting from the Mesa Boogie Rectifiers, because, due to their headroom, they can handle the low resonant frequency.
Tube 6V6

The Low Powered American: The 6V6

The 6V6, for the most part, is thought of having a close tonal characteristic to the 6L6 “California” and “Tweed” kind of tone associated with Fender amplifiers. The main difference is that the 6V6 is a lower output tube that breaks up quicker than a 6L6. It has less headroom than the 6L6, and so, the 6V6 is typically not used for ultra high gain metal amp heads.

When Should I Change My Tubes?

When you notice your tone and sustain starting to suffer, it can be two different things. It cab be an indication that the tubes need to be changed, or that one or more tubes may be failing. Listen for micro phonic ringing or a dying tone. If you hear it, look to see if any of the tubes is not glowing like the others. Never pull out a tube until the tubes have cooled down for at least twenty minutes. To be safe, you should take the amp to an authorized service center, if your amp needs to be re-tubed.

To Bias Or Not To Bias?

A common question regarding tubes is when to “bias” or not to “bias” a set of tubes. Biasing an amp is essentially calibrating the tubes to work properly with your amp. If your amp requires biasing and you neglect to have the procedure administered, the amp will not perform to its potential. It may sound brittle, not emit the tones expected, and the saturation will not be as musical sounding. The amp will not “feel right”. You should bias your power tubes when you change your tubes.

Your 12ax7 preamp tubes do not need to be biased. If you have an amp that features a “cathode” bias, you will not need to bias the power tubes. Think of cathode bias as an automatic bias. VOX AC30 and AC15 amplifiers are examples of cathode biased amps. Fender Twin Reverb’s or a Marshall Plexi’s are not cathode biased and would need the biasing procedure performed. Biasing should be done at an authorized amp repair shop.

The Encore

When choosing a new amp, or trying to better understand an amp you already may own, keep in mind the characteristics of tubes as discussed above. With this information you can choose the tube configuration that will best reflect your personal tastes and requirements.

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Written by: Freddy Demarco


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